The Fasting Mimicking Diet: Valter Longo's Science, ProLon Protocol, and Clinical Results

The FMD as studied by Longo and commercialised through the ProLon kit follows a specific five-day protocol. Day 1 provides approximately 1,090 kcal, composed of 11 % protein, 46 % fat, and 43 % carbohydrate — deliberately low in protein and with a specific macronutrient ratio designed to suppress mTOR and PKA signalling (the cellular nutrient-sensing pathways that regulate cellular growth and inhibit autophagy). Days 2 through 5 each provide approximately 725 kcal with approximately 9 % protein, 44 % fat, and 47 % carbohydrate. The foods provided are plant-based: soups, kale crackers, olives, herbal teas, nut bars, and a supplement pack containing vitamins, minerals, and omega-3 fatty acids. The specific food composition matters because the fasting response is triggered not just by caloric restriction per se, but by the absence of specific nutrient signals — particularly amino acids (leucine and other branched-chain amino acids that directly activate mTOR) and glucose. The low-protein, plant-dominant composition of the FMD keeps these signals low enough to engage fasting-like cellular programmes, while the calories (primarily from healthy fats and complex carbohydrates) prevent the hypoglycaemia, metabolic stress, and lean mass catabolism associated with true water fasting. The protocol is intended to be used once per month for 3–6 months as a clinical intervention, or once every 1–3 months for ongoing health maintenance in healthy adults.

Autophagy — from the Greek for 'self-eating' — is the cellular recycling process by which cells degrade and reuse damaged organelles, misfolded proteins, and other intracellular debris. It is an evolutionarily ancient stress-response mechanism: when nutrients are scarce, cells downregulate biosynthesis and upregulate degradation and recycling, clearing cellular damage accumulated during periods of nutrient abundance. Yoshinori Ohsumi's Nobel Prize-winning work (2016) elucidated the molecular machinery of autophagy; subsequent research has established that reduced autophagy is a feature of ageing, neurodegeneration, cancer progression, and metabolic disease. The primary regulators of autophagy are two nutrient-sensing kinases: mTOR (mechanistic target of rapamycin) and AMPK. mTOR is activated by amino acids, growth factors, and glucose — when mTOR is active, autophagy is inhibited. AMPK is activated by cellular energy depletion (low ATP:AMP ratio) and activates autophagy. Caloric restriction and fasting suppress mTOR and activate AMPK through both lower glucose and lower amino acid availability, releasing the brake on autophagy. The FMD's specific low-protein composition is designed to minimise amino acid-driven mTOR activation even in the presence of some calories. Animal studies from Longo's group demonstrate robust increases in autophagy markers (LC3-II, p62 clearance) during FMD cycles, alongside regeneration of haematopoietic stem cells, intestinal stem cells, and neurogenesis markers — findings that were partly replicated in the human pilot trials. Critically, autophagy is a non-linear, time-dependent process: it typically ramps up significantly after 24–48 hours of nutrient restriction and continues through the five-day protocol, before normalising rapidly on refeeding.

Insulin-like growth factor 1 (IGF-1) is a growth hormone signalling molecule produced primarily by the liver in response to growth hormone and dietary protein. Chronically elevated IGF-1 is associated with increased cancer risk (it promotes cellular proliferation and inhibits apoptosis), accelerated biological ageing, and reduced longevity in multiple species — findings that are mechanistically supported by the remarkable longevity of individuals with Laron syndrome (growth hormone receptor deficiency, resulting in very low IGF-1 levels), who show near-zero rates of cancer and diabetes. Protein restriction is the most potent dietary suppressor of IGF-1. Caloric restriction without protein restriction does not robustly reduce IGF-1 in humans — a finding that distinguishes the metabolic effects of the FMD (which is protein-restricted) from simple calorie-reduced diets. In the landmark 2017 Science Translational Medicine study (Wei et al.), three monthly FMD cycles in 100 healthy participants produced significant reductions in IGF-1 (averaging a 24 % decrease), fasting glucose, and blood pressure, along with reductions in inflammatory markers (CRP, IGF-binding protein 1 changes), and modest but significant reductions in visceral abdominal fat. These changes occurred in participants who completed three cycles; the single-cycle group showed smaller and less consistent effects, suggesting that cyclical repetition matters. The study also showed preservation of lean body mass despite significant fat loss — a distinctive feature compared to standard caloric restriction, possibly related to the growth hormone pulse that occurs during fasting facilitating muscle protein synthesis preservation.

The human clinical evidence for FMD has developed substantially since 2015. The pivotal Wei et al. (2017) trial in healthy adults showed consistent effects across three FMD cycles on: fasting glucose (reductions meaningful in the context of pre-diabetic range), systolic blood pressure (average reduction of 4–6 mmHg), total and LDL cholesterol (modest reductions), IGF-1 (24 % average reduction), C-reactive protein (reduction in those with elevated baseline), and body composition (visceral fat reduction with lean mass preservation). A subsequent 2020 randomised controlled trial in pre-diabetic and diabetic adults (Wilkinson et al. from a different group, testing time-restricted eating rather than FMD specifically, though with overlapping mechanisms) reinforced the metabolic marker improvements from structured caloric restriction patterns. The multiple sclerosis pilot data (Choi et al., 2016) is notable: a small study of relapsing-remitting MS patients using an FMD protocol showed reductions in symptoms and disease activity scores alongside evidence of enhanced remyelination markers — findings consistent with the animal model data showing oligodendrocyte precursor activation during FMD cycles. This has driven larger ongoing clinical trials. For cancer applications, the FMD is being studied as an adjunct to chemotherapy — the hypothesis (supported by animal data) being that FMD-induced downregulation of IGF-1 and glucose, combined with the differential stress response between cancer cells (which cannot downregulate growth) and healthy cells (which can), may increase chemotherapy efficacy. Human clinical trials are in progress; no clinical recommendations exist yet.

Appropriate candidates for the FMD as a self-directed monthly health practice include: generally healthy adults without chronic metabolic conditions, who want periodic metabolic reset and potential longevity benefits; people with moderate overweight or metabolic syndrome markers who have struggled with sustained caloric restriction (the discrete 5-day intervention format may suit them better than ongoing restriction); and individuals interested in evidence-based longevity interventions beyond standard dietary advice. The FMD is not appropriate — and requires medical supervision or avoidance — for the following groups: people with type 1 diabetes (the caloric and carbohydrate restriction can produce dangerous glycaemic instability without careful insulin adjustment); people on insulin or sulfonylurea medications for type 2 diabetes (hypoglycaemia risk is real; doses require physician-guided adjustment); pregnant or breastfeeding women (caloric restriction is contraindicated; fetal growth and milk supply depend on adequate nutrient intake); individuals with a history of eating disorders (structured very-low-calorie protocols can trigger restrictive eating relapses); underweight individuals (BMI below 18.5); people with advanced kidney or liver disease; and anyone currently undergoing chemotherapy or other active cancer treatment without specific physician clearance (while the FMD-chemotherapy combination is being investigated, it is not standard of care). People taking metformin for type 2 diabetes are in a nuanced category — metformin activates AMPK through a mechanism that partially overlaps with fasting, and their combination may require monitoring, though is not necessarily contraindicated.