Fasting

Growth hormone secretion increases 200 to 300% during 24-hour fasts. This is because insulin and growth hormone operate in opposition. When insulin drops during fasting, growth hormone rises to preserve lean tissue whilst mobilising fat.

This leads to simultaneous fat loss and muscle preservation. Eating every few hours suppresses this response entirely.

Wilkinson randomised 19 metabolic syndrome patients to a 10-hour time-restricted eating window without calorie counting for 12 weeks. Participants reduced body weight by 3%, waist circumference by 4%, visceral fat, blood pressure, total and LDL cholesterol and HbA1c. Those on statin or antihypertensive medication showed additional reductions beyond drug effects alone. No participant withdrew.

The intervention required only that all food and caloric beverages be consumed within a self-selected 10-hour window. No food types were restricted. The mechanism involves extending the overnight fasting period sufficiently to activate AMPK, initiate fatty acid oxidation and lower fasting insulin. The results were achieved in patients already on pharmaceutical treatment, suggesting the metabolic benefit of a fasting window is additive to and independent of medication.

Cheng et al. (2014, Cell Stem Cell) demonstrated that prolonged fasting (48 to 72 hours) in mice triggered a switch from a quiescent to a self-renewing state in haematopoietic stem cells, producing a sixfold increase in stem cell regeneration upon refeeding. The mechanism involved reduction in IGF-1 signalling and PKA activity during the fast, which removed the suppressive signals that keep stem cells dormant. Upon refeeding, the released stem cells repopulated the immune system with new, functional cells. The authors described fasting as "flipping a regenerative switch."

In human subjects, Longo's group showed that a 72-hour fast reduced circulating white blood cells by approximately 40 per cent (as old, damaged immune cells were recycled via autophagy) and that refeeding triggered a burst of haematopoiesis that reconstituted the immune system with newly generated cells. This has implications for ageing (immunosenescence), chemotherapy recovery (immune reconstitution) and autoimmune disease (resetting a malfunctioning immune system). The finding that the body possesses a built-in mechanism for immune system regeneration, activated simply by temporary food withdrawal, was described by Longo as "something you could not have predicted."

Beta-hydroxybutyrate, produced during fasting or carbohydrate restriction, serves at least three signalling functions independent of its role as an energy substrate. Shimazu demonstrated it acts as a class I histone deacetylase inhibitor, directly modifying gene expression to upregulate oxidative stress resistance genes including FOXO3A and MT2. Youm showed BHB suppresses the NLRP3 inflammasome, reducing IL-1beta and IL-18 inflammatory cytokine release.

Additionally, BHB stimulates brain-derived neurotrophic factor expression in the hippocampus, supporting neuronal survival and synaptic plasticity. These three functions, epigenetic regulation, anti-inflammatory action and neurotrophic support, are activated by the same metabolic state that chronic eating patterns prevent. The modern human rarely produces meaningful ketone concentrations because the fasting window never extends long enough.

Krikorian et al. (2012, Neurobiology of Aging) randomised 23 older adults with mild cognitive impairment to either a low-carbohydrate diet (5 to 10 per cent of calories from carbohydrate, sufficient to induce ketosis) or a high-carbohydrate diet (50 per cent carbohydrate) for 6 weeks. The low-carbohydrate group showed a 25 per cent improvement in verbal memory performance (paired associate learning), which correlated with urinary ketone levels (r = 0.45). No improvement was observed in the high-carbohydrate group. Weight loss was similar between groups, suggesting the cognitive benefit was from ketones, not weight reduction.

Ketone bodies (beta-hydroxybutyrate and acetoacetate) serve as an alternative brain fuel to glucose. The brain cannot use fatty acids directly but can derive up to 70 per cent of its energy from ketones during fasting or carbohydrate restriction. Ketones are a more efficient fuel than glucose, producing more ATP per unit of oxygen consumed and generating fewer reactive oxygen species. Beta-hydroxybutyrate also acts as a signalling molecule: it inhibits histone deacetylases (epigenetic regulators), increases BDNF expression (brain-derived neurotrophic factor, essential for synaptic plasticity and memory formation) and activates the Nrf2 antioxidant pathway. The ageing brain often develops "type 3 diabetes" (insulin resistance in the brain), reducing its ability to use glucose. Ketones bypass this metabolic block entirely.

Alirezaei et al. (2010, Autophagy) measured autophagy markers in mouse brain tissue and demonstrated that a 24-hour fast induced a dramatic increase in autophagosome formation, approximately 300 per cent above fed baseline. Significant autophagy induction began at 12 to 16 hours of fasting, when hepatic glycogen stores became depleted and AMPK (the cellular energy sensor) activated. Yoshinori Ohsumi received the 2016 Nobel Prize in Physiology or Medicine for discovering the fundamental mechanisms of autophagy: the process by which cells degrade and recycle their own damaged organelles, misfolded proteins and intracellular pathogens.

Autophagy is the cell's internal quality control system. When nutrients are abundant, the mTOR pathway suppresses autophagy and promotes growth. When nutrients are scarce (fasting), mTOR is inhibited and AMPK activates, triggering the formation of double-membraned vesicles (autophagosomes) that engulf damaged mitochondria, aggregated proteins, viral particles and other cellular debris, delivering them to lysosomes for degradation and recycling. The amino acids and lipids released are used to build new cellular components. Modern eating patterns (three meals plus snacks, eating from waking to sleeping) maintain mTOR activation continuously, suppressing autophagy and allowing damaged cellular components to accumulate. This accumulation is a hallmark of ageing, neurodegeneration and cancer.

Halberg et al. (2005, Journal of Applied Physiology) subjected 8 healthy non-obese men to alternate-day fasting (eating normally one day, fasting the next) for 15 days. Insulin-mediated whole-body glucose uptake increased by 75 per cent (measured by hyperinsulinaemic-euglycaemic clamp, the gold standard for insulin sensitivity assessment). Adipose tissue insulin sensitivity improved, with enhanced insulin-mediated suppression of lipolysis. These improvements occurred with no significant change in body weight, confirming that the benefit was from the fasting protocol itself, not weight loss.

Insulin resistance is the metabolic defect underlying Type 2 diabetes, metabolic syndrome, PCOS, non-alcoholic fatty liver disease and is implicated in Alzheimer's disease, cardiovascular disease and cancer. The primary driver is hepatic fat accumulation: excess intrahepatic triglyceride impairs insulin signalling in the liver, causing increased glucose output and hyperinsulinaemia. Fasting depletes hepatic glycogen and then hepatic triglyceride, directly reversing the pathological process. Two weeks is sufficient to demonstrate the effect. Standard medical treatment for insulin resistance (metformin) improves insulin sensitivity by approximately 20 to 30 per cent. Alternate-day fasting produced a 75 per cent improvement with no medication and no cost.

Over 4.3 million people in the UK are diagnosed with type 2 diabetes, with an estimated 850,000 additional undiagnosed cases. A further 13.6 million are estimated to be at increased risk of developing the condition. Diabetes UK projects 5.5 million diagnoses by 2030. The condition costs the NHS approximately £10.7 billion annually in direct treatment, representing roughly 10% of the entire health budget.

Type 2 diabetes is a disease of chronic insulin resistance, driven primarily by sustained caloric excess, ultra-processed food consumption and physical inactivity. Insulin resistance precedes diagnosis by 10 to 15 years, during which the condition is detectable but rarely detected. The pancreas compensates by producing more insulin until it cannot, at which point blood glucose rises and the diagnosis is made. The metabolic damage accumulated over the preceding decade is already substantial.

Yoshinori Ohsumi received the Nobel Prize in Physiology or Medicine for discovering the mechanisms of autophagy. When nutrient intake stops, cells recycle damaged proteins and defective organelles.

This is because the cell prioritises survival by cannibalising its weakest components first. Autophagy peaks during extended fasts of 16 to 24 hours. It is suppressed by insulin. Every time you eat, the process stops.

The mechanistic target of rapamycin is a kinase that integrates nutrient signals to drive cell growth, protein synthesis and proliferation. Harrison and colleagues showed that rapamycin, an mTOR inhibitor, extended maximum lifespan by 9% in males and 14% in females when administered to genetically heterogeneous mice starting at 600 days of age. It is the only pharmacological intervention consistently extending lifespan across multiple species.

Chronic mTOR activation from constant nutrient availability suppresses autophagy, the cellular recycling process essential for removing damaged proteins and organelles. Saxton and Sabatini described mTOR as the central node connecting overnutrition to cancer, neurodegeneration and metabolic disease. Fasting temporarily inhibits mTOR, activating the same longevity pathways that rapamycin targets without the drug.

Halagappa et al. (2007, Neurobiology of Disease) showed alternate-day fasting in APP transgenic mice reduced amyloid-β plaque burden by 65% and improved cognitive performance on Morris water maze by 40% compared to ad libitum controls. Mattson (2005, Annals of Neurology) reviewed the mechanisms: fasting upregulates BDNF (brain-derived neurotrophic factor) by 50 to 400% depending on duration, activates neuronal autophagy via AMPK-ULK1 signalling and increases ketone body β-hydroxybutyrate which serves as both fuel and HDAC inhibitor.

This is because the brain possesses its own waste clearance system, the glymphatic pathway, which operates primarily during sleep and is enhanced by metabolic states that reduce interstitial fluid viscosity. Fasting reduces brain glucose and insulin, lowering interstitial fluid osmolality and enhancing glymphatic flow. Simultaneously, AMPK activation triggers autophagic degradation of misfolded proteins (tau, α-synuclein, amyloid-β) that accumulate during normal neuronal metabolism.

Alzheimer's disease has been called "Type 3 diabetes" (de la Monte and Wands, 2008, J Diabetes Sci Technol) because brain insulin resistance is a consistent early feature. A population that eats from waking to sleeping, never allowing blood insulin to fall below postprandial levels, never activates the neuronal housekeeping that clears the proteins whose accumulation defines neurodegenerative disease.

Hartman et al. (1992, Journal of Clinical Endocrinology and Metabolism) measured growth hormone (GH) secretion in healthy men under fed and 24-hour fasted conditions using frequent blood sampling. Fasting produced a 20-fold (2,000 per cent) increase in integrated 24-hour GH concentration. The increase was driven by both increased pulse frequency and increased pulse amplitude. The GH surge began at approximately 16 to 18 hours of fasting and peaked at 20 to 24 hours. Intermittent fasting protocols (alternate-day fasting) produced similar but smaller increases in GH pulsatility.

Growth hormone in adults promotes lipolysis (fat breakdown), muscle protein synthesis, bone density maintenance, collagen production and immune function. The fasting-induced GH surge is a survival mechanism: when food is unavailable, GH mobilises stored fat for energy while preserving lean tissue. This is why extended fasting does not produce the muscle wasting seen in chronic caloric restriction. The modern pattern of continuous eating suppresses GH secretion through constantly elevated insulin (insulin is the primary inhibitor of GH release). A person who eats from 07:00 to 22:00 daily never experiences the 16 to 18 hour threshold that triggers the GH surge. The hormonal environment of modern eating patterns is fundamentally different from the intermittent feeding that characterised human evolution.