44 terms every faster should know

16:8 is the most popular intermittent fasting protocol. You fast for 16 hours and eat within an 8-hour window — typically noon to 8 p.m. Most of the fast happens overnight. 16:8 is sustainable for years, fits most social schedules, and reliably produces fat loss, improved insulin sensitivity, and lower fasting glucose within weeks. It is the protocol most clinicians recommend as a starting point for healthy adults.

18:6 tightens 16:8 to an 18-hour fast with a 6-hour eating window — typically 1 p.m. to 7 p.m. The extra two fasting hours deepen fat oxidation and push the body toward mild ketosis. Many practitioners find 18:6 easier than 16:8 once ghrelin adapts. Two substantial meals, sometimes with a snack, fit comfortably into six hours. 18:6 suits people who have already adapted to 16:8 and want accelerated fat loss or sharper focus.

20:4 — the Warrior Diet pattern — compresses eating into a 4-hour window with a 20-hour fast. It produces deep ketosis, significant fat loss, and pronounced autophagy, but demands careful meal construction to hit protein and micronutrient targets in one or two meals. 20:4 is not for beginners. Hydration, electrolytes, and protein density in the eating window are essential. Many practitioners use it cyclically — a few days per week — rather than daily.

5:2 is a modified fasting protocol popularized by journalist Michael Mosley. You eat normally for five days and restrict intake to about 500 calories on two non-consecutive days each week. The low-calorie days are not true fasts; a small meal of lean protein and vegetables is typical. Studies show 5:2 produces comparable weight loss and insulin improvements to daily time-restricted eating. It appeals to people who prefer not to restrict their eating window every day.

An absolute fast (also called a dry fast) abstains from both food and water. Scripture records several absolute fasts: Esther's three-day fast (Esther 4:16, ESV), Paul's three-day fast after encountering Christ (Acts 9:9, ESV), and Moses' and Elijah's miraculous 40-day fasts. Absolute fasting is spiritually intense but physiologically risky. Dehydration can develop within one to three days. Most pastors and clinicians counsel against extended absolute fasting, recommending water fasts or partial fasts for typical believers.

Alternate-day fasting (ADF) alternates 'feed days' (eating ad libitum) with 'fast days' (no calories, or about 25% of maintenance calories). Studies show ADF produces weight loss comparable to daily calorie restriction, with potentially better preservation of lean mass. ADF is intense and can be socially disruptive; many find it harder to sustain than daily time-restricted eating. Modified ADF, allowing roughly 500 calories on fast days, is more tolerable and equally effective.

AMP-activated protein kinase (AMPK) is the cell's master energy sensor. When cellular energy is low — as during fasting or exercise — AMPK rises and triggers processes that produce energy (fat oxidation, glucose uptake) while suppressing energy-consuming processes. AMPK activation improves insulin sensitivity, reduces inflammation, and supports longevity. Fasting and metformin both activate AMPK. AMPK and mTOR work in opposition: when AMPK is high, mTOR is low, and the cell shifts from growth to repair mode.

Apple cider vinegar (ACV) is fermented apple juice containing acetic acid. One or two teaspoons diluted in water before meals may modestly improve insulin sensitivity and lower post-meal glucose spikes. During a fast, a small amount of ACV in water has negligible calories and is generally considered fast-safe. ACV may help with digestion and appetite regulation. Always dilute ACV — undiluted vinegar can damage tooth enamel and the esophagus. Use a straw to protect teeth. Avoid ACV gummies, which contain sugar.

From the Greek for 'self-eating,' autophagy is the cellular process of degrading and recycling damaged proteins and organelles. Japanese cell biologist Yoshinori Ohsumi won the 2016 Nobel Prize in Physiology or Medicine for identifying the genes and mechanisms that govern it. Fasting, calorie restriction, and exercise all trigger autophagy. It is implicated in protection against neurodegeneration, cancer, infection, and aging. Animal studies suggest meaningful autophagy begins after 12 to 16 hours of fasting and peaks at 24 to 48 hours.

Brain-derived neurotrophic factor (BDNF) is a protein that supports the growth, survival, and plasticity of neurons. Low BDNF is associated with depression and neurodegenerative disease; higher levels correlate with learning, memory, and resilience. Fasting and exercise both raise BDNF. Mark Mattson's research, summarized in the 2019 New England Journal of Medicine review, suggests intermittent fasting increases BDNF and may protect the brain against Alzheimer's, Parkinson's, and stroke. Ketones also signal BDNF production, linking ketosis to cognitive benefits.

Biblical fasting is the voluntary abstinence from food (and sometimes water) for spiritual purposes — prayer, repentance, mourning, seeking God's direction, or intercession. Scripture records Moses, David, Elijah, Daniel, Esther, Ezra, Nineveh, Jesus, and the early church fasting. Jesus assumed his disciples would fast (Matthew 6:16, ESV, 'when you fast'). Biblical fasting is always paired with prayer and humility, never performed for show. It is distinct from dietary fasting, though the practices can overlap.

Black coffee — about 2 calories per cup — does not break a fast. It contains caffeine (90 to 120 mg per cup) that suppresses appetite, boosts metabolism modestly, and may stimulate autophagy. Coffee also increases lipolysis, helping the body release fat. Adding cream, milk, sugar, or syrups breaks the fast by triggering insulin. Bulletproof coffee (with butter or MCT oil) contains fat calories and also breaks a fast, though it does not raise insulin much. For a clean fast, drink coffee black or with a pinch of cinnamon.

Bone broth is a slow-simmered stock made from animal bones and connective tissue, rich in collagen, gelatin, amino acids, and minerals. It provides electrolytes and is often used to break extended fasts or to sip during modified fasts. Bone broth contains calories and protein, so it technically breaks a fast — though many practitioners use it during longer fasts (48+ hours) for electrolyte support and to ease digestive load. Choose high-quality, sugar-free bone broth. For a clean fast, stick to water, black coffee, tea, and unsweetened yerba mate.

To 'break your fast' is to consume the first calories after a fasting window — literally the origin of the word breakfast. How you break a fast matters: a small, low-carb, protein-rich meal (eggs, nuts, fish, vegetables) is gentler on digestion and insulin than a large carbohydrate-heavy meal. After extended fasts (24+ hours), break slowly with bone broth or fermented food, wait, then eat a normal meal. Aggressive refeeding after long fasts risks refeeding syndrome.

The circadian rhythm is the body's internal 24-hour clock, regulating sleep, hormone release, body temperature, and metabolism. Insulin sensitivity peaks in the morning and declines through the evening, so eating earlier in the day tends to produce better metabolic outcomes. Time-restricted eating aligned with daylight — for example, eating between 8 a.m. and 4 p.m. — is sometimes called 'early time-restricted feeding' and may improve glucose control and blood pressure beyond the effects of fasting itself.

A corporate fast is a fast undertaken by a group — a family, church, city, or nation — for shared repentance, intercession, or seeking God. Scripture records corporate fasts by Israel (Judges 20:26, ESV), Nineveh (Jonah 3:5, ESV), and the Jews of Susa (Esther 4:16, ESV). The early church fasted corporately before sending missionaries (Acts 13:2-3, ESV). Corporate fasting unites believers, intensifies collective prayer, and has historically accompanied revival, repentance, and major decisions.

The Daniel Fast is a 21-day partial fast based on Daniel 1 and Daniel 10. Practitioners eat only whole, plant-based foods — fruits, vegetables, whole grains, legumes, nuts, and seeds — and avoid meat, dairy, refined sugar, caffeine, alcohol, and processed foods. Daniel 10:3 (ESV) records, 'I ate no delicacies, no meat or wine entered my mouth.' The Daniel Fast is a fast of consecration, focused on prayer and dependence on God, not on calorie restriction or metabolic outcomes.

The dawn phenomenon is a normal rise in blood glucose that occurs in the early morning hours (typically 3 to 8 a.m.), driven by a surge in cortisol, glucagon, and growth hormone. The liver releases glucose to prepare the body for waking. In fasting practitioners, a higher morning glucose reading does not necessarily indicate metabolic dysfunction — it may reflect the dawn phenomenon. Insulin, glucose, and ketones together give a more accurate metabolic picture than glucose alone.

The Day of Atonement (Yom Kippur) is the holiest day in the Jewish calendar, instituted in Leviticus 16:29-31 (ESV): 'You shall afflict yourselves.' Jewish tradition understands 'afflict' as a complete 25-hour fast from food and water. The Day of Atonement is the only mandatory fast in the Mosaic Law. It was a day of national repentance, sacrifice, and reconciliation with God. Christians see its fulfillment in Jesus, the ultimate atonement (Hebrews 9), but the principle of fasting as repentance continues.

A dry fast (or absolute fast) prohibits both food and water. It is practiced in some religious traditions and by a small subset of biohackers, who claim it accelerates autophagy and fat loss. Dry fasting carries significantly higher risk than water fasting — dehydration, kidney strain, and electrolyte derangement can develop rapidly. Most clinicians advise against dry fasting beyond 24 hours. Scripture records Moses and Elijah undertaking 40-day absolute fasts, but these were miraculous events, not prescriptive models.

The eating window is the period of the day during which you consume all calories. In 16:8, the eating window is 8 hours (e.g., noon to 8 p.m.); in 18:6, it is 6 hours; in OMAD, it is roughly 1 hour. A well-constructed eating window includes enough protein, fiber, healthy fats, and micronutrients to support the day. Most practitioners find that two substantial meals and an optional snack fit comfortably in an 8-hour window.

Electrolytes — sodium, potassium, magnesium, and calcium — are minerals that carry electrical charges and regulate hydration, nerve signaling, and muscle contraction. During fasting, insulin drops and the kidneys excrete sodium (along with potassium and magnesium), causing headaches, fatigue, cramps, and 'keto flu.' A simple fasting electrolyte protocol: 1/2 teaspoon of sea salt in water daily, plus a sugar-free electrolyte mix with potassium and magnesium. Avoid sugar-laden sports drinks. Electrolytes are essential for fasts longer than 18 hours.

The Esther Fast is a three-day absolute fast modeled on Queen Esther's call to the Jews of Susa: 'Go, gather all the Jews... and fast for me; do not eat or drink for three days, night or day' (Esther 4:16, ESV). Esther and her people fasted before she risked her life by approaching King Xerxes unbidden. The Esther Fast is observed today as a season of intensified prayer and intercession, particularly before major decisions, deliverance, or crisis. It is one of Scripture's clearest examples of corporate, absolute fasting.

The fasting window is the period of the day during which you abstain from caloric intake. In 16:8, the fasting window is 16 hours — most of it overnight. During the fasting window, you may drink water, black coffee, plain tea, and unsweetened yerba mate. The fasting window is when insulin drops, fat oxidation rises, and (after 12 to 16 hours) autophagy begins. Most of the metabolic benefits of intermittent fasting accrue during the fasting window.

Glucagon is a hormone produced by the alpha cells of the pancreas. It is insulin's counter-regulatory partner: when blood glucose drops (as during fasting), glucagon rises and signals the liver to release stored glycogen and to produce new glucose. Glucagon also stimulates lipolysis and ketogenesis. The insulin-to-glucagon ratio, low during fasting, is a primary driver of the body's shift from storage mode to mobilization mode. Eating raises insulin and lowers glucagon; fasting reverses the ratio.

Glycogen is the storage form of glucose, found primarily in the liver (about 100 grams) and skeletal muscle (about 400 grams). During fasting, the liver breaks down glycogen to maintain blood glucose — typically exhausting reserves within 12 to 24 hours. Once glycogen is depleted, the body shifts to gluconeogenesis (making glucose from amino acids and glycerol) and ketogenesis (making ketones from fat). Each gram of glycogen stores roughly three grams of water, which is why fasters shed water weight early on.

Human growth hormone (HGH), produced by the pituitary gland, supports muscle preservation, fat metabolism, and tissue repair. Fasting can raise HGH levels dramatically — some studies report a two- to fivefold increase within 24 hours. This surge helps explain why short-term fasting preserves lean muscle even in a caloric deficit. HGH works in part by mobilizing fatty acids for fuel and by counteracting the muscle-breakdown effects of cortisol. Sleep, exercise, and protein intake also influence HGH.

Insulin sensitivity describes how responsive your cells are to insulin's signal to take up glucose. High sensitivity means small amounts of insulin effectively clear glucose from the blood. Low sensitivity (insulin resistance) is a precursor to type 2 diabetes. Fasting improves insulin sensitivity by giving the pancreas a rest and allowing insulin levels to fall, which restores cellular responsiveness. Time-restricted eating has been shown to reduce fasting insulin by 20 to 30 percent in some studies.

The 'keto flu' is a cluster of symptoms — fatigue, headache, irritability, brain fog, muscle cramps — that occurs in the first one to two weeks of fasting or ketogenic diet adaptation. It is caused primarily by sodium, potassium, and magnesium depletion as the body sheds glycogen-bound water. The keto flu is not the flu and is not dangerous. Adding electrolytes (a pinch of sea salt in water, a sugar-free electrolyte mix), staying hydrated, and easing into the protocol resolves symptoms within days.

Ketones (or ketone bodies) are water-soluble molecules produced by the liver from fatty acids during fasting, carbohydrate restriction, or prolonged exercise. The three ketones are beta-hydroxybutyrate, acetoacetate, and acetone. Beta-hydroxybutyrate is the most abundant and is the preferred brain fuel during ketosis. Ketones provide energy to the brain, heart, and muscles when glucose is scarce, and they signal adaptive responses including reduced oxidative stress and increased BDNF production.

Ketosis is a metabolic state in which the body burns fat as its primary fuel and the liver produces ketone bodies (beta-hydroxybutyrate, acetoacetate, acetone) from fatty acids. Nutritional ketosis typically begins after 12 to 24 hours of fasting or after several days of a very-low-carbohydrate diet. Blood ketones between 0.5 and 3.0 mmol/L indicate nutritional ketosis. Ketones are an efficient fuel for the brain and heart and are associated with appetite suppression and mental clarity.

Lent is the 40-day liturgical season of fasting, prayer, and repentance observed by many Christian traditions between Ash Wednesday and Easter (excluding Sundays). It commemorates Jesus' 40-day fast in the wilderness (Matthew 4:1-2, ESV). Practitioners often give up a specific food, habit, or luxury as a partial fast. Lent is one of the oldest Christian observances, with roots in the second-century pre-Easter fast. It is paired with almsgiving and prayer as the three classical Lenten disciplines.

Lipolysis is the breakdown of stored triglycerides in adipose (fat) tissue into free fatty acids and glycerol, which the body uses for energy when glucose is scarce. Fasting, exercise, cold exposure, and low insulin levels all stimulate lipolysis. Catecholamines (adrenaline, noradrenaline) and glucagon are key hormonal drivers. The free fatty acids released enter the bloodstream and are taken up by muscles and the liver, where they are either oxidized for energy or converted into ketones.

Medium-chain triglyceride (MCT) oil is a concentrated fat extract (usually from coconut oil) that the liver rapidly converts into ketones. MCT oil is popular in ketogenic and low-carb circles for a quick energy boost. Strictly speaking, MCT oil contains calories (about 120 per tablespoon) and breaks a fast metabolically, though it does not raise insulin significantly. Some practitioners use small amounts of MCT oil during fasting for cognitive focus, accepting a partial fast. For a fully clean fast, avoid MCT oil.

Metabolic flexibility is the body's ability to efficiently switch between burning glucose (fed state) and burning fat and ketones (fasted state). A metabolically flexible person can move between fuel sources without energy crashes or cravings. Modern constant eating erodes this flexibility, leaving many people 'sugar-burners' who struggle to access body fat. Fasting, low-carbohydrate eating, and exercise train metabolic flexibility. It is one of the most-cited benefits of intermittent fasting and underlies the sustained energy many practitioners report after adaptation.

Mammalian target of rapamycin (mTOR) is a cellular nutrient sensor that promotes growth and protein synthesis when amino acids and insulin are abundant. When you fast, mTOR activity drops, which suppresses growth pathways and allows the cell to focus on repair and recycling — including autophagy. mTOR is the inverse of AMPK: eating (especially protein) turns mTOR on; fasting turns it off. Periodic mTOR suppression is associated with longevity in animal models.

OMAD — One Meal A Day — is a 23:1 protocol: a single large meal within roughly one hour, fasting the other 23. Advocates report deep ketosis, mental clarity, and significant fat loss. The single meal must contain adequate protein, fiber, healthy fats, and micronutrients. Risks include inadequate protein, micronutrient shortfalls, and binge-like eating patterns. OMAD is not recommended for beginners, pregnant or breastfeeding women, or those with eating-disorder histories.

Oxidative stress is the imbalance between free radicals (reactive oxygen species) and the body's antioxidant defenses, leading to cellular damage. It contributes to aging, inflammation, and chronic disease. Fasting reduces oxidative stress by improving mitochondrial efficiency, inducing antioxidant enzymes, and triggering autophagy, which clears damaged mitochondria. The 2019 New England Journal of Medicine review by Mattson highlights reduced oxidative stress as one of the key mechanisms through which intermittent fasting protects against chronic disease.

A partial fast restricts some foods but not all — typically avoiding delicacies, meat, or certain categories while still eating simply. The Daniel Fast is the most well-known biblical partial fast: Daniel 10:3 (ESV) records, 'I ate no delicacies, no meat or wine entered my mouth.' Partial fasts are sustainable for longer durations (e.g., 21 days) and are well-suited to those for whom absolute fasting is unsafe or impractical, including pregnant women, nursing mothers, and people with medical conditions.

Refeeding is the deliberate, gradual reintroduction of food after a fast. For fasts under 24 hours, refeeding is simple — eat a normal meal. For fasts of 24 to 72 hours, start with small, low-carbohydrate meals and add electrolytes. For fasts longer than 5 days, refeeding must be medically supervised to prevent refeeding syndrome, a dangerous shift of phosphate, potassium, and magnesium into cells that can cause cardiac and neurological complications.

Refeeding syndrome is a potentially fatal shift of fluids and electrolytes that can occur when food is reintroduced too quickly after a prolonged fast (typically five days or more) or severe starvation. As insulin rises, cells rapidly take up phosphate, potassium, and magnesium, depleting serum levels and causing cardiac and neurological complications. Risk is highest in malnourished patients. To prevent it, refeed slowly with small, low-carbohydrate meals, supplement electrolytes, and seek medical supervision after extended fasts.

Time-restricted eating (TRE) is the scientific term for consuming all daily calories within a defined window — typically 4 to 12 hours — and fasting the rest of the day. 16:8 and 18:6 are common TRE patterns. TRE aligns food intake with circadian rhythms, which improves insulin sensitivity and metabolic health beyond the effects of calorie restriction alone. Early TRE (eating in the morning and afternoon) appears particularly effective for glucose control.

A water fast is a fast in which only water is consumed — no food, no caloric beverages, no supplements with calories. Short water fasts (24 to 72 hours) are used for metabolic reset, autophagy, and spiritual focus. Extended water fasts (beyond 72 hours) carry risks including electrolyte imbalance, muscle loss, and refeeding syndrome, and should be supervised by a clinician. The Bible records Moses and Elijah undertaking 40-day water (or absolute) fasts under extraordinary circumstances.

Yerba mate is a traditional South American herbal tea made from the leaves of Ilex paraguariensis. It provides a smooth, sustained caffeine lift (roughly 85 mg per cup) without coffee's jitter, plus antioxidants including chlorogenic acid and theobromine. Unsweetened yerba mate has negligible calories and does not break a fast. It is a popular fasting beverage for those who find coffee too harsh on an empty stomach. Drink it plain, hot or cold; avoid commercially sweetened mate drinks, which contain calories.