mTOR / IGF-1

This Aging Cell paper directly addressed a paradox: rodent studies of caloric restriction reliably show IGF-1 reductions and longevity benefits, but the few existing human CR studies had not replicated the IGF-1 effect. Why? Fontana and colleagues compared three groups of human subjects: 28 long-term Calorie Restriction Society members (about 30 percent CR for 5+ years, but maintaining typical Western protein percentages around 24 percent of energy), 28 age-matched moderately protein-restricted vegans (around 10 percent of energy from protein), and 28 sedentary controls. The headline finding overturned the assumption that calories drive the IGF-1 effect: the strict CR group had no significant reduction in IGF-1 versus controls, while the vegans (heavier than the CR group, with more body fat) had significantly lower total and free IGF-1. The paper's conclusion is unambiguous: in humans, low protein intake — not low calorie intake — is what suppresses IGF-1. This finding helped explain why CR-induced longevity benefits in mice have not translated cleanly to humans on standard Western protein intakes, even at low calorie levels.

This Cell Metabolism paper combined a large NHANES-based human cohort (2,253 adults followed over 18 years) with mouse experiments to ask whether high protein intake — especially animal protein — drives cancer and mortality risk via IGF-1 and growth-hormone signalling. The headline finding is age-dependent. In adults aged 50–65, those reporting high protein intake (≥20 percent of calories from protein) had a 75 percent higher overall mortality and a fourfold higher cancer death risk over the next 18 years compared to low-protein eaters (under 10 percent of calories). The effect was largely abolished when the protein came from plant sources rather than animal sources. After age 65, the relationship reversed: high protein became protective for cancer and overall mortality — though high protein at any age was associated with a fivefold increase in diabetes mortality. Mouse experiments supported the mechanism: high-protein diets accelerated tumour growth and elevated IGF-1, while protein restriction did the opposite. The interpretation is that protein's relationship with longevity is not monotonic; it depends on age, on the protein source, and on what's being optimized for.

This Cell review by Saxton and David Sabatini — Sabatini being one of the original co-discoverers of mTOR — is the most-cited modern synthesis of mTOR signaling biology. The paper traces how mTOR (mechanistic target of rapamycin) integrates four classes of inputs: nutrients (amino acids, especially leucine and arginine), growth factors (insulin, IGF-1), cellular energy state (AMPK senses ATP:AMP), and stress signals. mTOR exists as two complexes: mTORC1, which controls protein synthesis, lipid synthesis, and inhibits autophagy; and mTORC2, which controls cytoskeletal organization and Akt phosphorylation. The review explains how mTORC1 activation drives anabolic programs (cell growth, protein synthesis) while suppressing catabolic programs (autophagy, lipolysis). Conversely, mTORC1 inhibition — by fasting, by rapamycin, by amino acid restriction, or by genetic loss — releases autophagy, increases lipolysis, and engages stress-resistance programs. The paper documents how dysregulated mTOR signaling drives cancer (mTOR is hyperactivated in most tumors), diabetes (mTORC1 contributes to insulin resistance), and aging (mTOR inhibition extends lifespan in every model organism tested). Therapeutic targeting of mTOR is an active drug-development area.

This Cell Metabolism paper from Valter Longo's USC group introduced the fasting-mimicking diet (FMD) — a 5-day periodic dietary protocol designed to deliver fasting's molecular benefits while keeping participants able to consume modest amounts of plant-based food. The paper has two parts. In aged mice, monthly FMD cycles for several months produced multi-system regeneration: hippocampal neurogenesis rose, IGF-1 dropped, PKA activity decreased, NeuroD1 expression increased, and cognitive performance improved on standard mouse cognition tests. In a 38-participant pilot human RCT, three monthly FMD cycles (each 5 days) produced reductions in body weight, body fat, blood pressure, fasting glucose, and IGF-1 without significant adverse events. The paper is foundational because it bridged rodent CR research and practical human protocol design — providing a structured, safe framework for delivering fasting benefits without continuous calorie restriction. Longo subsequently commercialized the protocol as ProLon, a packaged 5-day FMD product. The paper's data quality is solid but the commercial development complicates how it should be cited.

This NEJM review summarizes evidence that intermittent fasting regimens — alternate-day fasting, time-restricted eating, and periodic multi-day fasts — engage a "metabolic switch" from glucose-derived energy to fat- and ketone-derived energy after hepatic glycogen is depleted, typically within 12–36 hours of fasting depending on the individual and the protocol. The authors argue that repeated exposure to this switch produces adaptive responses across organ systems, including improved insulin sensitivity, reduced inflammation, increased mitochondrial biogenesis, enhanced autophagy, and improved stress resistance in cells. The review compiles findings from animal models alongside the available human trials at the time of publication. The review notes that, despite preclinical signals being strong and consistent, the human evidence base is more heterogeneous: the largest gains in metabolic markers (fasting insulin, HOMA-IR, lipid profile, inflammatory markers) appear in adults with obesity or metabolic syndrome, while effects in lean, metabolically healthy individuals are smaller. The authors flag practical issues — adherence over months, the early-fast hunger and irritability phase, and the lack of long-term outcome data — as the main barriers to clinical adoption rather than safety in healthy adults.

This 2024 Nature Cell Biology paper from the Madeo lab identified spermidine — a polyamine found in many foods (wheat germ, soybeans, mushrooms, aged cheeses) and produced endogenously — as the essential mediator of fasting-induced autophagy. The authors ran experiments across multiple model systems: yeast, nematodes, mouse cells, and human cell lines (U2OS osteosarcoma cells and H4 neuroglioma cells). Across all systems, blocking spermidine synthesis with the inhibitor DFMO suppressed fasting-induced autophagy — and supplementing exogenous spermidine (100 µM) rescued the autophagy response. The paper also reports human-cohort metabolomics: across multiple cohorts of fasting participants (61 to 109 volunteers per cohort, fasting durations 3 to 16 days), serum spermidine levels rose during fasting. Human PBMCs showed increased hypusination of eIF5A — a downstream effect linking spermidine to translation control and autophagy machinery. The paper's mechanistic claim is significant: spermidine is not just correlated with fasting-induced autophagy; it is required for the response to occur.

Before this paper, the dominant view was that the brain was metabolically privileged — protected from the autophagy-inducing effects of food restriction so that neurons could maintain function during starvation. Alirezaei and colleagues at the Scripps Research Institute overturned that assumption. Using mice fasted for 24 to 48 hours, they directly measured autophagy markers in cortical neurons and Purkinje cells (the large output neurons of the cerebellum). They found dramatic upregulation: increased numbers of autophagosomes, altered autophagosome characteristics, and decreased neuronal mTOR activity (measured via reduced phosphorylation of S6 ribosomal protein). Transmission electron microscopy directly visualized the autophagosome accumulation. The paper's interpretation: short-term fasting is a simple, non-pharmacological intervention that produces measurable brain autophagy responses. The authors speculated that periodic fasting could be a low-cost approach to engaging neural autophagy as a therapeutic mechanism for protein-aggregation neurodegenerative diseases. The paper has been cited heavily in subsequent fasting-and-brain-health literature and in popular science writing on fasting's neurological benefits.

This is the most-cited review of whether fasting and calorie restriction actually trigger autophagy — the cellular self-cleaning process that recycles damaged proteins and organelles. The authors surveyed studies across cell culture, rodent models, and human subjects, looking at autophagy markers such as LC3 lipidation, p62 turnover, ATG7 expression, and mTOR signalling under various fasting and calorie-restriction protocols. Their headline conclusion is that fasting and calorie restriction reliably upregulate autophagy across a wide variety of tissues and organs — liver, muscle, brain, heart, kidney — and that the effect is robust. They also note that autophagy is mechanistically central to the longevity and disease-prevention benefits of caloric restriction: blocking autophagy in animal models attenuates those benefits. The evidence base, however, leans heavily on rodent and cell-culture work; direct measurement of autophagy in living humans is limited because most autophagy markers require tissue biopsy.

Seyfried and Shelton restate and develop the metabolic theory of cancer first proposed by Otto Warburg, arguing that the origin and progression of cancer is best understood as a mitochondrial-respiratory dysfunction that drives the cellular dependence on glycolysis — the Warburg effect — observed in the majority of tumors. The review compiles evidence from cancer cell biology, tumor metabolism, and animal models suggesting that interventions which restrict glucose availability (caloric restriction, ketogenic diets, multi-day fasting) or that pressure tumor cells through mitochondrial dysfunction may slow tumor growth or sensitize tumors to conventional therapy. The authors propose specific therapeutic implications and discuss the evidence base for ketogenic and caloric-restriction interventions as adjunctive cancer therapy. The review has been influential among researchers exploring metabolic approaches to cancer and is cited heavily in popular content connecting fasting and ketogenic eating to cancer outcomes — sometimes carefully, often less so.

This is the Nature review that brought autophagy to mainstream biomedical attention. Authored by four of the field's most prominent researchers — Mizushima, Beth Levine, Ana Maria Cuervo, and Daniel Klionsky — the paper synthesizes what was known by 2008 about cellular self-digestion as a regulated, disease-relevant process. The authors lay out three core ideas. First, autophagy operates at a basal level in all eukaryotic cells and can be induced by environmental stress — most notably nutrient deprivation, but also hormonal signals, hypoxia, and pathogens. Second, the regulatory pathway centers on mTOR (target of rapamycin), which inhibits autophagy when nutrients are abundant; when mTOR is suppressed (by fasting, by rapamycin, or by genetic loss of function), autophagy is unleashed. Third, autophagy plays both protective and harmful roles depending on context: it prevents neurodegeneration, fights infection, and clears damaged proteins, but cancer cells and some pathogens can hijack the process to survive. The review remains the foundational citation for almost any modern paper on autophagy's role in disease.