Fasting Basics

What Fasting Is

Fasting is the deliberate suspension of food intake for a defined period, allowing the body to shift from processing incoming nutrients to mobilising stored reserves. Water is generally permitted (though dry fasting variants exist), as are non-caloric beverages like coffee and tea. The defining feature is the absence of food calories.

The body has two governing metabolic states, fed and fasted, and fasting is the deliberate extension of the fasted state long enough to access its characteristic adaptations. The transition isn’t instantaneous; it unfolds over hours and days through a sequence of well-characterised stages, each with its own hormonal signature and metabolic priority.

The Metabolic Transition: What Happens When You Stop Eating

The phase-by-phase progression has been studied extensively since George Cahill’s foundational work on starvation metabolism in the 1960s and 70s at Harvard. Cahill and colleagues established the basic phases that remain accurate today, though subsequent work has refined the details substantially.

• Hours 0-4 – The Fed State: You’ve just eaten. Blood glucose rises, insulin is released, nutrients are absorbed and distributed. Glucose is the primary fuel; whatever isn’t immediately used gets stored as glycogen in the liver and muscles, or as fat in adipose tissue. Insulin levels are elevated. Anabolic processes (protein synthesis, glycogen storage, fat storage) are running. Fat oxidation is suppressed.
• Hours 4-12 – Postabsorptive State: Digestion completes. Blood glucose starts to drop. Insulin levels decline. Glucagon (insulin’s counterpart, produced by the pancreas) starts to rise. The liver begins releasing stored glycogen as glucose to maintain blood sugar. You’re now drawing on yesterday’s meals.
• Hours 12-18 – Glycogen Depletion: Liver glycogen stores (roughly 100-150 grams in a well-fed adult) deplete progressively. The liver continues releasing glucose, increasingly through gluconeogenesis (producing new glucose from amino acids and glycerol) rather than glycogenolysis (breaking down stored glycogen). Adipose tissue starts releasing fatty acids into the bloodstream more substantially. Insulin continues falling; glucagon continues rising. You’re starting to mobilise body fat.
• Hours 18-24 – The Metabolic Switch: Glycogen is largely depleted. The liver begins producing ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone) from incoming fatty acids. Blood ketone levels begin to rise, typically reaching 0.3-0.5 mmol/L by the 24-hour mark. The body is shifting toward fat as primary fuel. Growth hormone secretion patterns change, with pulse amplitudes substantially elevated. Catecholamine (epinephrine, norepinephrine) levels rise modestly.
• Days 1-3 – Ketogenesis Ramps Up: Ketone production accelerates. Blood BHB typically reaches 1-3 mmol/L by day 3 in healthy adults. The brain, which can’t use fatty acids directly, begins shifting toward ketones as an alternative fuel source. Protein breakdown, initially elevated to supply gluconeogenic amino acids, starts to slow as ketones spare the need for as much glucose. Most people report substantial appetite reduction by day 2-3 as ketones suppress hunger signalling.
• Days 3-7 – Adapted Fasting: The body has substantially shifted toward fat metabolism. The brain may now derive 50-70% of its energy from ketones. Protein sparing is at its peak (the body is preferentially burning fat rather than breaking down muscle). Many of the deeper adaptations (autophagy upregulation, immune cell turnover, growth hormone elevation) are running at their highest sustained levels. This is the metabolic zone where the most dramatic therapeutic effects (Longo’s stem cell research, Fung’s clinical diabetes reversal work) have been documented.
• Beyond 7 days: The body continues drawing on fat reserves, but the marginal benefit per additional day plateaus and the risk profile changes. Extended fasts beyond 7 days carry meaningful risks around electrolyte balance, refeeding syndrome, and muscle protein loss that warrant medical supervision. Most of the well-characterised therapeutic effects are accessible within the 3-7 day window.

The hour and day markers above are approximations, and there is always individual variation. Metabolically flexible people (those with regular exposure to lower-carbohydrate eating, exercise, and previous fasting experience) progress through the phases faster. Metabolically inflexible people (those whose primary fuel has been carbohydrate-heavy with frequent eating) progress more slowly and may experience more pronounced “keto flu” symptoms during the transition.

Autophagy: Cellular Cleanup

Of all the fasting-triggered adaptations, autophagy is the one that has captured the most popular attention. The word combines Greek roots meaning “self-eating,” and it describes the process by which cells dismantle and recycle their own damaged components: misfolded proteins, dysfunctional organelles, and accumulated metabolic debris.

The Nobel-recognised foundational work: Yoshinori Ohsumi at the Tokyo Institute of Technology was awarded the 2016 Nobel Prize in Physiology or Medicine for his work in the 1990s, identifying the genes responsible for autophagy in yeast. His group identified the ATG (autophagy-related) genes, the molecular machinery of the autophagosome, and the regulatory pathways that determine when autophagy is triggered. Subsequent work, particularly from Noboru Mizushima’s group at the University of Tokyo and Daniel Klionsky’s group at the University of Michigan, has extended these findings into mammalian systems.

What autophagy does: Cells accumulate damage over time: proteins that have misfolded and lost function, mitochondria that have stopped producing ATP efficiently, lipid droplets that have become oxidised, and various other cellular detritus. Without a way to clear this debris, the damage accumulates and cellular function declines. Autophagy is the body’s mechanism for cleaning house: damaged components get tagged, packaged into a double-membrane structure called an autophagosome, transported to the lysosome, and broken down into their constituent amino acids, fatty acids, and nucleotides, which become available for new construction.

Why it matters for health and longevity: When autophagy is impaired, the consequences are substantial. Neurodegeneration (Alzheimer’s, Parkinson’s, Huntington’s, ALS) involves accumulated misfolded proteins that adequate autophagy would have cleared. Type 2 diabetes involves dysfunctional pancreatic beta cells that better autophagy might have repaired or recycled. Cardiovascular disease involves accumulated cellular damage in the vasculature. Cancer is mechanistically complex, but autophagy can both suppress early tumour formation and protect established tumours from chemotherapy. Most age-related conditions have an autophagy component.

What activates autophagy? Nutrient deprivation is the most reliable trigger. When cells sense insufficient amino acids and energy, the mTOR pathway (mechanistic target of rapamycin, a master regulator of cell growth) is suppressed, and autophagy is upregulated.

Specifically:

• Insulin (which signals nutrient abundance) suppresses autophagy
• Glucagon (which signals nutrient scarcity) upregulates autophagy
• Amino acid deprivation (especially leucine) suppresses mTOR and upregulates autophagy
• AMPK (the cellular energy sensor that activates when ATP is low) upregulates autophagy through ULK1
• Exercise upregulates autophagy in working tissues
• The pharmaceutical rapamycin upregulates autophagy by directly inhibiting mTOR

The timing question is uncertain: A common claim in the popular fasting literature is that “autophagy kicks in at 16 hours” or “peak autophagy occurs at 48-72 hours.” Most direct measurements of autophagy come from yeast, mice, and isolated human cells. Direct measurement of human autophagy in vivo is technically difficult. The widely-cited human timing thresholds are largely extrapolated from mouse studies, where 24-hour fasting produces substantial autophagy changes in multiple tissues, scaled to human metabolic rates. What we can say with reasonable confidence:

• Autophagy is upregulated by fasting in mammals
• The longer the fast (up to a point), the more autophagy activity is detectable
• Even time-restricted eating (16-20 hour daily fasts) produces measurable autophagy increases in animal models
• Pharmaceutical interventions (rapamycin) and exercise produce autophagy independent of fasting
• The exact human kinetics aren’t precisely characterised

Fasting upregulates autophagy, but the specific thresholds claimed in the popular literature (24 hours, 48 hours, 72 hours as discrete activation points) are practitioner-friendly approximations rather than well-mapped biological transitions.

Tissue-specific autophagy: Worth noting that autophagy isn’t uniform across the body. Macroautophagy occurs throughout most tissues. Mitophagy specifically targets damaged mitochondria. Chaperone-mediated autophagy (CMA) is a distinct process that selectively degrades specific proteins. Different tissues (liver, brain, muscle, and immune cells) have different autophagy dynamics and respond differently to fasting. The “more autophagy is better” framing is too simple; what we actually want is appropriate autophagy in the right tissues at the right times.

The Hormonal Cascade

Fasting produces predictable changes across the major metabolic hormones. The cascade is well-characterised in primary research, though the magnitudes have sometimes been overstated in the popular literature.

• Insulin: Drops substantially during fasting. Within 24 hours, fasting insulin can decrease by 50% or more in metabolically healthy individuals. By 3 days, it may approach baseline levels seen in lean, insulin-sensitive populations. The drop is part of why fasting improves insulin sensitivity over time: the receptors that respond to insulin get “reset” by periods of low circulating insulin, similar to how any receptor system becomes more responsive when not chronically stimulated.
• Glucagon: Rises substantially during fasting. The insulin-to-glucagon ratio shifts dramatically, which is the actual driver of much of the fasted-state metabolism. Glucagon stimulates glycogenolysis, gluconeogenesis, and ketogenesis. The shift in ratio matters more than either hormone’s absolute level.
• Growth hormone: Here’s where popular framing often overstates the case. The Ho et al. 1988 Journal of Clinical Investigation paper is the foundational human study showing that fasting amplifies growth hormone pulse amplitudes. After 5 days of fasting, GH pulse amplitude increased roughly 5-fold (500%), and across the full day, the integrated GH secretion was substantially elevated. The often-cited “1300-2000% increase at 20-24 hours” figure traces back to misreadings of this and similar research; the actual finding is that peak pulse amplitudes can reach those multiples of baseline, but this is the peak of brief secretory pulses, not a sustained hormonal state. The day-averaged increase is meaningful but more modest than the popular framing suggests. The effect is real (fasting genuinely elevates GH), but not as dramatic as the marketing implies.
• Cortisol: Rises modestly during fasting, particularly in the morning. This is part of the fasted-state metabolic response (cortisol promotes gluconeogenesis and lipolysis). Most healthy people tolerate this fine. People with already-elevated cortisol due to chronic stress, sleep deprivation, or HPA-axis dysfunction may not tolerate fasting well because they’re adding cortisol elevation to an already elevated baseline. The pattern is one of the more reliable markers for “fasting isn’t working for this person right now.”
• IGF-1 (Insulin-like Growth Factor 1): Drops substantially during prolonged fasting. Valter Longo’s research at USC has shown that 5 days of fasting can decrease IGF-1 by approximately 30-60%, with corresponding increases in IGFBP1 (a protein that further suppresses IGF-1 signalling). IGF-1 is a major pro-growth, pro-cancer signalling molecule; chronic elevation is associated with accelerated ageing and increased cancer risk. The fasting-induced drop is hypothesised to contribute to several of fasting’s protective effects.
• Catecholamines (epinephrine, norepinephrine): Rise modestly during fasting, contributing to the mild metabolic rate elevation seen in the first 48-72 hours.
• Leptin: Drops during fasting. Leptin is the “fullness” hormone produced by adipose tissue; its drop is part of what eventually triggers hunger signals strongly enough to break the fast. Importantly, leptin sensitivity tends to improve after fasting periods, which is part of the mechanism by which fasting can recalibrate appetite over time.
• Ghrelin: The “hunger” hormone has a more interesting pattern than people expect. Ghrelin rises in anticipation of expected meal times rather than rising linearly with hours of fasting. If you typically eat at noon, ghrelin will spike around noon whether or not you eat. Skip the meal, and ghrelin drops back down. This is part of why hunger during fasting is often wave-like rather than continuously building, and why people who maintain consistent eating windows experience less subjective hunger than people whose eating times are erratic.

Fasting and the Brain

The brain runs primarily on glucose, but it can run on ketones too, and the transition has neurological consequences. Mark Mattson’s group, formerly at the National Institute on Ageing and now at Johns Hopkins, has done much of the foundational work on the neuroscience of intermittent fasting.

• Ketones as brain fuel: Once the body shifts to producing ketone bodies, the brain can derive a substantial portion of its energy from beta-hydroxybutyrate rather than glucose. After 4-7 days of fasting (or sustained ketogenic eating), ketones can provide 50-70% of the brain’s energy needs. The brain still requires some glucose (certain regions and cell types can’t use ketones directly), but the demand is reduced and supplied by gluconeogenesis from amino acids and glycerol. Many people describe a characteristic “mental clarity” during fasting once the keto-adaptation has progressed, which appears to be related to the more stable energy supply ketones provide (no blood sugar swings) and possibly to direct neurological effects of BHB.
• BDNF (Brain-Derived Neurotrophic Factor): Fasting increases BDNF, a protein critical for neuronal growth, survival, and synaptic plasticity. Mattson’s work has shown BDNF increases of 50-400% in animal models with intermittent fasting, with smaller but measurable increases in human studies. BDNF is sometimes described as “fertiliser for the brain”; chronically low BDNF is associated with depression, cognitive decline, and reduced neuroplasticity. The exercise literature shows similar BDNF elevation, which is part of why exercise and fasting share many cognitive benefits.
• Neuroprotection: In animal models, intermittent fasting has been shown to protect against neurodegeneration in models of Alzheimer’s, Parkinson’s, Huntington’s, and stroke. The proposed mechanisms include increased autophagy (clearing accumulated misfolded proteins), reduced inflammation, improved mitochondrial function, increased BDNF, and the direct anti-inflammatory effects of ketone bodies (covered below). Human data is more limited but growing; epidemiological studies suggest that adults with intermittent fasting practices show reduced rates of cognitive decline.
• Alzheimer’s as “type 3 diabetes”: A working hypothesis in current Alzheimer’s research is that the disease has substantial metabolic components, with insulin resistance in the brain contributing to the energy crisis that precedes cognitive decline. People with type 2 diabetes have a 50-65% increased risk of Alzheimer’s. Fasting, which improves insulin sensitivity and provides alternative brain fuel via ketones, has been proposed as a potential intervention. The clinical evidence base is still developing; the mechanistic case is reasonable.
• The role of BHB beyond fuel: Beta-hydroxybutyrate isn’t just an alternative fuel. Eric Verdin and John Newman’s work has established BHB as a signalling molecule with effects on gene expression. BHB inhibits histone deacetylase 1 and 2 (HDAC1, HDAC2), changing the expression of multiple genes involved in oxidative stress resistance and metabolic regulation. BHB also inhibits the NLRP3 inflammasome, a major pro-inflammatory protein complex. Some of fasting’s anti-inflammatory effects may trace specifically to BHB’s signalling properties rather than to the absence of food per se.

Fasting and the Immune System

Valter Longo’s group at USC has done much of the foundational work on fasting and immune function. The key findings:

• Stem cell regeneration: Prolonged fasting (typically 3-5 days, or the equivalent fasting-mimicking diet protocol) triggers a process in which the body dismantles a portion of its older or damaged immune cells (specifically, white blood cell counts drop measurably) and then, upon refeeding, regenerates the immune cell population from hematopoietic stem cells. The effect was first demonstrated in mice and has been replicated in human subjects. Longo’s interpretation: the body sees prolonged fasting as a signal to “clean house” of older immune cells, and then refeeding triggers the rebuilding from stem cells, producing a fresher immune cell population.
• The PKA pathway: The mechanism Longo’s group identified involves the suppression of cAMP-dependent protein kinase A (PKA) during fasting. PKA suppression is what triggers the stem cell-mediated regeneration. The pathway is conserved from yeast through mammals.
• Reduced chronic inflammation: Multiple human trials have shown that intermittent fasting and prolonged fasting reduce inflammatory markers (C-reactive protein, IL-6, TNF-alpha) in people with chronic inflammation, including those with metabolic syndrome, autoimmune conditions, and cardiovascular disease.
• The Fasting Mimicking Diet (FMD): Longo developed a 5-day protocol that produces many of the metabolic effects of water-only fasting while allowing approximately 750-1,100 kcal per day across the 5 days. The protocol was designed to make prolonged fasting tolerable and safer for clinical applications. Multiple clinical trials have shown FMD produces reductions in CRP, IGF-1, blood glucose, and cardiovascular risk markers, with effects persisting for weeks to months after the protocol concludes. The commercial product ProLon (sold by L-Nutra, in which Longo has a financial stake) implements the protocol; the underlying science is sound, and the commercial dimension warrants noting.

Caveats: The immune effects of prolonged fasting are well-documented. The immune effects of shorter time-restricted eating (16:8) are more modest. Anyone with an active infection should not fast aggressively; the immune system needs energy and nutrients to mount effective responses to active pathogens. Anyone immunocompromised should approach fasting cautiously and ideally with medical guidance.

Fasting and the Gut

Digestion is energetically expensive (roughly 10-25% of caloric intake goes to digesting and processing the food itself), and the gastrointestinal tract benefits from periodic rest. Several mechanisms operate during fasting:

• The Migrating Motor Complex (MMC): Between meals, the small intestine produces a wave-like contractile pattern called the migrating motor complex, which sweeps undigested food particles and bacteria from the small intestine into the large intestine. The MMC operates in approximately 2-hour cycles when no food is being digested. Frequent eating suppresses the MMC because the digestive system is constantly busy processing incoming food. Regular fasting periods (overnight fasts of 12+ hours, or longer time-restricted windows) allow more complete MMC cycles and may contribute to reduced small intestinal bacterial overgrowth (SIBO) and improved gut motility.
• Microbiome cycling: The gut microbiome responds rapidly to changes in feeding patterns and food composition. Fasting periods appear to reshape the microbiome in ways that include increased microbial diversity and shifts toward bacterial species associated with metabolic health (Akkermansia muciniphila, certain Bacteroidetes species). The microbiome’s circadian rhythms, which track host eating patterns, may benefit from clearer feeding-fasting cycles than the constant low-grade snacking common in modern eating.
• Intestinal barrier integrity: Some animal and limited human research suggests fasting periods may help repair compromised intestinal barrier (“leaky gut”) by reducing the constant inflammatory burden of digestion. The mechanisms include autophagy of damaged intestinal epithelial cells and the anti-inflammatory effects of ketones and reduced insulin.

Caveats on gut effects: This area has more practitioner enthusiasm than clinical evidence behind it. Most claims about fasting “healing” specific gut conditions exceed what the research directly supports. The animal data is promising; the human data is more limited. Anyone with active inflammatory bowel disease, a history of gastric surgery, or ongoing significant GI symptoms should approach fasting cautiously and with medical guidance.

Fasting and Mitochondria

Mitochondria, the cellular energy-producing organelles, are central to many of fasting’s beneficial effects. Several adaptations operate:

• Mitochondrial biogenesis: Fasting upregulates the production of new mitochondria, primarily through the PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha) signalling pathway. PGC-1α is the master regulator of mitochondrial biogenesis, and it’s activated by AMPK (the cellular energy sensor that responds to low ATP), by exercise, and by caloric restriction. The result is more mitochondria per cell, particularly in metabolically active tissues like the liver, brain, and muscle.
• Mitophagy: A specific form of autophagy that selectively removes damaged mitochondria. Damaged mitochondria produce more reactive oxygen species (free radicals) and less ATP than healthy ones; their accumulation contributes to age-related decline. Fasting upregulates mitophagy, allowing damaged mitochondria to be cleared and replaced with new ones.
• Sirtuins and NAD+: Fasting raises levels of NAD+ (nicotinamide adenine dinucleotide), an enzyme cofactor critical to mitochondrial function. NAD+ activates sirtuins (especially SIRT1 and SIRT3), which in turn promote mitochondrial biogenesis, reduce oxidative stress, and support DNA repair. NAD+ levels decline substantially with age, and the loss of NAD+ contributes to mitochondrial dysfunction in older adults. Fasting is one of the few interventions reliably shown to elevate NAD+. The supplement industry around NAD+ precursors (NMN, NR) is built on this finding; the actual evidence for supplemented NAD+ precursors producing the same effects as endogenous elevation from fasting is more limited than the marketing suggests.
• Fatty acid oxidation efficiency: Ketone bodies (and the underlying fatty acid oxidation that produces them) generate more ATP per oxygen molecule than glucose does, and produce less oxidative stress per unit ATP. The compound effect: cells fueled by ketones tend to have healthier, more efficient mitochondria. This is part of the rationale for ketogenic diets in mitochondrial disease and neurodegeneration.

Fasting vs Caloric Restriction vs Starvation

These three states are often conflated in popular discussions but represent meaningfully different things.

1. Starvation is a state of severe, sustained energy deficit where the body lacks sufficient calories and nutrients to maintain function. Vital organs begin to break down. Muscle wasting becomes severe. Hormonal systems shut down. The body cannibalises itself in ways that produce permanent damage. Starvation is what happens in true food deprivation, anorexia nervosa, or extreme protein-calorie malnutrition (kwashiorkor). Starvation is the breakdown of the systems that make fasting possible.
2. Caloric restriction is a sustained reduction in calorie intake (typically 15-25% below maintenance needs) without micronutrient deprivation. The CALERIE trial at multiple US universities has been the major human research program studying long-term caloric restriction. Findings: caloric restriction in metabolically healthy adults produces measurable improvements in cardiovascular risk markers, reductions in inflammatory markers, slowing of certain biological ageing markers, and improvements in insulin sensitivity. The effects are modest, and many participants find sustained caloric restriction difficult to maintain. The metabolism slows somewhat to accommodate the reduced intake (some adaptive thermogenesis), which is the body’s normal response and isn’t the “starvation mode” panic some popular discussions invoke.
3. Fasting is the deliberate cycling between fed and fasted states, not the chronic reduction of total calories. A 16:8 time-restricted eating pattern with normal caloric intake during the 8-hour window doesn’t reduce total calories meaningfully for most people but does produce many of the metabolic effects of caloric restriction (improved insulin sensitivity, reduced inflammation, autophagy upregulation, mitochondrial biogenesis). The cycling appears to produce some effects that sustained caloric restriction doesn’t, including the substantial fed-state recovery periods that allow muscle protein synthesis and other anabolic processes.

Fasting protocols don’t require a chronic energy deficit to produce their benefits. The cycling itself is the active ingredient. This is why fasting can work for body composition (sustained or modest deficit during fasting periods, normal eating otherwise) without producing the metabolic adaptations associated with chronic dieting.

Potential Side Effects and What to Expect

Most people transitioning to fasting experience some combination of these short-term effects, particularly during the first 1-2 weeks of any new fasting practice:

• Headaches: Usually related to caffeine withdrawal, electrolyte shifts, or the body’s adjustment to lower blood sugar. Typically resolves within 2-3 days. Adequate hydration and sodium intake (covered in detail in the Cheatsheet) substantially reduces headache risk.
• Fatigue and lethargy: Common during the metabolic transition before keto-adaptation. The body is shifting fuel sources, and the transition isn’t always smooth. Usually peaks at days 2-4 of a longer fast or in the first 1-2 weeks of adopting a regular intermittent fasting practice, then resolves as keto-adaptation progresses.
• Lightheadedness, dizziness, low blood pressure: Related to fluid shifts and electrolyte changes. Fasting causes substantial water loss (glycogen binds three times its weight in water; as glycogen depletes, that water is excreted), which can drop blood pressure if not compensated for with adequate sodium intake.
• Irritability and mood changes: Common during the first week of a new fasting practice. The brain is adapting to a less consistent glucose supply. Usually resolves with continued practice and metabolic adaptation.
• Cold sensitivity: The body produces less metabolic heat during fasting, particularly in the deeper fasted state. Many people report feeling cold, especially in the extremities. Dress warmly; this isn’t a problem that requires intervention.
• Digestive changes: Constipation is common (less material moving through the GI tract). Diarrhoea can also occur, particularly when breaking longer fasts. Some people experience temporary “keto breath” related to acetone excretion through the lungs.
• Sleep disruption: Some people experience initially disrupted sleep when adopting fasting, particularly fasts that extend past dinner. Most people adapt within 1-2 weeks. Eating too close to bedtime can also disrupt sleep.
• Hunger waves: As noted above, hunger during fasting is typically wave-like rather than continuously building. The waves correspond to expected meal times. They pass within 20-30 minutes. The repeated experience of hunger arising, peaking, and resolving without eating is part of how the relationship with hunger gets rewired through fasting practice.

More serious side effects warranting attention:

• Persistent dizziness or fainting
• Heart palpitations or irregular heartbeat
• Severe muscle cramping
• Confusion or significantly impaired cognitive function
• Persistent severe fatigue that doesn’t resolve with adequate electrolytes

These typically signal electrolyte imbalance, dehydration, or overdoing fast duration relative to your current state of metabolic flexibility. Break the fast, replenish, and reassess. If symptoms persist after refeeding, see a physician.

Flare-ups of certain medical conditions: Fasting can precipitate gout attacks (uric acid rises during fasting), gallstone symptoms (gallbladder contraction patterns change), and other conditions where the body’s metabolic shifts have specific consequences. People with these conditions should approach fasting cautiously.

The toxin release framing: A common claim in the practitioner literature is that fasting “releases stored toxins” from adipose tissue as fat is mobilised, producing some of the discomfort of longer fasts. The mechanism has limited primary research support, but it isn’t unreasonable: lipophilic compounds (heavy metals, persistent organic pollutants, certain pharmaceutical residues) do accumulate in adipose tissue and would be released as fat is mobilised. Whether this produces meaningful symptomatic effects is less clear. The framing is plausible but speculative.

The IGR and GKI: Tools for Tracking State

Siim Land’s Metabolic Autophagy introduces two tracking metrics that have become popular in the practitioner literature: the Insulin-to-Glucagon Ratio (IGR) and the Glucose-Ketone Index (GKI). Both are useful approximations rather than established clinical measurements, and both have specific limitations worth understanding.

The Insulin-to-Glucagon Ratio (IGR): The ratio of these two opposing hormones determines whether you’re more anabolic (building, storing) or catabolic (mobilising, autophagic). The actual measurement requires blood tests for both hormones, which isn’t routinely done.

Approximate IGR values across different states (from Land’s synthesis of the research):

• 1:1 balanced ratio = 1.0
• Fasting, no food = ~0.8
• Western diet, mixed carbs = ~4.0
• Low-carb diet, mixed eating = ~1.3
• Fasting plus protein consumption = ~0.5 (glucagon-induced)
• Low-carb plus protein = ~1.3
• Carbohydrates plus protein = ~70 (highly anabolic)
• Amino acids plus carbohydrates = highly elevated

Combining protein and carbohydrates produces the most anabolic response (which is what you want post-resistance-training); fasting with no food maintains a low ratio that’s friendlier to autophagy and fat mobilisation; protein alone without carbs is more catabolic than commonly assumed because it raises glucagon strongly.

The Glucose-Ketone Index (GKI): Formula: (Blood glucose in mg/dL ÷ 18) ÷ Blood ketones in mmol/L. If your meter reads in mmol/L, skip the division by 18.

Interpretation ranges:

• GKI below 1.0: Deep ketosis, typically only reached during extended fasting
• GKI 1-3: Strong ketosis
• GKI 3-6: Moderate ketosis
• GKI 6-9: Mild ketosis
• GKI above 9: Not in ketosis

Thomas Seyfried’s research on cancer metabolism suggests a target GKI of 0.7-2.0 for therapeutic protocols. This recommendation comes from the metabolic theory of cancer (covered in Ketosis and the Rabbit Hole), where the goal is to deprive cancer cells of glucose while maintaining ketones for healthy tissues. The clinical evidence for this as an established cancer therapy is limited; the mechanistic case is strong enough to warrant ongoing research.

Limitations of both tools: The IGR requires lab testing that most people won’t routinely access. The GKI requires both blood glucose and blood ketone monitoring, which is expensive over time. Both are practitioner-grade approximations rather than clinically validated diagnostic tools. They’re useful for self-experimentation if you’re inclined toward that level of measurement; they’re not necessary for general fasting practice.

Summary

The biology of fasting is rich enough that no single page can cover it comprehensively. 

When you stop eating, your body undergoes a sequence of hormonal and metabolic shifts that move you from processing incoming nutrients to mobilising stored reserves. The shifts are well-characterised, they happen on a relatively predictable timeline, and they trigger adaptations (autophagy, mitochondrial biogenesis, immune cell turnover, BDNF elevation, anti-inflammatory signalling) that have meaningful long-term health implications.

The adaptations evolved because our ancestors regularly experienced periods without food. Our biology expects this cycling and seems to work better when it gets some of it. Modern food abundance has removed the need for cycling for most people in the developed world. Deliberately reintroducing fasting practices restores the cycling and accesses adaptations that constant feeding doesn’t.

The practical applications of this biology, who should and shouldn’t fast, and the specific protocols for different goals are covered in the Game Plan and the Cheatsheet. The deeper metabolic state of ketosis is covered in Ketosis. The deeper dives into specific research areas are in the Rabbit Hole.