Rebooting the System: The Benefits of a Fasting Mimicking Diet

on March 15, 2017 by Chris Kresser

While fasting has many health benefits, prolonged calorie restriction can take a toll on patients, both physically and mentally. Diets that mimic the physiological benefits of fasting without the burden of food restriction may be a good alternative. Read on to learn more about fasting mimicking diets, their health benefits, and which patients are likely to benefit most from them.

Prolonged water-only fasting provides many health benefits, including regeneration of the immune system, reduced blood glucose, and autophagy (1). However, prolonged fasting is difficult for most people and can cause adverse effects due to its extreme nature. Researchers have therefore been trying to design diets that might mimic the beneficial effects of prolonged fasting without the stress of complete food restriction.

Previously, I have written about the benefits of intermittent fasting. While intermittent fasting approaches share some characteristics with fasting mimicking diets (FMDs), there are some key distinctions. In this article, I will discuss these differences, the evidence for FMDs in both animal and human studies, and which conditions respond best to FMDs.

Terminology: FMD vs. IF

Before we dive into the evidence, let’s clear up some terminology. My guess is that you’ve probably heard of intermittent fasting (IF) but are less familiar with the concept of a fasting mimicking diet (FMD). Intermittent fasting is a general term used to describe various approaches that use short-term fasting with the overall goal of improving health. Examples of IF approaches include skipping one meal a day, shrinking the eating window (also called time-restricted feeding), or alternate-day fasting.

Under this loose definition, fasting mimicking diets can be considered a type of IF. However, FMDs are much more detailed than most IF approaches and are designed in order to achieve specific biological outcomes. Researchers have developed a very-low-calorie, low-protein diet that causes changes in markers associated with stress resistance and longevity (insulin-growth factor, ketone bodies, and glucose) in a similar manner to prolonged fasting. One FMD cycle on this diet usually lasts for several days (three to five) and cycles are fairly infrequent (one to two times per month). Between cycles, eating resumes as usual without any restrictions (2).

Animal studies show FMD improves health across the board

Since FMD is a relatively new concept, most studies to assess the associated health benefits have been performed in animal models. The physiology that allows organisms to respond and adapt to fasting or starvation conditions first appeared billions of years ago and is present in virtually all organisms (3). Even in S. cerevisiae, a type of yeast, periodic fasting extends lifespan and increases resistance to oxidative stress (4).

Five health benefits of the fasting mimicking diet

Animal models allow researchers to analyze a wider range of health outcomes, such as the regeneration of tissues. Mice are among the most commonly used animal models for studying FMD and are typically given two four-day-long FMD cycles per month, with ad libitum (unrestricted) intake between cycles. In the remaining part of this section, we’ll explore the evidence for FMDs in animal studies.

Metabolic disorders and diabetes

In one mouse study, FMD cycles had profound effects on visceral fat, glucose, and IGF-1 levels. IGF-1 is an endocrine hormone produced primarily in the liver. IGF-1 signaling has been associated with biological aging in some organisms, but whether high levels are good or bad is somewhat controversial (5). After return to an ad libitum (eat as desired) diet, both glucose and IGF-1 levels returned to baseline, but visceral fat remained lower. Mice also showed evidence of significant liver and muscle regeneration as a result of FMD (4). In another study published in February 2017, FMD cycles were shown to reverse late stage Type 2 and even Type 1 diabetes! The FMD triggered epigenetic changes that resulted in expression of prenatal development genes in the adult pancreas. This led to pancreatic β cell regeneration and the return of insulin secretion (6).

Cognitive function

An FMD has been shown to promote hippocampal neurogenesis and improve motor learning and memory in old mice (4). In a mouse model of Alzheimer’s, protein restriction cycles have been shown to alleviate the age-dependent impairment in cognitive performance and reduce levels of phosphorylated tau, a protein that forms the tangles characteristic of Alzheimer’s and several other neurological diseases (7).

Autoimmune disease

In a mouse model of multiple sclerosis, FMD reduced clinical severity in all mice and completely reversed symptoms in 20 percent of animals (8). At the molecular level, FMD increased corticosterone levels and regulatory T cells and promoted remyelination in axons. FMD also reduced levels of pro-inflammatory cytokines and immune cells involved in promoting inflammation.


FMD has been shown to rejuvenate the immune system and protect against cancer in mice. One study found a 45 percent reduction in neoplasia incidence in the FMD group relative to the control group. Cycles of FMD beginning at middle age also delayed tumor onset and reduced the number of lesions, which may indicate a switch from malignant to benign tumors. FMD has also been shown to reduce the number of tissues with inflammation and protect against inflammation-associated skin lesions (4).


Immunosenescence is the age-associated decline in hematopoiesis (the creation of new blood cells), resulting in diminished or altered production of adaptive immune cells. FMD causes a rejuvenation of the blood profile and a reversal of this decline by increasing the number of hematopoietic and mesenchymal stem and progenitor cells. Four months of FMD cycles resulted in a significant increase in RBC count and hemoglobin levels compared to baseline. FMD also increased median lifespan of mice by 11 percent and either attenuated age-dependent bone mineral density loss or induced bone regeneration in mice.

Interestingly, there were some contraindications to fasting in older mice. The authors suggest that in older animals, a less severe low-calorie and low-protein diet may be preferable to continue to provide beneficial effects while minimizing malnourishment (4).

FMD: human clinical trials

While most studies on FMD have been done in animal models, there have been a few clinical trials in humans. In one of the most substantial studies to date, performed by Wei et al. (9), they had 71 subjects complete three FMD cycles. Each FMD cycle lasted for five days, and was repeated once every month. During the five-day period, subjects in the FMD group consumed about 34 to 54 percent of their normal caloric intake. The diet composition was approximately 9 to 11 percent protein, 43 to 47 percent carbohydrate, and 44 to 46 percent fat by energy.

The researchers measured several markers in the blood after three FMD cycles, some of which were maintained after returning to their normal diet:

  • Subjects who completed three full FMD cycles and returned to their normal diet for five to seven days had significantly reduced body weight, total body fat, trunk fat, absolute lean body mass, waist circumference, IGF-1, systolic and diastolic blood pressure, total cholesterol, LDL, and HDL.
  • The effects of FMD on body weight, BMI, waist circumference, IGF-1, and diastolic blood pressure persisted for at least three months after the final FMD cycle.

FMD was found to have especially profound effects in patients at high risk for cardiovascular, metabolic, and age-related disease.

  • Individuals who had high baseline levels (>1 mg/liter) of C-reactive protein, a potent marker of inflammation, had significantly lower CRP levels three months post-FMD.
  • Subjects with low baseline HDL showed significantly increased three-month follow-up HDL levels.

Altogether, these data suggest that FMD causes many beneficial changes in risk factors for chronic, age-related diseases in both animals and humans. Further studies will only contribute to our understanding of the mechanism behind these changes.

When to prescribe a fasting mimicking diet

FMD clearly has numerous health benefits, as I have outlined in this article. However, it should be noted that fasting does have the potential to make some patients worse, depending on their condition.

Patients who are good candidates for FMD include: those who are fighting chronic infections or trying to lose weight; those who have a weak immune system, neurological issues, type 2 diabetes, or other metabolic problems; and those who are healthy and are simply trying to optimize longevity.

Patients who are typically not good candidates for FMD include: those who are pregnant or have HPA axis dysregulation (“adrenal fatigue”), an eating disorder, or thyroid problems. Fasting is also not usually recommended for children or teens.

Always be sure to monitor patients closely when they start to implement FMD or any type of fasting. If patients start to exhibit symptoms of HPA-D or hypothyroidism, you will need to reconsider their fasting schedule. Remember that each patient will respond to fasting differently.

The FMD, as studied, is currently available from Prolon as a specific package of prepared foods and micronutrients intended to be administered under a doctor’s supervision.

It’s likely that a “homemade” version with similar macronutrient ratios and foods would have the same effects, but this hasn’t yet been studied in a clinical trial. Stay tuned for more discussion about the benefits and practical application of fasting in the future, as this is a topic I’m exploring deeply.

That’s all! Now it’s your turn. Had you heard of fasting mimicking diets? Could your patients benefit from an FMD? Share your thoughts in the comments!

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  1. Chris,
    I look to you for unbiased and scientific information. Disappointing that you are promoting this product. The advertising is misleading with very small improvements in blood pressure, cholesterol, and CRP. I know it’s all the rage, but I expected better from you.

    • I like the one size fits all approach and fasting or calorie restriction is not for everyone.
      On the other hand, while this approach seems to be reasonable from an evolutionary standpoint (some “scarcity” alternated with hunting success), I’m really concerned about the fasting “kit”. We always end up with eating processed food. Furthermore, nature is not planned and predictive. It’s merely about business.
      While I perfectly agree that it may be wise to be monitored for issues like HPA and tyroid disregulation, it makes no sense to me that I should buy a processed junk kit to eat less or fast.
      Researchers shouldn’t drive business and this is my main complain I move to dr. Longo and others.
      Research should be aside from business, we blame Big Pharma to do that and then we support a more or less “alternative” idea but business driven exactly like the former we criticized.
      Is it about a real health seeking or about stealing business to the others?
      Moreover, human trial about FMD are surely helpful in the short term.
      People come from decades of chronic inflammation that everything you do slightly better makes a huge impact.
      But does it mean that it’s the way to go on the long run?
      Is it reasonable to think that you can eat junk for most time and it just takes to switch for some days to a fake processed hypocaloric stuff to erase everything?
      I sincerely doubt about it and I don’t know if I’m more angry or disappointed about the direction that is taking research.

    • All he said was: “The FMD, as studied, is currently available from Prolon as a specific package of prepared foods and micronutrients intended to be administered under a doctor’s supervision.” That way, lazy people who refuse to do their own research can garner some of these benefits.

      Having said that, I completely agree that eating such prepackaged foods is entirely unnecessary. I just spent three weeks at a fasting clinic — a two-week medically-monitored water-only fast, followed by a one-weed re-feeding period. I am fairly healthy, but my sister (who has Lupus) had near-miraculous results. We met people at the clinic who cured themselves of diabetes, heart disease, and other serious ailments.

      The doctors work with fasting patients on a daily basis and they have decades of experience. They recommend a fasting mimicking diet once a month for three to five days with a 500 calorie per day restriction. Guess what? Google the amount of calories in a pound a spinach. About 100 calories. So, you can eat about five pounds of various leafy greens per day and still stay within the 500 calorie limit for a fasting mimicking diet. It is the calorie restriction that provides most of the benefits, so forget about the carb/protein/fat ratios. Having said that, spinach is about 30% protein and 15% fat. Look it up.

      Another huge plus: unlike eating Prolon processed food, there no need to administer a 500 calorie diet consisting of five pounds of leafy greens per day under a doctor’s supervision. Eating five pounds of leafy greens per day, you will certainly not go hungry. Let’s face it, you are not going to improve on Mother Nature. This is not rocket science, folks.

      Best of luck to those on the road to good health! Mr. Kresser, thank you for these articles. I do not always agree with everything in them, but they do make me think.

  2. I would be curious to find out more about how to adapt this to a home-based program in terms of correct nutrient ratios, which foods to eat, at what times, and in what quantities.

    • I know folks who have done this many times. They lose some muscle mass, mostly WBC, then upon refeeding their muscle mass goes back slightly higher than it was. Longo talks about how the fasting excites stem cells, but refeeding activates them, leading to the regeneration of tissues in the liver, muscle, etc.

  3. I recently just fell into something like this by listening to my body cues and I’m actually feeling like I need to do this again. Basically, I just limited my protein intake for 3 or 4 days. I ate lots of clean carbs and vegetables with fat on them. I felt great, but at day 4 I felt a bit worse so I reintroduced more protein and went on my way. Now, I feel like I need to start over, my digestion feels sluggish and I’m tired and have brain fog.

    I think that for me, I need to limit the amount of protein I eat from time to time or my whole system slows down to a crawl. This is regardless of how much I supplement with enzymes, ox bile, HCL, ACV etc. I can down a bunch of all of these things, but in the end removing some protein for a while is what works.

    My problem is that I DO have HPA axis issues (low cortisol all day) and low T presumably from CIRS caused by mold exposure. Keyword: presumably…… I think the jury is still out on that diagnosis. I do everything I can otherwise to try and nurture my HPA axis like: Meditate, sleep, eat moderate carbs, clean diet, reduce toxin exposure, easy exercising, no alchohol or caffeine, eat Himalayan salt, take b vitamins and full spectrum mineral, etc etc etc.

  4. It is my opinion that patients – sick people – are dancing with the devil if they do any low calorie fasting; they need all the nutrition they can get. Healthy people struggle with fasting, mainly with the toxic shock and withdrawal from glucose addiction. If patients must fast, it needs to be a low-volume, nutrient dense diet without unrestricted carbs (sugars and grains that metabolise to glucose) and it needs significant preparation plus a gradual implementation. There’s a big difference between low calorie fasting and reduced food fasting – calories have nothing to do with diet or fasting, it’s all about nutrition, digestion and metabolism.
    I’m quite disappointed that you haven’t made the leap to the irrelevance of cholesterol measurements as an indicator of anything to do with health. The HDL and LDL numbers are not measures of cholesterol. If anything, they are indicators of the glucose triglyceride level. Happy to explain if you’re interested

  5. i am willing to try the FMD because i have fasted in the past year for 3 days every 3 weeks and got miraculous results in terms of pains and aches. but recently i haven’t been able to do it anymore, so I am curious to find out more about how to adapt this to a home-based program in terms of correct nutrient ratios, which foods to eat, at what times, and in what quantities.
    many thanks

  6. Regardless of what the “experts” say “It’s good for you” “No, it’s bad for you” fasting is a natural process and should be pursued. Ditto for many, many other things. Too many people are hanging on every word of the experts and are not using their own mind. But to use your own mind you have to become physically healthy, and there’s the “catch 22”. Makes me sad when persons trust doctors when most doctors (Dr. Kresser, Mercola, et al. excepted) don’t know what’s good and are trained by big pharma, because they, the patients, are ignorant.

  7. Would a bone broth diet be more beneficial than the packaged food contained in Prolon? I would assume that would have sufficiently low enough carbs to mimick a FMD and a ton more valuable nutrients to support immune function.

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