The Effects of Intermittent Fasting on Human and Animal Health

Intermittent Fasting

Three years.

It has been three years since starting my study on Intermittent Fasting (IF) and its proclaimed health effects.

I am finally ready to go public.

A quick background story…

Why IF? Because for almost a decade I’d been interested in the subject. Can we really lose weight, improve health and even live longer by staring at an empty plate every now and then? It was against everything I’ve learned, in the gym, in bars, even in med school.

Seemingly, there was a lot of confirming evidence to be found online, showing amazing body transformations and improved health for many individuals. I spoke with many of those people. – Wow, to hear the stories was such an inspiration. But from a scientific standpoint the stories were highly biased and not doing much to further improve our knowledge. You think anyone ever changed their feeding frequency and kept all other things equal? Of course not. How than can we know what Really improved their life? IF or the fact that they also changed their training routine, stopped smoking and started to walk to work? Or a combination?

I wanted to see what we knew at this point. How much was the subject really studied and what did the evidence say?

I finished the work in about one year and the result was awesome. The audience loved it.

Intermittent Fasting

I got my MD and felt on top of the world.

But where do I go next? First, the idea was publication in a scientific magazine.  For many reasons, taking time from becoming a good clinician being the most important, but far from single one, I eventually decided not to go that route. Ambivalentia led me to sit on it for way too long, sharing it only with personal friends and people I respect…but not much more. I just could not decide what I wanted to do with it. Since finishing it I’ve had several offers from various ‘important’ people wanting to use my paper. The problem was always integrity. This is my baby. I’m not interested in other people making a quick buck on something that was my life for over a year and giving me a piece of the cake.

Finally, after some recent online pressure – I decided to give it away for free – right here. There is no use in it sitting on my dropbox-account.

Please show your appreciation for the enormous work that this required from me, a hyperactive person that normally can’t sit for more than 10 minutes. And the fact that you are getting it for free, on a sliver plate, without having to spend 300 nights, bent over scientific papers, pulling your hair out.

Share it with your friends – the social media buttons are on your left. If you like it, please drop a comment below, let me know. Don’t be anonymous.

If you want to know more about IF I’d recommend you to buy Brad Pilons book Eat-Stop-Eat.  The book, in my opinion, is not perfect, and I don’t agree with all the conclusion drawn in there – but it is by far the most complete recourse available for purchase out there with practical implications for anyone interested in the IF lifestyle, and it has helped many people get leaner and healthier with a relaxed mindset.

Without hesitation the best online recourse on the topic is my dear friend Martin Berkhan’s . Martin is the original popularizer of the IF method  and has researched the subject extensively. He also helped me get the project started with some pointers and connections.  What’s more, he is a great dude and I made a friend in the process, and that I am very grateful for. He is a guy I’d share my last beer with. Martin, I know you’re reading this so thanks, bud.

Now, grab a (big) cup or coffee and enjoy the read.

Your friend,


Intermittent Fasting

The effects of intermittent fasting on human and animal health –

a systematic review

Thesis January 2012, University of Lund by Bojan Kostevski

Center for Primary Health Care Research, Lund University, CRC, SE-205 02 MALMÖ, Sweden

Supervised by: Bengt Zöller, Staffan Lindeberg

Examination: Daniel Arvidsson

Keywords: intermittent fasting, calorie restriction, obesity, aging, cardiovascular health, glucose metabolism, cancer, neurodegenerative disease.




An increasing number of animal studies have shown altered markers for health in subjects exposed to intermittent fasting, i.e. regularly and repeatedly abstaining from eating during 12-36 hours per period. It has been hypothesized that the reported beneficial health effects from caloric restriction on excess body weight, cardiovascular risk factors, glucose metabolism, tumor physiology, neurodegenerative pathology and life span can be mimicked by alternating periods of short term fasting with periods of refeeding, without deliberately altering the total caloric intake. Therefore, a systematic review of available intervention studies on intermittent fasting and animal and human health was performed. In rodents, intermittent fasting exhibits beneficial effects including decreased body weight, improved cardiovascular health and glucose regulation, enhanced neuronal health, decreased cancer risk and increased life span – some of the effects independent of the effects attributed to calorie restriction alone. The human studies performed to date are generally of low-quality design. Beneficial effects such as weight loss, reduced risk for cardiovascular disease and improved insulin sensitivity have been observed, but conflicting data exists. The potential health promoting effects of intermittent fasting in humans and applicability to modern lifestyle are discussed.


Calorie restriction and intermittent fasting

Almost a century has passed since Osborne and colleagues in 1917 observed that reducing calorie intake in rats increased the animal’s life span (1). In 1935, McCay et al. were first to describe that calorie restriction – deliberately reducing calories without causing malnutrition – prolongs mean and maximal lifespan in rats compared with rats fed ad libitum (2). Numerous subsequent studies have confirmed that a calorie restriction of 30 to 60 percent of ad libitum intake increases the life span by similar amounts in a range of organisms including yeast, roundworms and rodents, while simultaneously decreasing or delaying the occurrence of age related diseases such as numerous cancers (including lymphomas, breast and prostate cancers), hypertension, stroke, diabetes, nephropathy, autoimmune disorders and other risks factors for cardiovascular disease (3,4). Furthermore, it is suggested that calorie restriction can display beneficial effects in rodent models of various neurodegenerative diseases such as Alzheimer’s, Parkinson’s and Huntington’s disease (5). Accordingly, overeating is considered a risk factor for the majority of the conditions mentioned above, further supporting the hypothesis that calorie restriction can be beneficial (6,7). To further explore the relevance of these findings in rodents on primate health, a study was initiated at the Wisconsin National Primate Research Center (WNPRC) in 1989, studying a 30% calorie restriction in rhesus monkeys (8). The incidence of diabetes, cancer, cardiovascular disease and brain atrophy was reduced in animals on the calorie restricted diet compared with monkeys on the control diet. Twenty years into the study, 80% of the calorie restricted animals were still alive, compared with 50% of the control fed animals. The data obtained to date suggest that calorie restriction slows aging in primates and improves health.

Calorie restriction in humans is associated with weight loss, reduced inflammation and improved markers for cardiovascular and metabolic health in obese{Formatting Citation} (9,10) as well as non-obese (11,12) subjects, proposing a novel therapy for increasing life span. However, adherence to the recommended calorie reduced/low fat diet remains an issue for some people in the long term (13,14).

To improve compliance in human subjects, a model in which calories are periodically restricted has been proposed. Intermittent fasting is a paradigm where periods of fasting are cycled with periods of over-eating where subjects are fed ad libitum. Alternate-day fasting, one model of intermittent fasting has been widely used in animal calorie restriction research because it has shown to result in reduced food intake over time and decrease body weight in rats (15). In human trials, intermittent fasting has been shown to be equally effective as daily calorie restriction for causing weight loss in obese subjects (16).

While alternate-day fasting leads to calorie restriction over a two-day period in many rodent species, in some strains of mice, the animals managed to compensate for the calorie deficit created on fast days by increasing their intake on feast days twofold and thus keeping the total calorie intake over a two day period at the same level as in mice fed an ad libitum diet (17). These mice managed to maintain constant body weight but, interestingly, still acquired some of the health benefits as rats on daily calorie restriction. This lead to the hypothesis that by implementing periods of fasting, one could improve health without deliberately reducing calorie intake. My objective was to review relevant intervention studies on the effects of intermittent fasting on energy balance, cardiovascular risk factors, glucose metabolism, neurodegenerative pathology, tumor physiology and life span.

Significance: The study will be important for the understanding of excess caloric intake and the management of obesity, and identify ways to alter cardiovascular, metabolic and neuronal health.


A systematic review of intervention studies in mammals, including humans was performed. PubMed between 1973 and 2011 was searched by use of relevant MeSH terms related to the effects of intermittent fasting on excess body weight, energy balance, aging physiology, cardiovascular risk factors, glucose metabolism, tumor physiology and neurodegenerative pathology. For each of the MeSH terms, the search process was restricted by use of non-MeSH terms such as “intermittent fasting”, “periodic fast”, “alternate-day fasting” and other relevant terms. All terms used are listed in Table 1. Furthermore, relevant review articles on calorie restriction and intermittent fasting were reviewed for additional relevant studies to include in the review. Studies were included in the review if short term fasting was the primary intervention and studied any of the above mentioned outcomes. Studies that purposely restricted calories in the intermittent fasting group were excluded.

Animal trials

Energy intake and body composition

A total of 36 studies were found. When an alternate-day fasting diet is implemented, overall calorie restriction and weight reduction occurs in most rodent species, indicating that the restriction on the fasting day isn’t compensated fully on feasting days when food is offered ad libitum (18-34). Consequently, alternate-day fasting is a widely used model for studying the effects of calorie restriction in rodent species (15). This however is not a universal finding and numerous studies have reported no alterations in energy intake and body weight (17,35-39). In general, studies using Sprague-Dawley and Wistar rats show decreased energy intake and reduced body weights (15,25). However, C57BL/6 mice maintained on the same alternate-day fasting regimen consume similar food quantities in a 48-hour time period and maintain body weights similar to that of mice fed ad libitum (17). The effect of intermittent fasting on body weight thus seems largely dependent on the animal genotype but could also be affected by the age of initiation, with optimal age varying in the various rodent strains (40).

Modified alternate-day fasting is one alternative model sometimes used in the intermittent fasting research. In this model, the animals are not completely fasted every other day, but allowed a small energy intake of 15-25% of the daily intake consumed by ad libitum fed animals. Modified alternate-day fasting could allow for better maintenance of body weight than true alternate-day fasting protocols (a complete every other day fast) (30,38). The complete compensation and increased energy intake does not appear to be dependent on the calorie density of the food, since neither a high-fat or low-fat 85% modified alternate-day fasting diet alters body weight compared to ad libitum feeding over a four week period (41).

Total body weight however does not reflect alterations in body composition, and there could be changes in lean mass to fat mass ratio or altered fat distribution which of course would not be reflected in the animal’s body weight alone. Alterations in fat distribution were demonstrated in one study in which mice on both true and modified alternate-day fasting diets showed a redistribution of adipose tissue from visceral to subcutaneous depots without altering body weight overall (39).

Cardiovascular health

Four rodent studies that examined the effect of alternate-day fasting on cardiovascular disease were included. In general, rats maintained on an alternate-day fasting regimen lose bodyweight and display reduced blood pressure and heart rate, and improved insulin sensitivity, compared to rats fed ad libitum (28,29,42). Reduced blood pressure was also demonstrated in diabetic rats, proposing that alternate-day fasting can have a preventive effect on the progression of diabetes nephropathy (32).This data is suggesting that intermittent fasting may reduce the risk of cardiovascular disease.

Furthermore, when myocardial infarction was induced in rats maintained on an alternate-day fasting diet, reduced infarction size, improved cardiac function, and increased survival was observed, compared to rats fed ad libitum (24,33,43). More interestingly, the effects on infarction size, survival rates and cardiac function can be observed even if the dietary intervention is induced after the ischemic event, by increasing the expression of angiogenic factors and increased vascularization of the damaged myocardium, proposing a novel non-pharmacological therapy for subjects with chronic heart failure (43).

A possible contributing factor for the cardio protective effects of intermittent fasting is increased levels of adiponectin, a hormone that exhibits both anti-athrogenic and insulin sensitizing effects and has been shown to protect cardiac myocytes against ischemic injury (44,45). Interestingly, alternate-day fasting demonstrates increased adiponectin levels in numerous rodent studies, even in the absence of calorie restriction and weight loss (39,46).

Glucose metabolism

A total of seven studies were found. Increased insulin sensitivity, as indicated by decreased fasting concentrations of glucose and insulin, has been demonstrated in rodents on alternate-day fasts both with (19,28,33) and without (17) decreased calorie intake. Anson et al. showed that mice on alternate-day fasting regimen who consume the same amount of food in a 48-hour period as mice fed ad libitum, decreased glucose and insulin concentrations to a similar degree as did mice on daily calorie restriction despite maintained energy intake and body weight (17). In another study, as little as two 24 hour fasts per week, without calorie reduction overall, were sufficient to improve insulin sensitivity in mice (46). In diabetic rats, alternate-day fasting reduces blood pressure, normalizes HDL levels, protects against glomerular damage and prevents development of diabetes nephropathy (32). These findings suggest beneficial effects on glucose metabolism and improved markers associated with obesity and the metabolic syndrome.

Brain pathology

A total of 17 studies were included. Numerous aspects of intermittent fasting and neuronal health have been examined in rodent species. Compared to rats fed ad libitum, alternate day fasted rats showed protection of age-related changes in dendritic spine number and morphology (20). Other rodent experiments have showed increased neurogenesis in brains of rats maintained on an alternate-day fasting diet, as evident by increased number of newly generated neural cells in the hippocampus (21). These results suggest that intermittent fasting could hinder morphological neuronal changes seen with normal aging and could thus slow down the neuronal aging process. Other observed effects in mice include increased synaptic plasticity in the hippocampus and enhancement of learning abilities and other cognitive functions (47).

Intermittent fasting potentially exhibits desirable effects in manifest neuronal diseases. Rats maintained on alternate-day fasting diets show reduced brain damage and mortality rate in rodent models of stroke (19,31). After a period of 2–4 months on alternate-day fasting, a neuroprotective effect against induced hippocampal excitotoxic damage was observed (25). Epileptic seizures in animals maintained on an alternate-day fasting diet lead to decreased brain damage (22,26,34). Beneficial effects have been demonstrated in animal models of neurodegenerative diseases such as Alzheimer’s (25,48) and Parkinson’s (18) disease. Furthermore, in an animal model of Huntington’s disease, prolonged survival, reduced disease-associated weight loss and improved motor function was observed in animals on an alternate-day fasting diet compared to animals fed ad libitum (49). Interestingly, the protective effect of intermittent fasting against induced excitotoxic brain damage has been demonstrated in mice despite no reduction in calorie intake or weight loss. Furthermore, mice on alternate-day fasting diets showed greater resistance to excitotoxic injury than mice on daily, controlled calorie restriction (17).

When mice with progressive demyelinating disorders of the peripheral nervous system were put on an alternate-day fasting diet regime, hampered disease progression was observed as indicated by improved nerve morphology and performance compared to mice fed ad libitum (37). Furthermore, alternate-day fasting leads to increased functional recovery after experimentally induced spinal cord injuries in rats, independently if the alternate-day fasting regimen is implemented prior or after the spinal cord is injured (27,50). If this effect is demonstrated in humans, intermittent fasting could potentially serve as a non-pharmacological therapeutic alternative in the rehabilitation process in subjects with spinal cord injuries. The effect in mice was greater with alternate-day fasting compared to daily calorie restriction, suggesting that increased time span in the fasted state has additive effects other than those attributed to calorie restriction alone (27).

The beneficial effect does however not appear universal to all neurologic disorders. No desirable effect was observed in an animal model of amyotrophic lateral sclerosis (ALS), indicating that intermittent fasting has no beneficial effect on the development of this motor neuron disease (51).

Cell proliferation and cancer

To study the potential anti-carcinogenic effect of intermittent fasting, three different aspects of tumorgenesis have been studied: circulating markers of insulin-like growth factor-1 (IGF-1), cell proliferation rates, and direct effect of intermittent fasting on carcinogenesis in animal models. Seven studies were included.

Subjects with elevated IGF-1 levels have been reported to exhibit increased risk of several cancer types. Furthermore, high circulating levels of insulin and IGF-1 in combination are often seen in subjects with obesity, insulin resistance and type 2 diabetes, patient categories that are also more likely to be affected by cancers (52). Rats on alternate-day fasting diets showed decreased levels of IGF-1 and proliferation rates of T-cells and prostate cells (30). Cell proliferation rates are considered a central element in the development of cancers (53). Decreased cell proliferation has previously been demonstrated with reduced feeding frequency alone, despite matched calorie intake (54). Mice put on a  85% modified alternate-day fast (eating 15% of ad libitum daily energy intake on fasting days) reduced IGF-1 levels and decreased proliferation rates of epidermal, prostate, splenic T and liver cells, despite no weight change (41). In a third study, true but not modified alternate-day fasting decreased IGF-1 levels in mice. Cell proliferation rates were however reduced in both groups, even in the absence of weight loss (38).

There is however some conflicting data in regard to intermittent fasting and IGF-1. Two 24 hour fasts/week without overall calorie restriction showed increased levels of IGF-1 and no effects on tumor size or survival in rats with prostate cancer (46). One might suspect that two 24 hour fasts per week would be insufficient to exhibit the anti-carcinogenic effects. However, Anson et al. displayed increased levels of IGF-1 in mice on alternate-day fasting diets with maintained body weight compared to controls, in contrast to mice on daily calorie restriction who showed decreases in bodyweight and decreased IGF-1 (17). The authors suggested a difference in the way intermittent fasting and calorie restriction influence the growth hormone -IGF-1 axis and insulin signaling pathways. The relevance of IGF-1 for tumor growth in intermittently fasted animals, with or without calorie restriction remains thus a subject for further clarification.

Recent research has also examined intermittent fasting and its direct effect on tumor development. OF1 is a strain of mice that spontaneously develops age related lymphomas at a high rate. In a 16 week trial, none of the mice of this particular strain fed on alternate days developed lymphomas compared to 33% of mice in the control group fed ad libitum (36). There was no difference in food intake or body weight between the two groups, suggesting that intermittent fasting has a protective effect on lymphoma development in this mouse strain, and that the effect was independent of the total calorie intake. The effect of intermittent fasting on induced hepatocarcinogenesis has also been examined. When rats were put on a 48 hour fasting regimen once per week, they developed less preneoplastic lesions compared to rats fed ad libitum over a 48 week period (55). The effects of shorter, more frequent fasts, such as alternate-day fasting on hepatocarcinogenesis remains a subject for future research.

Consequently, studies to date indicate that intermittent fasting hampers cell proliferation rates in a variety of cell types, and that it could potentially protect against direct development of some cancer types. 

Life span

Two studies looked at survival per se. They propose that animals on alternate-day fasting diets increase life span compared to those fed ad libitum (15,40). The magnitude of life span enhancement seems to be dependent on animal strain and age of initiation (40). Furthermore, in one study, only rats on alternate-day fasting diets survived to 30 months of age compared to a mean lifespan of 22-24 months for rats fed ad libitum (20).

It is merely speculative if the effect on longevity is secondary to the above described effects such as decreased body weight, improved insulin sensitivity, improved cardiovascular health, decreased tumor growth and improved neuronal health, or if intermittent fasting might have some distinctive effect on the aging process. No study to date has specifically studied the effect of intermittent fasting without calorie restriction on lifespan, although the effects that have been described are expected to increase life span. Interestingly, the largest magnitude of life span expansion (25 percent increase in mean life span) is seen in C57BL/6J mice, the same strain that in many of the studies on alternate-day fasting maintain a constant total energy intake and body weight (40).

Other effects

Some other interesting effects than the primary addressed in this review were observed in various studies. Many strains of laboratory rats develop spontaneous progressive kidney failure with development of proteinuria and glomerulosclerosis. Rats fed on alternate days showed preserved kidney function as demonstrated by preserved glomerular filtration rate and renal plasma flow, compared to rats fed ad libitum (56). Another surprising finding in rats maintained on intermittent fasting is increased testicular mass and testosterone/estrogen ratio compared to control rats or rats on a calorie restriction diet (57). Analgesia, which may be attributed to negative modulation of synaptic transmission in nociceptive neurons in the dorsal horn of the spinal cord, has also been reported in rats maintained on an alternate-day fasting diet (35). This finding opens up the question whether intermittent fasting alone or in combination with a pharmacological agent could serve as a useful new therapeutic approach for treating pain.

Human studies

The Ramadan fast

Fasting is one of the five pillars of Islam. During the holy month of Ramadan, Muslims restrain from fluid and food intake during daytime for the whole month. Worldwide, there are more than one billion Muslims, of whom the majority fast annually (58). The holy month of Ramadan could thus potentially be a good period to study prolonged short term intermittent fasting in humans on a large scale. A total of 17 studies were found. Conclusions are however very hard to draw from these studies. Apart from the obvious difficulties with doing randomized controlled trials there is a number of confounding factors (59,60). Such confounding variables include:

  • Altered food choices and macronutrient distribution during the fasting month
  • Dehydration and the difficulties with reliable lab tests
  • Changes in activity patterns
  • Reduced sleep due to nighttime eating and socializing
  • Differences in fasting length and hydration status in different geographical locations and time of year

Furthermore, the studies were generally of poor study design with few participants and lack of control group. As a result the studies are highly inconclusive with the effects on body weight and blood lipids with some studies showing unchanged body weight (59,61) while others show weight loss (62). Therefore, no objective conclusions could be made about this type of short term intermittent fasting and cardiovascular and metabolic risk factors, and further research of higher quality is warranted.

In one observational study, young competitive soccer players were sent to a training camp 3 week’s prior to, and during, the Ramadan fast. The fasting participants were compared to the non-fasting participants and all food was delivered from the same kitchen, thus eliminating some of the confounding factors above (60,63). Apart from a small difference in body weight (0,7 kg) that could be explained by hydration status between the two groups, no differences were observed in blood glucose levels, hematocrit, cortisol levels, inflammation markers or physical performance. In another study, fasting healthy men and women were compared to a matched non-fasting group with regard to inflammation markers and blood lipid status (61). No differences were observed in body weight, total cholesterol, triglycerides or LDL levels. There was however an increase in HDL levels and decreased inflammation – proposing a beneficial effect in the fasted subjects.

Thus, there are some data suggesting altered health markers during the month of Ramadan, but more research is needed if any objective conclusions about this type of intermittent fasting and the factors studied in this review ought to be drawn.

Alternate-day fasting

To date, very few human intervention studies have tried to replicate the reported effects of alternate-day fasting seen in rodent studies. Only six such studies were found, with somewhat disappointing study designs (64-69). The sample size in these studies was rather small, ranging from eight to sixteen participants, and the study period was often very short. Only one trial included a control group. The results are summarized in Table 2.

In both true alternate-day fasting trials, a decreased body weight was observed (66,67). In modified alternate-day fasting trials, maintained bodyweight was observed in lean (65,69) but not obese (64,68) subjects. In obese subjects, a modified 8-10 week alternate-day fasting regimen resulted in weight loss, reduced blood pressure and heart rate, and improved markers for cardiovascular health, such as decreased total cholesterol, decreased LDL and triglycerides, increased HDL concentrations and decreased oxidative stress and systemic inflammation, suggesting that alternate-day fasting might be a novel strategy for decreasing body weight and improving cardiovascular health in the obese population (64,68).

To examine the effects of alternate-day fasting on glucose metabolism, eight healthy men were maintained on a 20h modified alternate-day fast for two weeks. Despite unaltered body weight and habitual physical activity, insulin dependent glucose uptake increased, and increased adiponectin levels were observed (65). In another trial, the insulin sensitizing effect of true alternate-day fasting was observed through reduced insulin response to a standardized meal in men, but not women – suggesting a potential sex difference in the effect of alternate-day fasting on glucose metabolism (66). Although not demonstrated in all human studies (68,69), these results indicate that alternate-day fasting might mimic the insulin sensitizing effects observed in rodents on alternate-day fasting diet, and that the effect might be due to increased adiponectin levels.

Sex differences were also observed in another study where healthy men and women were fasted on alternate days. In this study, HDL levels were increased in women only, and triglycerides were decreased in men but not women (67). Increased insulin sensitivity was suggested by decreased insulin levels with unaltered glucose levels. In this study, blood pressure was unaltered, but the study duration was merely 22 days. In contrast, one trial showed decreased blood pressure and resting heart rates in subjects on modified alternate-day fasting regimens for 10 weeks, suggesting that longer intervention periods might be needed for this effect to occur (64). There is, however, conflicting data from another study that utilized a two week crossover study design and randomized eight healthy men to a modified alternate-day fasting diet or a standard diet. No differences were observed in body weight, blood lipids, glucose metabolism or hormone levels, and there was a decrease in energy expenditure after the 2 week period in the alternate-day fasting group (69). More controlled studies, with larger sample sizes and longer study durations are thus needed to bring clarification in this matter.

No human trial has directly examined intermittent fasting and tumor physiology. A single two day fast increases endogenous GH-production fivefold, reflecting the metabolic adaptation to fasting, including increased hepatic glucose production, lipolysis and nitrogen conservation (70). However no significant changes in IGF-1 are seen after a single fast period in human subjects, suggesting that repeated fasts and longer intervention periods might be necessary to mimic the changes in IGF-1 and altered cancer growth observed in some rat studies. Whether a prolonged alternate-day fasting regimen can alter IGF-1 levels in humans remains an area for future research. Furthermore, no human trials to date have examined the effects of intermittent fasting on neuronal health or life span.

Mechanisms of calorie restriction and intermittent fasting

The exact mechanism by which calorie restriction and intermittent fasting exhibits its effects on various organ systems remains unknown. The main hypothesis includes a stress preconditioning response mechanism, in which it is believed that periods of nutrient deprivation displays a beneficial mild stress that results in molecular adaptive changes in various tissues, which increases the organism’s resistance to bigger stressors such as excitotoxic and oxidative injury, including ischemia (33,71). Alternating periods of anabolism and catabolism during intermittent fasting might further increase the cellular stress resistance. Other displayed effects are increased production of neutrophilic factors and antioxidant enzymes, ketone body formation and altered metabolism enzyme production (5).

Potential adverse effects from fasting

Blood glucose levels, mood and cognition

A variety of questions often arises when discussing intermittent fasting and human health. It is often believed that blood sugar levels will fall to pathological levels if prolonged fasts are implemented. A characteristic decline in mood and energy levels before lunch among humans is often attributed to a drop in blood sugar. However when actually testing blood sugar levels in healthy subjects prone to this phenomena, no actual decline in blood sugar to pathologic levels was seen during a 24 hour fast (72). In healthy human subjects, a 24 hour fast decreases liver glycogen stores no more than 57% and in absence of vigorous exercise does not lead to muscle glycogen consumption, suggesting that liver glycogen stores are sufficient after a 24 hour fast to keep blood glucose levels within normal range (73). Furthermore, a double-blind, placebo-controlled study of two days of calorie deprivation showed no adverse effect on cognitive performance, activity, sleep, and mood, when the subjects were unaware of the calorie content of the treatments (74).


The homeostasis of body weight regulation and hunger signaling is composed of complex circuits of both central signals including orexin, neuropeptide Y, melanin concentrating hormone and alpha-melanocyte, and peripheral signals from the gut and adipose tissue, such as ghrelin, peptide YY and leptin (75). The interplay between these and other endocrine signaling systems and its effect on body weight regulation and subjective feelings of hunger and satiety remains largely unknown. The hunger response however seems to be highly adaptive in different meal patterns. Ghrelin, a gut derived hormone, is considered a meal-initiation signal. It increases during fasting and usually peaks in concentration before an anticipated meal, paired with increased feelings of hunger, and decreases after feeding. Interestingly, the rise in ghrelin is independent of meal timing as demonstrated by similar peaks before an anticipated meal in various meal frequencies, thus suggesting that subjective feelings of hunger and energy intake is highly dependent on the individual’s preferred meal pattern (76).

Increases in subjective feelings of hunger might be the single most important factor to consider when discussing the applicability of intermittent fasting as a therapeutic or preventive intervention in human subjects. In obese patients, a 14 day total fast lead to strikingly decreased body weights and decreased blood pressure, without causing increased hunger sensations. Thus a hunger suppressing effect of prolonged fasting was demonstrated (77). This anorexic effect might be attributed to the evolutionary purpose of seeking for nutrients in absence of food. The experiment, dating back to 1962, was effective and well tolerated.

Only one study has directly examined the feelings of hunger and fullness in non-obese subjects on an intermittent fasting diet, by using a 100 mm visual analog scale (67). The subjects were fasted on alternate days and reported an increased feeling of hunger from 37 to 56 mm and decrease in feeling of fullness from 43 to 23 mm when the dietary intervention was initiated. The magnitude of hunger did however not change during the intervention period as repeated measurements were taken, and feelings of fullness actually increased some over time. The duration of this study was only 22 days and it is still purely speculative whether and adaptation to the new meal pattern would occur in a longer time span. In contrast, modified alternate-day fasting in obese asthmatic patients did not significantly increase the subjective perception of hunger from baseline during the eight week long intervention period (68).

Whether repeated bouts of short term fasting can alter hunger hormone signaling or demonstrate the same anorexic effect as the long term fast described above is highly speculative and an interesting area for future research.

Decreased metabolic rate

It is commonly believed that multiple small meals increase metabolism and lead to increased overall energy expenditure. Following every meal there is an increase in expenditure due to the processing of the nutrients, commonly referred to Thermic Effect of Food (TEF) (78). A common belief therefore is that increased meal frequency leads to increased TEF and increased overall energy expenditure with multiple meals, and that intermittent fasting accordingly would decrease metabolic rate and lead to increased fat accumulation and possibly obesity. According to current research though, TEF is proportional to the calorie content and vary with macronutrient composition (with the highest increase in energy expenditure observed with a high protein diet) and not meal frequency per se, as demonstrated by the equal TEF in different meal patterns under iso-caloric conditions (79,80). Furthermore, one study examined alterations in resting metabolic rate in human subjects on alternate-day fasting diets, and found no changes after a 22 day period (67). According to these findings, any potential decreases in metabolic rate would be due to decreased total calorie intake and not fasting per se.

Increased stress

Increased levels of both ACTH and corticosteroids can be noted in rodents maintained on alternate-day fasting diets compared with rats fed ad libitum (28,29,42). Apart from the obvious notion that cortisol is one of the major hormones responsible for glucose utilization during fasting, the question arises whether the increased stress in any way could be harmful to the human organism. The molecular stress response in intermittently fasted subjects seems markedly different from the one associated with uncontrolled stress. In fasted rodents there is actually a down regulation of glucocorticoid receptors in the brain, with maintained expression of mineralocorticoid receptors, suggesting that fasting might alter the brain’s responsiveness to glucocorticoids (81). In contrast, in uncontrolled stress, down regulation of the mineral corticoid receptor has been noted. Furthermore, deleterious stress responses are associated with a decrease in the expression of brain-derived neurotrophic factor (BDNF), a response quite the opposite of calorie restriction and intermittent fasting, where increased concentrations of BDNF have been observed in numerous studies (4). In conclusion, the controlled stress response from intermittent fasting seems fundamentally different from the one by uncontrolled physiological and psychological stress. Conversely, In line with the mechanisms described above, the increased stress might be one of the necessary factors for initiating molecular resistance for larger stressors, and thus promote some of the beneficial effects of intermittent fasting.

Loss of muscle mass

One potential serious side effect of intermittent fasting would be loss of muscle mass. Theoretically, food deprivation would result in depleted hepatic glycogen stores, leading to increased proteolysis and flux of amino acids from skeletal muscle for hepatic de novo gluconeogenesis, to maintain healthy blood glucose concentrations. As discussed previously though, a 24 hour short term fast is insufficient in duration to deplete liver glycogen stores in healthy subjects (73). Up to 40 hours of total fasting does not stimulate catabolic processes and lead to skeletal muscle atrophy (82). Modified alternate-day fasting and loss of lean body mass was investigated in only one study in the systematic search. No loss of fat free mass in the absence of weight loss was observed compared to a control group fed a standardized diet (69). Furthermore, an increase in ketone body concentrations has been observed in subjects on alternate-day fasting diets in both human and animal studies (17,68). Ketone bodies spare skeletal muscle from breakdown by providing non-glucose energy substrate for various tissues, of which the brain is the most important, and thus decrease the need for protein-derived substrates for gluconeogenetic conversion to maintain glucose homeostasis (83). Available data thus suggests that short term fasting does not deplete hepatic glycogen stores to the extent that markedly increased proteolysis and gluconeogenesis becomes necessary to maintain healthy glucose concentrations. Still this notion needs to be clarified in future research of longer duration.


Alternate day fasting as a model for calorie restriction

Intermittent fasting in the form of alternate day fasting in many instances reduces overall energy intake, with no obvious adverse effects, and thus becomes a model of calorie restriction in both human and animal subjects. Secondary to reduced energy intake and weight loss, effects such as reduced risk factors for cardiovascular disease, and improved glucose metabolism have been demonstrated in both animal and human subjects on true and modified alternate-day fasting diets.

In rats, protection against ischemic injury and improved survival has been demonstrated in both myocardial and cerebral ischemic events. Other beneficial effects, such as slowing the neuronal aging process and increasing cognitive functions and memory, have been observed. In line with animal studies on daily calorie restriction, alternate-day calorie restriction has shown beneficial effects in neuronal disorders such as stroke, epilepsy and neurodegenerative disorders, including Alzheimer’s, Parkinson’s and Huntington’s disease. Additionally, calorie restriction can reduce cancer risk and increase life span in rodent models on alternate-day fasting diets.

Intermittent fasting and health in the absence of calorie restriction

Some effects occur even if the subject maintains body weight, suggesting that the reduced meal frequency or prolonged time in the fasted state might have some additional effects regardless of overall calorie restriction and weight loss. In humans, modified alternate-day fasting diets might be easier to adhere to and they seemingly lead to less pronounced weight loss than true alternate-day fasting. Without causing weight loss, effects such as improved fasting insulin have been demonstrated in both animals and humans. In line with these findings, adiponectin increases in rats and humans on both true and modified alternate-day fasting diets in the absence of calorie restriction. Additionally, in mice, fat redistribution from visceral to subcutaneous stores has been observed despite unaltered overall body weight. If this effect proves to be true in human subjects it could propose reduced disease risk despite unaltered body weight.

Animal data further indicate some beneficial effects of intermittent fasting diets even without calorie restriction. Neuronal health improvements such as resistance to excitotoxic injury have been observed. Resistance to oxidative stress could be beneficial in the pathogenesis of epilepsy and various neurodegenerative diseases such as Alzheimer’s disease. Alternate-day fasting in animals also leads to improved recovery after induced spinal cord injuries and progressive demyelinating disease of the peripheral nervous system, in the absence of calorie restriction. Furthermore, in animal studies, changes associated with retareded tumorgenesis, such as decreased cell proliferation rates in various cell lines and decreased incidence of lymphoma, have been observed. Whether these observations are valid in human subjects as well remains an interesting area for future research.

Future research is warranted to test whether the health promoting effects described in animal studies have some validity in humans. We are in the very infancy of research on intermittent fasting in human subjects and future studies with larger sample sizes, longer durations and of better study design must be completed before any definite conclusions can be made regarding intermittent fasting and human health and the applicability to modern lifestyle.


My sincere gratitude to Staffan Lindeberg and Bengt Zöller for helping me set up the systematic search, for all the intellectually stimulating discussions and for the guidance in writing this review.



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Table 1: Used search terms

The complete search term used was: (Hunger/physiology[mesh] OR Cholesterol/metabolism[mesh] OR Blood Pressure/physiology[mesh] OR Body Composition/physiology[mesh] OR Blood Glucose/metabolism[mesh] OR Energy Metabolism/physiology[mesh] OR Cardiovascular Diseases/prevention[mesh] OR Diabetes Mellitus, Type 2/prevention[mesh] OR Neoplasms/prevention[mesh] OR Caloric restriction[mesh] OR Caloric restriction/methods[mesh] OR Fasting[mesh] OR Fasting/physiology[mesh] OR Fasting/blood[mesh] OR Feeding behavior[mesh] OR Obesity[mesh] OR Energy intake[mesh] OR diet/methods[mesh] OR eating/physiology[mesh] OR Ghrelin/blood[mesh] OR Ghrelin/metabolism[mesh] OR Glucagon/blood[mesh] OR Glucagon/metabolism[mesh] OR Insulin/blood[mesh] OR Insulin/metabolism[mesh] OR Insulin Resistance/physiology[mesh] OR Leptin/blood[mesh] OR Leptin/metabolism[mesh]) AND (Short term fasting” OR “Short term fast” OR “Intermittent fasting” OR “”Intermittent fast” OR “Periodic fasting” OR “Periodic fast” OR “Alternate-day fasting” OR “Alternate-day fast” OR “Alternate day modified fasting” OR “Alternate day modified fast” OR “Every other day feeding” OR “Reduced meal frequency” OR Nibbling gorging OR Hormesis OR “Omitting breakfast” OR “Omit breakfast” OR “Skipping breakfast” OR ramadan OR fasting refeeding OR Alternate day caloric restriction).

Table 2: Alternate day fasting and body weight, glucose metabolism and cardiovascular health in humans

Reference Subjects (n) Protocol Trial length

Body weight

Glucose metabolism

Cardiovascular health

Soeters et al, 2009 (69) 8, non-obese 20 h mod ADF 2wk crossover


-glucose, -insulin


Varady et al, 2009 (64) 16, overweight 75% mod ADF 10wk

Not studied

↓BP, ↓HR

↓TCL, ↓LDL, ↓TG,  – HDL

Johnson et al, 2007 (68) 10, overweight, asthmatic 80% mod ADF 8wk

-glucose, -insulin



Heilbronn et al, 2005a (67) 16, non-obese ADF 22d

-glucose, ↓insulin


↑HDL(women), ↓TG (men)

Heilbronn et al, 2005b (66) 16, non-obese ADF 22d

↑insulin sensitivity (men), -insulin sensitivity (women)

Not studied

Halberg et al, 2005 (65) 8, non-obese 20 h mod ADF 15d


↓glucose, -insulin,

↑insulin sensitivity

Not studied

Abbreviations: ADF, alternate-day fasting; mod ADF, modified alternate day fasting; BP, blood pressure; HR, heart rate; TCL, total cholesterol; LDL, low-density lipoprotein; HDL, high-density lipoprotein; TG, triglyceride. -, unaltered; ↑, increase; ↓, decrease.

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91 comments on “The Effects of Intermittent Fasting on Human and Animal Health
  1. Michel says:

    Very interesting stuff, I love that even though some of the references were a bit over my head (the latin names for some illnessess and the implications of these, etc) most of it was completely comprehensible eventhough it was made for medical purposes.

    You are one helluva writer Bojan, thanks for sharing this.

  2. Jason ross says:

    Wow…that is a lot of work. Thanks for sharing. Been IF for 2 days a week for a year.

  3. Great summary of some of the work that has been done to date. Some more recent studies showing timing (i.e. dinner instead of breakfast) may be more beneficial (Kahleova et al., 2014; Fernemark et al., 2013; see for nice summary). Keep up the nice work.

  4. Steve says:

    Fantastic job! I’ve been doing leangains for almost 2 years and love it! Appreciate you (and Berkhan) sharing your work with us!

  5. Lee says:

    Thanks for taking the time to write this up, Bojan. It was a very nice, in-depth look at the studies surrounding IF and written in a way that can appeal to the medical-savvy people, and the general public interested in health like myself. Appreciate the hard work you’ve put into this, and let’s hope there’s some proper, high quality studies done on IF in humans in the years to come!

  6. Jeff says:

    There used to be a time when people were trained to write and communicate, and from the above article this time has clearly passed.
    Too long, too much jargon, entire paragraphs that don’t move the topic forward, and worst of all-the important stuff (the “lead”) is so buried and muddled that it was just confusing. I can see that you are using this article to try to bring people in and eventually sell them something. The way you are approaching it wont work-unless you really think people are going to read 10,000 words of science crap instead of just closing your site and going elsewhere. Just sayin’the truth.

    • Notjeff says:

      Obviously, you’ve never read a peer-reviewed journal article before. While it may seem like jargon to you, it is the preferred lexicon for scholarly publications. Also, “just sayin'” is an absolutely douchey thing to say.

      • Noteither says:

        The paper isn’t peer-reviewed and the author indicates he doesn’t feel inclined to attempt to get the paper reviewed/published. Red flag.

        • John Smith says:

          First of all, this is not a paper to be published in a journal and I don’t think you have made a thesis before. Second, when you present it, you have a panel of professor/researchers evaluating your work, exactly like a peer-review. Do you think a professor/researcher would easely sign your thesis without reading it?

    • Adam says:

      With all due respect Jeff, this is a thesis paper and by you saying that it’s too long with too much jargon is a statement of your lack of education. This is proper academia and if you can’t digest this then that is truly unfortunate.

      • Vicki Brown says:

        Just, no.

        This is being published On The WWW. With the note “I am finally ready to go public.”

        Do not dis people who don’t have a PhD or MD in the Biological Sciences. Do NOT presume their level of education.

        I once listened to an Immunology professor at a University complain that the only articles he could understand in Science (the journal) were the ones on Immunology.

        Trust me; if you were to pick up and try to read a scholarly academic article outside of your field, you would react the way Jeff is reacting.

    • Liam Bailey says:

      Oh dear Jeff, you have clearly shown that this level of academia is lost on you. He isn’t trying to sell you anything lol, this is his thesis that he has kindly shared (not a fitness blog from a run-of-the-mill personal trainer), of which the ‘scientific jargon’ is absolutely necessary. This is an excellent review of the available literature, written by a highly educated person. This is an extremely disrespectful comment that shows your lack of intelligence. I’m a little embarrassed for you. Think things through before you comment in future.

      • Vicki Brown says:

        No. In fact, I would be embarrassed for _you_ if I weren’t simply irritated.

        It is Your comment that is extremely disrespectful comment and shows a lack of intelligence and knowledge of how many different styles there are in the world.

    • Jay says:

      Jeff, I’d tell you how incorrect and down right stupid your comment was, but two things:

      #1 You obviously wouldn’t understand

      #2 Others have beat me to it.

      Carry on.

    • Jay says:

      Jeff, I’d tell you how incorrect and down right stupid your comment was, but two things:

      #1 You obviously wouldn’t understand.

      #2 Others have beat me to it.

      Carry on.

    • Scooby says:

      Jeff pls go

    • Vicki Brown says:

      Note: I have an MS in Microbiology and have been employed as a programmer and technical writer for over 25 years. Both Jeff and Notjeff (seriously?) are correct.

      Jeff is correct. The paper is nearly incomprehensible to a layman.

      Notjeff is (sadly) also correct. The language and formatting are perfectly correct “preferred” for a scholarly scientific paper.

      However, Audience is Everything. If you’re writing for MDs, this paper is probably a great start. If you’re writing for _people_, this paper is not good enough.

      Unfortunately, this is the WWW, not Nature or Science. If you want this information to be read and understood by people who are not your thesis committee or people with an MD or PhD in your field, you need to re-write it.

  7. Brenda Rose says:

    This study confirms much of the anecdotal evidence that I had already heard about the possible long-term effects of intermittent fasting. I’m delighted and relieved to now know that although it may be an unconventional weight loss method for many people, it isn’t harmful.

  8. Jessica says:

    Thanks! Amazing work.

  9. John A Hill says:

    Interesting timing of this post as I was just talking with a co-worker about the benefits of intermittent fasting and the general practice of fasting.
    Although it has been a while (many years)since I have practiced fasting, it is a ritual that I have studied and used its healthy benefits in the past. I have used short fasts of less than 36 hours and longer fasts of 7 days for personal research,purposes and practice.
    After having lived several years in an unhealthy way, I have re-tooled my lifestyle into a sustainable healthy way of living and have lost 55 pounds in the past year. The subject of intermittent fasting came up as I believe that I will begin a regular practice of IF as a part of this lifestyle.
    I am looking forward the time when I can read your complete work as I have only scanned it this morning.
    Thanks for sharing your work with me.
    John <

  10. Chaya says:

    Thank you for sharing your research. The popular models of IF being 5:2 with the 2 days being 500 calories a day and 18:6 (over a 24 hr period) have received a lot if attention. Do you have any results on their benefits from your research?

  11. Boris says:

    I need to leave a quick comment just as I started reading :) The best and biggest recource base for the IF is by far Martin Berkhan, and his website, not Brad Pilon, whose book is good, to be fair. Now back to the reading…

  12. Boris says:

    Oh god, sorry, I’ve posted that comment before reading the next sentence, ignore that :(

  13. Keegi says:

    It is a great summary and I do appreciate your work on this. However, I wanted to ask whether you came upon studies that researched IF’s effects on female reproductive health? I’ve seen anecdotal articles in blogosphere where women claim that IF has destroyed their hormone profiles and thus reproductive health and this was the reason I went off IF at some point, just in case. Still, I’ve kept on being curious about it, but haven’t found evidence in either direction about it.

  14. Shane says:

    Thanks for sharing this. I have been practicing IF (Leangains) for a bit over a year now, and I’m also a type one Diabetic. I’m curious about the effects and benefits of insulin sensitivity on a type one Diabetic with fasting? It seems like most of the research in this area has been done on type two Diabetics. Has there been any research and/or findings in regards to fasting and type one Diabetes?

  15. Mark says:

    Hello Boyan,
    I’d like to know if you have some data on how Intermittent Fasting acts on libido.
    Thanks in advance

  16. Micke says:

    Thank you for putting in the work that I don’t have the stamina for. Great work!

  17. Steve says:

    Great article, also a big fan of Martin’s site and message. However, as someone who has been an IFer for seven years, Ori Hofmekler (author of the Warrior Diet) deserves lots of credit for the IF movement.

    Ori may in fact be the first well-known proponent of IF and Warrior Diet should be recognized as one of original mainstream publications advocating IF for general health and physique improvements…

  18. Andrew Smith says:

    Great article. I spend much of my time researching information on fasting from peer reviewed journals through my university. I find the health benefits of IF, especially autophagy, are profound but tend to be poorly understood. I have the article you posted. You may also want to look at a review article “Effects of Diet on Brain Plasticity in Animal and Human Studies: Mind the Gap” by Tytus Murphy et al at King’s College London published May 12 2014. Cheers.

  19. Thank you for sharing, quite extensive work I must say.

    It would be interesting to understand how fasting affects our hormone profile, and as lads primarily testosterone (you do mention testosterone/estrogen ratios) and growth hormone. Perhaps there is not enough research, perhaps I would just need a more “layman-like” explanation. What do you know about this topic?

    Again, thank you for sharing.

  20. Scott says:

    Lots of writing but I was conflicted.. You talked in the beginning about this being your baby.. and how you didn’t even care to share it for a while because of the pride of piecing it together.. but then after reading it, it’s just a collection of all previously released studied and ideas. You made it sound like you had stumbled on some gold nuggets of previously unknown information. By the time the end finally arrives, there’s nothing new at all and even the idea of IF being superior is still in question.

    Nice resource of information overall but I still fail to understand why so many people regard IF as some unique idea or think of it as a new way to solve an old problem. I tried it for a full year before realizing that my results were identical to when I ate 6 meals a day for a decade. Is it staving off disease in me a little longer and giving me a future benefit in that respect? Possibly but not conclusively.

    In the end.. it’s still all just calories in/out and and IF is just a fancy term for not eating all damn day. It matters little and this just seems to justify that. I’ll continue to be blown away by people who so firmly believe eating all their food in an 8 hour window is greatly affecting their body composition. It ain’t. Eat 6 meals if you like or eat 1 or 2. It matters not.

    All that said.. great work putting this much information together. I can see great benefit in people linking to the article as a place to find many important studies on fasting.

  21. Jim DiGiovanni says:

    I don’t understand how a 24-hour fast for a human (about a 70-90 year lifespan) is comparable to a 24-hour fast for a lab mouse/rat which lives 2-3 years. Wouldn’t a human need to fast for 30 days or more?

  22. Andy Morgan says:

    Most appreciate the hard work, and you choosing to publish this free for all to read and benefit after the hours of work. Will put several links up to it.

    You’re open to questions or sick of the subject by now?

  23. Andreas Berge says:

    A great article and a good read, appreciate all the hard work put into it, thanks.

  24. Andy Mc says:

    I’ve read everything on IF and this is decent. I did it when it was called Animalbolics, that’s Pre Warrior Diet stuff.

    I especially liked the differentiation between normal stress and the stress of fasting because that’s basically the biggest argument against IF right now.

    It is true that junk food can totally screw up any health benefits of IF and the people that bash it did just that, or did unhealthy low carb or too low calorie or had lots of stress in other areas of life.

    IF can also backfire if done to extreme. I’ve seen some very smart people do an extreme version of IF (like 1 meal a day for 6 months) and then bash it then go back to 6 meals a day. I mean come on. On the other hand I don’t believe one 24 hour fast a week is enough either but it doesn’t matter what I believe, what matters is you doing what you feel like doing.

    You can skip breakfast, push lunch back a few hours and skip the late night snack and you are automatically doing IF/8 Hour diet, the diet that Wolverine did for his last 3 movies so there is something more to the equation than just IF.

    IF is the only diet that allows me to eat white bread and pasta everyday and get the same results I got when I ate MUCH more strict and have a normal life and no one even knows I’m dieting at all when they see me eat. That’s NICE because I can’t stand talking diets with “muggles.”

  25. Anon says:

    Thank you for all the work of putting it together! If you don’t mind, couple of quick questions.

    From perspective of
    1. Mood and energy
    2. Long-term health and longevity
    3. Physical and mental performance

    All other things being equal, according to current data, should someone training two or three times a week fast once a week for the whole day or daily for about 14-16 hours?

    Also, assuming the three days workout regimen (Mon, Wed, Fri) or two days workout regimen (Mon and Fri) what would be the best day to fast to get most of the benefits and sacrifice as little anabolism as possible? Thanks a lot!

  26. Greg says:

    Thank you for sharing it! Is there any scientific evidence to support the answer to this question?

    What of the following types of fasting should one do if he or she is not interested in weight loss (but rather muscle gain), but is very interested in all other health benefits fasting might provide and specifically in the benefit below (related to glomerulosclerosis) as this one can be life-saving(!)?

    1. Intermittent fasting daily
    1.1 No food food for 16 hours a day
    1.2 No protein and no carbs for 16 hours a day, but some fat for breakfast

    2. Alternate day fasting
    2.1 No calories on a given day – one day a week
    2.2 No calories on a given day – two days a week
    2.3 No calories on a given day – every other day — this seems to be the method of the discussed rat study (below)
    2.4 Limited calories on a given day (max 800) – one day a week
    2.5 Limited calories on a given day (max 800) – two days a week
    2.6 Limited calories on a given day (max 800) – every other day

    Many strains of laboratory rats develop spontaneous progressive kidney failure with development of proteinuria and glomerulosclerosis. Rats fed on alternate days showed preserved kidney function as demonstrated by preserved glomerular filtration rate and renal plasma flow, compared to rats fed ad libitum.
    Gehrig JJ, Ross J, Jamison RL. Effect of long-term, alternate day feeding on renal function in aging conscious rats. Kidney international. 1988;34(5):620-630.

    Is there anything else to keep read and keep in mind?

    Thank you again!

  27. Homero says:

    Incredible article! Although scientific in its approach, quite accessible to the rest of us to understand and assimilate.

    Congratulations on your MD!

  28. Lucy says:

    I try most days, to push back my eating widow – not for reasons of IF, but usually because I just can’t be bothered to eat… Or I’m lazy. Or busy. And i don’t know i want because really I don’t fancy anything.

    I can usually go to somewhere between 2 -4 pm. But should I push it too long – I often do! – I start to get shakey, nauseaus, and feeling terrible…

    It’s interesting that you mentioned blood glucose levels, because that’s what I’ve long thought it was. But if it’s not!? What is causing it?!

    My husband goes even longer than I do, but he never has the same shaky feelings.

    It’s been much better for me since giving up most sugar and adopting a paleo style diet. But yeah, if I go too long, it’s awful… :(

    • The symptoms you describe are associated with detoxing. You say the better your diet the less you experience them. This is key. The quality of your food matters, as the more toxins your body has to eliminate the more symptoms you will feel during your food-free periods. You should be able to push through these symptoms and stretch the length of time without food.

    • Brad says:

      It seems like the carb reduction in paleo is helping.
      Maybe try being more strict with carb intake, 20g daily.

  29. Carrie says:

    Great research! I certainly appreciate a scientific paper on weight loss versus the advertisement-laced, fad based articles. As a woman, I am curious about the physiological/hormonal differences between males and females and does IF effect genders differently? I noticed when mentioning research you state “humans” or “rodents” but do not specify whether males or females, or a combination of both, were used. (Unless, I didn’t read carefully enough, if so, I apologize.) I have read articles that state while research shows benefits for male subjects (rodent and human alike), that longer periods of fasting were detrimental for females. I’ve also heard there isn’t enough research on females to draw a conclusion.

    If anyone has insight on this topic, I’d be very appreciative to hear it.

  30. Cristi Vlad says:

    I would say that besides the fact that ketones spare lean tissue, there’s also the effect of growth hormone that prevents the loss of muscle under IF routine. what do you think?

    this is even more interesting in subjects (like me) who do not lose muscle on 48+ hours IF under very low carb ketogenic diets where glycogen storage is significantly reduced.


  31. nomi says:

    Very informative article. I started practicing Martin’s leangains style intermittent fasting about 6 months ago and its my life style now. Already lost 28 lbs and broke all my personal best in the major lifts.
    Thank you for sharing your hard work.

  32. Such great work that you are sharing with us!

    A phenomenal read. Gave you a little shout-out on my site:

    Keep up the great work!

  33. Jake Sacino says:

    Great read Bojan, thank you for posting it online. I especially liked the section regarding the Ramadan fast. And congratulations on your MD!

  34. Ben Brock says:

    Great work. I personally feel that IF has changed my life, and taught me what it really means to be hungry as opposed to just “time to eat.” After being exposed to LeanGains several years ago I begain doing a 16hr daily fast, just by skipping breakfast and avoiding late night snacking. It’s had minimal impact on my lifestyle, and adjusting to the no breakfast, no snacking ways were easy, because after only a few days of that I no longer felt hungry during the fasting period. While I’m still not as lean as I’d like to be, I began IF right about the same time I got out of the Army, and my activity levels decreased tremendously, yet thanks to IF I’ve gotten somewhat leaner and significantly decreased my food intake with minimal effort. I’ve also managed to gain plenty of muscle on IF through macro-cycling as described by Martin. I’m terribly undisciplined and my motivation ebs and flows throughout the year, but despite all that IF has been a very positive thing in my life. Oh, my blood work looks incredible as well, with my total cholesterol in the 180s despite very high HDL and my triglycerides in the mid 50s. Plus my blood pressure has decreased over the years from 200/120 at its peak, to currently sitting at 125/75 with no medication. Thanks guys. Your research changes lives. – Ben

  35. Anne Helene says:

    This was incredibly well written, fascinating and informative. Perhaps my news feed has been littered with illiterate ‘gym junkie’ material causing quite a negative perception of ‘fitness freaks’, yet I always kept an eye out in hope that I would one day satiate my craving for a rare breed of fitness intellectual. That day has finally come, it seems my fear of disappointment was just stopping me from looking a little harder. What’s more, you did it for free! I sincerely hope you are being fairly compensated for this gem that I will be keeping in my ‘understanding humanity’ arsenal alongside V.S Ramachandran, Dawkins and the Lancet Journal. Thank you, can’t wait to read more.

  36. sam says:

    Brilliant article. I met a trainer who has a giant of a sculpted body. He told me something about slingshot training which was based on eating and fasting cycles. He said the body releases growth hormones when we fast. I didn’t quite understand the concept then. Now it makes heck of a sense.

    Also, while fasting during Ramadan is mandatory, it is encouraged among Muslims to fast every Monday and Thursday. Very few people do it however. Like most good things in life.

    “O ye who believe! Fasting is prescribed to you as it was prescribed to those before you, that ye may (learn) self-restraint. And that you fast, it is better for you if only you know” – The Cow-verse 183- 184

  37. Elaina Love says:

    Thank you so much for sharing. I have been studying fasting for years on my own body as well as teaching it to others. I love the idea of IF because it is more doable than prolonged fasts and doesn’t interfere with ones work life/energy levels in general.
    Great research! I hope you write a book some day :)

  38. Matty B says:

    Great stuff, though a PDF would be nice.

  39. Pablo says:

    Thanks for sharing the stuff, Bojan!!

    I really appreciate not having to immerse myself in a stack of papers to understand some of the real science beyond IF.

    I don’t think IF is a shortcut to perfect health and body, specially if you are fed up with crap when not fasting, but it really seems a useful tool to promote optimal health.

    I’ve read some biologist claiming that nature provides us with the perfect fasting environment: darkness. Just studies in rodents by now.

  40. Thomas says:

    Muscle loss.
    With increased ketones due to short period fasting and Preventing the loss of non fat mass.
    Can you advise a optimal period in hours of fasting followed by hours not fasting?
    Also on none fasting days. recommend calorie intake whether it is normal or surplus?
    Would a ketogenic diet play some role in pre and post fast to minimise shock to the body and bring ketone levels up before a total nutritional cut off?
    And any thoughts on carb loading or cheat days relating to IF?

  41. Fiona Aleksoska says:

    Why have you not had your work published in a peer reviewed journal? I find that highly suspicious.

  42. Dimitris says:

    All that sounds great… but for a sec. please consider that.
    30-40 years ago people working heavy duty jobs like building constructions, agricultural works, livestock operations for 7/365 days, 12 or 14 hours per day carrying heavy weight loads or working in the sun, and finally during the brake they ate little bread, onions, tomatoes and a few olives, or a cup of soup and that’s it for the day. (I am from Greece and from family of building constructors).
    Those people were strong, lean, with stamina, poor, happy and optimistic.

    Now we are sick all the time, unsatisfied and with all my respect to the author work, with all that scientific data, – we forgetting that not only the quran reinforce fasting but the christianity too, vigorously i must say, – actually we exploring the proven results of fasting mechanism, without the way to reinforce the fundamental mentality of how to decide to live frugal.

  43. Alisa says:

    Just a thought for further research: Islam isn’t the only religion that practices their own brand of IF. The Church of Jesus Christ of Latter-Day Saints (aka Mormons) generally fast once a month. I’m not sure if that’s exactly the type of IF you’re looking to study, but it might also have health benefits.

  44. Notjeff says:

    Are the terms “Intermittent Fasting” and “Alternate Day Fasting” interchangeable? If so, why not settle on one unified term? If not, why are they used interchangeably throughout the article?

    • FooDog says:

      Intermittent Fasting is the general classification covering all forms of short regular fasting, Alternate Day Fasting is a specific form of Intermittent Fasting where food is only eaten every second day.

  45. Anwar says:

    Dr Kostevski,

    Thank you for sharing your work. Intermittent fasting seems to be a generally safe and effective regime; your year-long efforts help reveal the scientific underpinnings.

    I also read edited by another Swedish doctor, Dr Andreas Eenfeldt.

    Looks like we should look to Swedish doctors for leadership…!

    Kind regards,


  46. Teresa Murphy says:

    I was a half-assed pescatarian for many years. I went full vegan in March 2013. I’m a 5’8″ 57-year-old female. I’ve been moderately overweight (“you carry it well”) my whole life. Going vegan was hard, especially the beginning. Not anymore, though. I crave “real” raw fresh green foods these days. Weird. 2 years ago I was 185. In March I was around 175. Today, I’m 143 working towards 135 so that I can bounce around between 135 and 139. 139 is apparently, roughly the “ideal” weight for my stats. I’ve always known that I can’t eat at night. My partner gets home late, loves to cook and thinks nothing of eating a gigantic steak and full meal at 11pm. It was a tough change for me. Now, I limit my food to window of time. Mid-to-late morning thru till around 6pm at the very latest. I try to quit any big meal (and my meals aren’t that big anymore anyway) by 4pm. The better I hang during those hours AND the closer I stick to RAW VEGAN, the faster the weight just MELTS away. I’m not perfect and have some social times where, while still eating vegan of course, I eat too late or too much. It seems to me like I almost need those occasional times to re-jumpstart the downward trend. I swim for 20 minutes a day. I think dropping your body temp for a while also is good for kick-starting your body energy. I love to dance and dance around our condo all the time. I work from home. I realized that I was doing some kind of intermittent fasting and it sure seems to line up with my experience. I am ON FIRE with ENERGY —— ALL THE TIME!!!!!! I feel euphoric often. I feel so dang healthy I want to shout it from the rooftops! I haven’t had any cold or flu this season yet. Don’t expect to.

    Now, I eat for nutrition. I listen to my body more. I feel like a different person. I feel clean and wonderful inside! I know I’ve lost some muscle mass. I’m about to start a weight-focused routine focusing on my gluts and general core strength. Looking forward to reshaping what’s left into the “new me!”

    Anyway, just thought I’d share on the off chance it inspires anyone else!

    Thank you for the awesome info.

  47. Teresa Murphy says:

    Sorry, I meant so say I went Vegan in March 2014. My weight loss has been about a 10-month journey.

  48. Missy says:

    Amen! Thanks for sharing your work with us!!

    I am just now diving into IF, thanks to Dr Mercola occasionally promoting it.

    I haven’t combed all though your website yet. Is there a spot where I can get more basic how to details on the types of IF…and also figure our which is best more weight loss. I am not obese…just modern day over weight. I’ll do some more peaking around.


  49. Trinity says:

    Wow, absolutely amazing. Mind blown, thank you so incredibly much. I’ve been doing IM for about 3 weeks now, every day, based on a friend who recommended it and I’ve been loving it. I’ve been drinking lemon water and matcha green tea throughout my fasting period though, and I’m wondering if you think these will limit the benefits I get in terms of HGH / Testosterone. What’s your opinion on this? Can I still take lemon water & matcha green tea? (some people say matcha has no calories, some say it’s 3 calories per gram. I only take about 3 grams. And one small lime, which has about 5mL of juice). Would really appreciate your opinion on this =) Thank you SO much! Shared!

  50. This is excellent. Thank you very much.

    Congratulations on acquiring your MD. I will hopefully be following in your footsteps when I graduate in 2017.


  51. steve says:

    took some effort putting all this info into one place, appreciated

  52. Sebastian says:

    Thank you for the Great work you have done! Its a appreciated as much as I appreciat my kids! ;)

  53. Karen says:

    Thank you so much for sharing your research and information. It is refreshing to know that someone is giving the information to anyone that wants to access it for free. I have been so disappointed by all the teaser ads out there that link me to a lengthy video ending up with the “secret” cure not being revealed unless you buy their expensive product.

  54. abhayendu says:

    Thank you .

  55. Liam Bailey says:

    Superb review, thank you so much for sharing. I am an undergraduate studying nutrition and I am currently writing my dissertation on time to exhaustion in male subjects during intermittent fasting. The collation of literature here has helped me no end. Thank you so much again for sharing and congratulations on achieving your MD!

  56. Great Post and Nice Article.I like it.Thanks for sharing.

  57. Leon says:

    Thanks for sharing this.

  58. Great article. I added this to my website, which is all about intermittent fasting information, to share with others. Thanks!

  59. Very nice review, Sir; very useful to me. (I’d also like to say thank you, btw, for citing one of my own early papers on this topic. It’s nice to be remembered. :-) )

  60. Sigi says:

    So your awesome ‘paper’ wasn’t good enough for science (or med specific paper) and now you are so generious to share it with the world?

    The only result of your paper is the last sentenace:
    ‘We are in the very infancy of research on intermittent fasting in human subjects and future studies with larger sample sizes, longer durations and of better study design must be completed before any definite conclusions can be made regarding intermittent fasting and human health and the applicability to modern lifestyle.”

    You say basically: Everything I was able to find has no result.

    Shouldn’t you create real results or bring research further instead of only collecting non conlucive data and rewrite them in your own words?

  61. Pia says:

    Hej Bojan, thank you for sharing this informative collection of data!

    I have not tried the strict form of the method, but I usually don’t eat more that one proper meal per day. Very little in the morning, a glass of fresh juice; very little in the evening, a few nuts or some cheese. My proper meal consists of good protein and fat, a decent serving of veg and some fruit – and never forget your dark chocolate! No wheat, no added sugar – but also no actual rules.

    I simply don’t crave it, I do get hungry once in a while, but not for that crap, I want the nutrition, and clean flavor.

    So does that count as IF? Is there any science on that model? I’m losing weight fairly slowly, but not muscle according to the mirror. I stopped weighing myself many years ago. I also need less sleep. That could be an angle for a study. Did they check that with the rodents?

    Also, please let all those non science-reading people on Facebook know that eating non-stop is bad! My friend is eating from dawn till dusk – thinking that it’s healthy – always hungry, packing on the kilos, while thinking I have a huge problem with food:-)

    Take care, and thank you again;-)

  62. Santiago says:

    Thank you Bojan, the reeding was easy going and yet technical and informative.
    I appreciate your effort and thank you for motivating me and the rest of the readers into opening up to new possibilities whilst feeling confidente of the path we are taking.
    Keep it up, people will follow.

  63. Awesome blog. I enjoyed reading your articles. This is truly a great read for me. I have bookmarked it and I am looking forward to reading new articles. Keep up the good work.

  64. Anthony King says:

    Great read,

    Actually I am shocked I did not find this article years ago, my cousin sent it to me as he knows I have been on the “one meal per day” plan for more than five years now. the plan has had a significant impact on my energy level, recovery rate, youthful appearance, mental capacity and focus, as well as athletic performance, not to mention my food budget :)

    I have also been building a document more focused on the psychology of fasting, or more accurately, the resistance to fasting or what I call “Food Addiction and Withdrawal Symptoms” I looked at this because I started to notice a pattern in peoples responses to the subject of “one meal a day” that was very similar to the one someone gets from an “addict”. If fact some of the responses you received are the type of Knee Jerk reactions I see all the time.

    If you are still following this topic I would love to chat with you more on this.

  65. Anthony Soares says:

    I loved the read! I can’t thank you enough for putting in all the hard work and time into compiling all of this research.

    It is very much appreciated, and I can’t wait for the average consumer to understand that IF is healthy.

  66. Estelle A says:

    Thank you for sharing,

  67. vicky says:

    thanks for sharing! that’s a lot of hard work put into it

  68. Lane says:

    I know this is old, however I have no PhD, no analytical writing skills, or proper grammer. And I understood everything as it was ment to be understood. What’s so hard? Retards (& no political correctness ;-)~

  69. INTJ Dre says:

    Excellent Research. Proves that IF has many more benefits than just weight loss. I use IF everyday 17/7 split and will never stop.

  70. Lane says:

    Thank you Jeff I have searched in vain for a table of information on true ADF untill now. I haven’t looked at when the last time this was updated. However, have you seen any studies on continued ADF vs. 5day+ fasts for different metabolic changes? Thanks. Oh and I understood all of it w/o and MD PhD. Thanks for this!

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