The Fasting Mimicking Diet: Valter Longo's 5-Day ProLon Protocol Explained

The Science Behind Fasting Mimicking#

Valter Longo is the Director of the Longevity Institute at the University of Southern California and one of the most cited longevity researchers in the world. His work on caloric restriction, fasting, and the IGF-1/mTOR pathway has shaped how the field understands the relationship between nutrition and lifespan.

The core problem with extended fasting for most people: it is hard. 5-day water fasting is physiologically effective but practically brutal — loss of lean mass, difficulty working, social disruption, and safety concerns in people with certain conditions.

Longo's insight: if you design a diet with specific macronutrient ratios and calorie levels, you can keep the body in a fasting-like metabolic state — triggering autophagy, stem cell activation, and hormonal changes associated with fasting — while still consuming food. The body does not perceive the food as sufficient to exit the fasting state.

This is the fasting mimicking diet (FMD).

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The Exact Protocol#

Duration: 5 consecutive days, once per month (or once every 3 months depending on health goal)

Calorie targets:

• Day 1: ~1,100 kcal (10% protein, 56% fat, 34% carbohydrate)
• Days 2–5: ~800 kcal (9% protein, 44% fat, 47% carbohydrate)

Critical macronutrient features:

• Very low protein — keeps IGF-1 and mTOR suppressed (the key anti-ageing mechanism)
• High fat, low glycaemic carbohydrate — maintains ketosis and suppresses insulin
• No animal protein in the Longo protocol — plant-based protein sources only

What you eat:

• Vegetable soups (low sugar, low protein)
• Olives, nuts (fat sources)
• Nut bars (small amounts)
• Herbal teas and water (unlimited)
• Glycerol supplement (proprietary ProLon ingredient — helps preserve lean mass during caloric restriction)

Days 6–7 (refeeding):

• Gradually return to normal eating — day 6 is a Mediterranean-style transition day
• This transition period is critical: the cellular regeneration partly occurs during refeeding as stem cells activate to replace cleared cells

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The Clinical Evidence#

Phase 1/2 Trial (Brandhorst et al., 2015 — Cell Metabolism)#

Longo's landmark paper testing the FMD in humans: 71 adults randomised to either 3 cycles of FMD or control diet.

Results:

• Reduced IGF-1 (the primary growth hormone-related cancer and ageing risk factor) by 24%
• Reduced fasting glucose by 11.3%
• Reduced CRP (inflammatory marker) by 26%
• Reduced waist circumference and body fat (particularly trunk fat)
• Participants with elevated risk markers showed the greatest improvements

Phase 2/3 Trial (Wei et al., 2017 — Science Translational Medicine)#

100 participants, 3 monthly FMD cycles.

Results:

• Reduced systolic blood pressure by 4.5 mmHg
• Reduced IGF-1, IGFBP-1
• Reduced total and LDL cholesterol
• Reduced HbA1c in pre-diabetic participants
• Biological age reduction: Multiple markers of cellular age improved

Biological Age Study (2024 — Nature Communications)#

The most recent and significant clinical data: 3 cycles of FMD produced a measurable reduction in biological age using the PhenoAge algorithm (a validated biological age clock) of approximately 2.5 years vs controls.

This is one of the few interventions to show a measurable biological age reduction in a controlled clinical trial.

Cancer Context#

Longo's group has also demonstrated that fasting and FMD improve chemotherapy efficacy and reduce side effects in cancer patients — by sensitising cancer cells to chemotherapy while protecting normal cells. Multiple clinical trials are underway.

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Why It Works: Mechanisms#

1. Autophagy Induction#

Caloric restriction to ~800 kcal/day, particularly with low protein, creates the nutrient scarcity that activates autophagy — cellular self-cleaning. Within 24–48 hours of FMD initiation, autophagy markers rise significantly.

2. IGF-1 and mTOR Suppression#

IGF-1 (insulin-like growth factor 1) is a potent driver of both growth and ageing. Low protein intake dramatically reduces IGF-1 production. mTOR — the master nutrient sensor — is suppressed by low amino acid availability. Both pathways are central to longevity in every species studied.

3. Stem Cell Activation#

During the refeeding phase after FMD, stem cell proliferation increases — particularly haematopoietic (blood-forming) stem cells and intestinal stem cells. This "regenerative refeeding" effect is the most dramatic biological impact of the FMD cycle.

4. Metabolic Switching#

After 2–3 days, most people on FMD enter ketosis — producing ketone bodies that serve as neuroprotective fuel, reduce inflammation, and activate AMPK.

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DIY FMD vs ProLon#

ProLon is the commercial product Longo developed based on his research. At $249 for 5 days ($50/day), it is the most convenient option but not the only one.

DIY FMD — approximate daily menu:

Day 1 (~1,100 kcal):

• Breakfast: Handful of almonds + herbal tea
• Lunch: Homemade vegetable soup (broccoli, zucchini, tomato, olive oil)
• Dinner: Larger vegetable soup + a tablespoon of olive oil + 5–6 olives
• Snack: Small portion of mixed nuts

Days 2–5 (~800 kcal):

• Morning: Herbal tea, black coffee (allowed)
• Lunch: Vegetable soup (minimal potato/grain, mostly non-starchy vegetables)
• Dinner: Vegetable soup + 10 olives + handful of nuts

Key rules for DIY:

• Keep protein under 20–25g/day (the mTOR suppression threshold)
• No animal protein
• No high-glycaemic foods (bread, pasta, rice, sugar)
• Olive oil is your primary fat source
• Water and herbal teas ad libitum

The DIY approach can achieve similar caloric and macronutrient profiles for ~$15–20/cycle. The commercial ProLon advantage: precision, convenience, and the glycerol supplement for muscle preservation.

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Who Should Do the FMD#

Highest benefit population:

• People with metabolic syndrome (elevated glucose, triglycerides, blood pressure, waist circumference)
• Those over 50 wanting cellular rejuvenation
• Anyone seeking to reduce biological age markers
• Athletes in off-season looking for metabolic reset

Contraindications:

• Pregnancy and breastfeeding
• Underweight (BMI < 18.5)
• History of eating disorders
• Active cancer treatment (consult oncologist — may be beneficial but requires medical supervision)
• Insulin-dependent diabetes (hypoglycaemia risk without medical monitoring)
• Always consult your physician before starting — Longo himself recommends physician supervision for the first cycle

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Practical Tips for Completing the 5 Days#

The biggest failure mode is giving up on days 3–4 — the lowest energy period before ketosis fully establishes.

1. Start on a Wednesday or Thursday — days 4–5 fall on the weekend when social eating obligations are lower
2. Light exercise only — walking is fine, intense training is counterproductive
3. Expect day 2–3 fatigue — this is the transition period; it passes
4. Plan for low social commitments — restaurant meals and social eating are very difficult on FMD
5. Black coffee is allowed — a meaningful quality-of-life option during the 5 days
6. Track protein carefully — the mTOR suppression mechanism depends on keeping protein very low

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Frequency#

For healthy adults wanting longevity maintenance: Once every 3 months (4 cycles/year)

For metabolic improvement or biological age reduction: Once per month for 3–6 months, then quarterly maintenance

Longo's research used 3 monthly cycles as the primary intervention before switching to quarterly maintenance. Most of the benefits in his trials were established within 3 cycles.