The 5-day FMD cycle: structure and caloric targets

A 5-day protocol of ~800–1100 calories per day, designed to replicate the metabolic state of prolonged fasting.

Why it works

Sufficient caloric restriction over 5 days shifts the body into a metabolic state resembling multi-day fasting: circulating IGF-1 and insulin fall, glucagon rises, ketone production increases, and autophagy — the cellular self-cleaning process — is upregulated. The specific macro ratio (high fat, low protein, low carbohydrate) is designed to minimize the insulin and IGF-1 response while providing enough substrate to prevent lean tissue catabolism. Day 1 is higher calorie (about 1100 kcal) to ease the transition; days 2–5 drop to roughly 800 kcal.

How to do it

1. Day 1: ~1100 calories from roughly 10 % protein, 56 % fat, 34 % carbohydrate.
2. Days 2–5: ~800 calories with similar macros — the commercial ProLon kit operationalizes this without calorie counting.
3. Focus on plant-based fats (olive oil, nuts, avocado) and complex carbohydrates; animal protein should be minimal.
4. Maintain normal hydration; herbal tea and black coffee are typically permitted.
5. Plan the 5 days for a period of lower social and physical demand — this is not a performance week.

Evidence

A pilot RCT by Longo’s group found that 3 monthly FMD cycles reduced risk factors for cardiovascular disease, diabetes, and aging (including reduced IGF-1, blood pressure, body fat, and fasting glucose) in healthy adults. (rct)

The 2017 RCT was small (71 participants) and short (3 months). Longevity endpoints (actual lifespan extension) have not been demonstrated in human RCTs; the strongest longevity data remain from animal models.

Sources

• Wei et al. (2017), "Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease," Science Translational Medicine

Common mistake

Treating the FMD as a weight loss protocol and judging it by the scale. The mechanism is metabolic and cellular; weight lost during the 5 days partially returns as the body rehydrates, and that is expected.