What if five days a month were enough to make the body younger? That is the fascinating promise of the fasting mimicking diet (FMD), designed by longevity researcher Valter Longo to trick the body into a fasting state while it keeps eating. A study published in 2024 in Nature Communications found that three monthly five-day cycles lowered biological age by 2.5 years on average, independent of any weight loss. Here is what the science says about an approach that challenges how we think about fasting.
In brief – The fasting mimicking diet (FMD) is a five-day, plant-based eating plan designed by longevity researcher Valter Longo to reproduce the effects of fasting while still allowing a small amount of food. Low in sugar and protein but rich in healthy fats, it lowers growth signals such as IGF-1 and mTOR, triggers ketosis and autophagy, then lets the body regenerate once normal eating resumes. A 2024 study in Nature Communications found that three monthly cycles lowered median biological age by 2.5 years, alongside reduced insulin resistance, less liver fat and a rejuvenated immune system, independent of weight loss. The trial was small and the study’s authors note a commercial interest in the product, so the results call for confirmation rather than firm conclusions. Because of clear contraindications, the FMD should be tried under medical supervision.
What is the fasting mimicking diet (FMD)?
The FMD is a five-day plan designed to trick the body into believing it is fasting while it keeps eating. Valter Longo’s idea was to capture the benefits of a prolonged water fast without its difficulty or discomfort.
In practice, it is a plant-based diet with a sharply reduced calorie intake, low in sugar and, above all, low in protein, but rich in healthy fats. This specific profile keeps the body in a state close to fasting. Here are its main features:
- A short duration: five consecutive days, repeated once a month for a few months.
- Very little protein and sugar: to avoid reactivating cell growth pathways.
- A plant-based foundation with healthy fats: supplying energy without breaking the fasting-like state.
- A return to normal eating on the other days of the month.
A ready-to-use commercial version exists (ProLon, developed by Longo’s company), but the plan can also be supervised by a trained health professional.
How the FMD works in the body
The power of the FMD comes from the cascade of mechanisms it triggers, the same ones set off by an actual fast:
- Ketosis: deprived of sugar, the body draws on its fat stores and produces ketone bodies.
- Lower IGF-1 and mTOR: these growth signals, linked to aging, are put on standby.
- Autophagy: cells recycle their damaged components in a thorough internal cleanup.
- Regeneration: once eating resumes, the body activates its stem cells and renews itself.
This cycle of breakdown followed by regeneration is what sets the FMD apart from simple calorie restriction. Like intermittent fasting, it relies on alternating states, but in a deeper and more spaced-out way.
The study: a 2.5-year drop in biological age
The reference study, led by Longo’s team and published in Nature Communications in February 2024, analyzed blood samples from clinical trial participants who had completed three monthly cycles of the FMD (PubMed data).
The results stand out: a median biological age reduction of 2.5 years, measured with a validated indicator built from blood markers. The trial also found reduced insulin resistance, less liver fat visible on MRI, and a rejuvenated immune system. Notably, these benefits were independent of weight loss, which means this is not a weight-loss diet but an intervention on aging itself. The biological age measure used here is the same one you can estimate with our biological age calculator.
| Marker | Change after three monthly cycles |
|---|---|
| Biological age | Down 2.5 years (median) |
| Insulin resistance | Reduced |
| Liver fat (MRI) | Reduced |
| Immune system markers | Rejuvenated |
What the science says, and its limits
In animals, cycles of the FMD extend lifespan and support the regeneration of multiple organs. In humans, trials show real improvements in metabolic and inflammation markers (PubMed data), consistent with the known benefits of fasting on autophagy (PubMed data).
Caution is still warranted. The flagship study is a secondary analysis with a limited number of participants, and the 2.5-year figure rests on biomarkers, not on a directly measured lifespan. In the interest of transparency, Valter Longo has a financial interest in the company that sells the commercial product. This work is promising and rigorous, but it needs confirmation on a larger scale. The findings on diet and aging from the original study call for measured enthusiasm.
In practice, and precautions
The FMD is not something to improvise alone. A few essential points to keep in mind:
- Medical supervision: the FMD should ideally be supervised by a health professional, especially the first time.
- Clear contraindications: it is not advised for people with treated diabetes, pregnancy or breastfeeding, undernutrition, frailty, a history of eating disorders, or certain medical treatments.
- A reasonable frequency: a few cycles a year are enough, this is not meant as a permanent lifestyle.
- A foundation, not a replacement: the FMD complements, but does not replace, healthy daily habits.
This approach fits within the protein-moderation principle championed by Longo, which we cover in our article on protein and longevity, and among the major levers of longevity. Before starting, measuring your biological age lets you track your progress in concrete terms.
Frequently asked questions
What is the fasting mimicking diet?
It is a five-day, plant-based diet that is low in calories, sugar and protein, designed by Valter Longo to reproduce the effects of fasting while still allowing food. It is followed in cycles, a few times a year, not continuously.
Does the FMD really make you younger?
A 2024 study found a 2.5-year drop in biological age after three monthly cycles, measured through blood markers. That is promising, but it is a biological indicator, not proof of a longer lifespan in humans.
Can you do the FMD alone at home?
It is not advisable without guidance, especially the first time. Medical advice matters, and the FMD is contraindicated in several situations, such as treated diabetes, pregnancy or a history of eating disorders.
Is the FMD a weight-loss diet?
No. In the study, the benefits on biological age were independent of weight loss. The FMD targets metabolic and cellular rejuvenation, not weight loss, even though a slight weight reduction can occur alongside it.
⚕️ Medical disclaimer: This article is provided for informational purposes only and does not replace medical advice. Fasting and the FMD carry contraindications: consult a healthcare professional before starting either, especially if you have a medical condition, are pregnant, or have a history of eating disorders.
