FMD Diet: Fasting-Mimicking Diet, What It Is and the Science Behind It

Fasting has always been considered a powerful health tool, but also an exhausting one, dangerous for those unaccustomed to it, and nearly impossible for most of us to maintain over time. The question asked by Prof. Valter Longo from the University of Southern California (USC) was simple yet groundbreaking: What if you could activate the same biological mechanisms triggered by a prolonged water fast, without actually fasting? What if you could still eat, and yet trick the body into thinking it's fasting?

From this question, the FMD diet (Fasting-Mimicking Diet) was born. This is a five-day nutrition program, low in calories, low in protein, and plant-based, carefully designed to push the body into the same metabolic state it enters during a real fast, while you are still eating meals. This idea gained traction, was commercially marketed under the name ProLon, and sparked both great enthusiasm and healthy skepticism. In this article, we will honestly break down both sides.

What is the FMD Diet?

The FMD diet is not a daily diet, but a short, intensive cycle done several times a year. Here are the principles that define it:

• Five consecutive days. A typical cycle lasts five days, after which you return to a normal diet. It's not a daily lifestyle but a periodic intervention.
• Low calorie, gradually. On the first day, you consume about 1,100 calories, and on days 2 through 5, you drop to about 800 calories per day, far less than the normal requirement.
• Low protein and low sugar. This is perhaps the most critical feature. The protein level is intentionally low, to avoid activating the cellular growth pathways (like mTOR and IGF-1) that fasting is supposed to disable.
• Plant-based and healthy fats. Most calories come from healthy fats and complex carbohydrates from plant sources, not from animal protein.
• You still eat. Unlike a water fast, there is real food on the plate: soups, nut-based snacks, tea. This is what makes it feasible for many people who would never survive a full fast.

In short: A fasting-mimicking diet is an attempt to achieve the benefits of prolonged fasting with a much lower behavioral cost. The body enters a state of energy and protein deficiency, but the mind doesn't have to cope with five days of absolute hunger.

The Mechanism: How Can Food Mimic Fasting?

To understand why this works, you need to understand what actually happens during a real fast. When the body stops receiving food, especially sugar and protein, it undergoes a series of ancient survival responses developed over millions of years of food scarcity. The FMD diet is precisely designed to trigger those same responses.

The central lever is autophagy, literally "self-eating." This is a cellular process where the cell breaks down and recycles damaged components, misfolded proteins, and worn-out organelles. When food is abundant, the cell is busy with growth and building and doesn't bother cleaning. When food is scarce, the cell switches to maintenance mode and starts cleaning itself. Proper autophagy is considered one of the key mechanisms for cellular health and slowing aging.

A second lever is lowering IGF-1 levels (Insulin-like Growth Factor 1). IGF-1 is a growth hormone that, when chronically high, is linked to an increased risk of cancer and accelerated aging. A low-protein, low-calorie diet lowers IGF-1, thereby signaling the body to shift from a "growth" state to a "protection and repair" state.

The third lever, and perhaps the most intriguing, is stem cell regeneration. In Longo's studies, FMD cycles caused a temporary decrease in certain cell numbers during the fast, followed by a wave of stem cell activation and regeneration during the refeeding phase. The idea is that the cycle of "controlled destruction and then rebuilding" clears out old and damaged cells and replaces them with new ones. Additionally, FMD tilts the body towards metabolic resetting: improved insulin sensitivity, decreased blood sugar and inflammation, and reduced belly fat.

Current Evidence

The vision is beautiful, but what have the studies actually shown? Here are the two central works on which the entire field rests, honestly regarding what they show and what they still don't.

Study 1: Brandhorst and Longo from 2015 (Cell Metabolism)

This is the landmark study. A team led by Sebastian Brandhorst and Longo published a broad work in Cell Metabolism that included experiments on mice and humans. In mice, FMD cycles twice a month showed impressive results: reduced belly fat, stem cell regeneration in multiple systems, improved cognitive function, and even extended lifespan.

In the human part, a smaller sample was tested: 19 participants performed three FMD cycles (five days per month, for three months). The results showed a decrease in weight and belly fat, a decrease in IGF-1, a decrease in fasting blood sugar levels, and a decrease in CRP (an inflammation marker) among those who were initially inflammatory. It is important to emphasize: this is a small sample without a full control group, so this is promising but preliminary evidence.

Study 2: Wei and Longo from 2017 (Science Translational Medicine)

The second study strengthened the picture with a better design. Min Wei, Longo, and colleagues published a randomized, controlled trial in Science Translational Medicine with 100 healthy participants. The subjects were divided into two groups: one performed three FMD cycles over three months, the other continued with a normal diet, and then the groups switched.

The results: Among those who performed the FMD, there was a decrease in weight and waist circumference, a decrease in blood pressure, a decrease in fasting blood sugar, a decrease in IGF-1, and a decrease in cholesterol and CRP among those whose values were initially high. In other words, the improvement was greater precisely in people who started with worse markers, suggesting this is a better tool for correcting an existing problem than for optimizing an already healthy person. Still, this is an intervention of only three months, so long-term effects on morbidity and mortality in humans have not yet been proven.

FMD Diet vs. Intermittent Fasting: Don't Confuse Them

This is perhaps the biggest point of confusion, so we will clarify it well. Intermittent Fasting and the Fasting-Mimicking Diet are two completely different things, even if both play with the same mechanisms.

Intermittent fasting is a daily or weekly pattern. In the common version (e.g., 16:8), you eat within an 8-hour window and fast for 16 hours, every day. The fast is short, repetitive, and doesn't necessarily change what you eat, only when. The goal is mainly to extend the overnight fasting hours and improve metabolic control as a regular habit. For our full coverage on fasting and eating windows, you can refer to our article database.

The FMD diet is a periodic and intensive intervention. It is not done every day but several times a year, it lasts for five consecutive days, and it strictly defines what you eat (very low in calories, low in protein, plant-based) and not just when. The goal is to push the body deep into a fasting state, trigger autophagy and regeneration, and then return to normal eating and allow the body to rebuild itself.

In short: Intermittent fasting is a daily maintenance regimen, while the FMD diet is a seasonal "deep clean." The two are not competitors, and some people combine both. But describing FMD as "a type of intermittent fasting" is a mistake; they differ in frequency, duration, and the degree of control over food composition.

Should You Start the FMD Diet?

Here we need to stop and be cautious. Despite the promising results, there are some real and important warnings, and the FMD diet is definitely not suitable for everyone.

First, safety. A drastic reduction in calories for five days can be dangerous for certain populations. Diabetics taking insulin or blood sugar-lowering medications are at risk of dangerous hypoglycemia and must not do this without close medical supervision. The same applies to people who are underweight, pregnant or breastfeeding women, those who have or have had an eating disorder, frail and weak elderly individuals, and anyone suffering from a chronic disease or taking regular medications. For all of these, the clear recommendation is: consult a doctor beforehand, and do not go into this alone.

Second, cost. The commercial ProLon kit is not cheap: a single five-day cycle costs hundreds of shekels, and to get the effects, you need several cycles a year. It is possible to build a cheaper "do-it-yourself" version, but this requires meticulous nutritional planning to hit the exact caloric and protein targets, and it is best done with a professional. An improvised home version might miss the mark or be unbalanced.

Third, proportions. Most of the evidence in humans is based on small, short-term studies. We do not yet know if FMD extends lifespan in humans, if it is safe over decades, or what the optimal frequency is. This is a promising tool with a real scientific basis, but not magic and not a substitute for the fundamentals.

What Can You Take from the Research?

Even if you don't run to do an FMD cycle tomorrow morning, there are some practical conclusions you can apply right now:

1. If you are healthy and curious, consult first. If you are considering trying FMD or ProLon, talk to a doctor or clinical dietitian first, especially if you are taking any medications. Do not improvise a home version without guidance.
2. If you have borderline metabolic markers, this is a direction to discuss. The studies show that the benefit is greater precisely in those who start with high blood sugar, blood pressure, cholesterol, or inflammation. This doesn't mean run to buy a kit, but to raise the possibility with your doctor as part of a broader plan.
3. If you want the benefits without the cost and risk, start with the basics. A plant-based diet, reducing excess animal protein, cutting down on sugar and processed food, and physical activity all activate some of the same pathways (like autophagy and lowering IGF-1) in a gentler and safer way. See our guide to nutrition for longevity.
4. Don't confuse the tools. If you are already doing intermittent fasting, know that it is not the same as FMD. Both are legitimate, but they serve different needs and operate at different frequencies.
5. Integrate this into a broad picture. No single intervention beats a complete lifestyle. If you want to plan steps tailored to your age, condition, and goals, start by building a personal protocol.

Frequently Asked Questions

What is the difference between the FMD diet and intermittent fasting?

Intermittent fasting is a daily pattern of an eating window (e.g., eating in 8 hours and fasting for 16), which focuses mainly on when you eat. The FMD diet, in contrast, is a periodic and intensive five-day cycle done only a few times a year, and it strictly defines what you eat as well: very low in calories, low in protein, and plant-based. Intermittent fasting is daily maintenance, FMD is a seasonal deep clean.

How many times a year do you do FMD?

This depends on health status and goals, and should be determined with a professional. In research protocols, three five-day cycles were typically done over three months. After that, commercial recommendations usually range from one to several cycles a year for maintenance, depending on individual goals and health status. There is no single correct number for everyone.

Who should not do the FMD diet?

The FMD diet is forbidden or dangerous for diabetics taking insulin or blood sugar-lowering medications (risk of hypoglycemia), underweight individuals, pregnant or breastfeeding women, those who have or have had an eating disorder, frail and weak elderly individuals, and anyone suffering from a chronic disease or taking regular medications. Every person should consult a doctor before starting, and it should not be done without medical supervision in high-risk cases.

The Broader Perspective

The FMD diet represents an elegant idea in aging science: you don't always have to fight biology; sometimes you can harness it. Instead of forcing the body into a full and dangerous fast, Valter Longo and his colleagues found a way to speak to the body in its own language, put it into a fasting state through carefully planned food, and let it go through its cycle of destruction and rebuilding on its own.

But the most important message is one of humility. A fasting-mimicking diet is a tool, not a magic solution. The evidence is promising, the mechanisms are real, but the human studies are still small, short-term, and not suitable for everyone. The same mechanisms that FMD tries to activate—cellular cleaning, metabolic balance, and slowing growth pathways—are also activated, gently, by the things we say over and over: a plant-based diet, less sugar, more movement, and good sleep. The best fasting-mimicking diet, in the end, is simply a healthy lifestyle maintained over years.

References:
Wei M, Longo VD et al., Science Translational Medicine 2017 - Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease
Brandhorst S, Longo VD et al., Cell Metabolism 2015 - A periodic diet that mimics fasting promotes multi-system regeneration, enhanced cognitive performance, and healthspan