The Fasting Glossary

The 16:8 method is a form of time-restricted eating in which you fast for sixteen hours each day and eat within an eight-hour window. It...

A pattern of eating normally five days a week and sharply restricting calories (around 500–600) on the other two.

Body fat, the tissue that stores energy and releases it as fuel during a fast.

Alternating between days of normal eating and days of fasting or very limited calories.

The building blocks of protein. In meaningful amounts (whole protein, or BCAA and EAA supplements) they raise insulin and activate mTOR, which interrupts a fast....

AMPK, short for AMP-activated protein kinase, is the cell's main energy sensor. It monitors the ratio of spent to available fuel inside the cell, and...

Programmed cell death: an orderly process the body uses to clear away damaged or unneeded cells.

Apple cider vinegar does not break a fast in the small amounts people typically use. A tablespoon contains only a few calories, not enough to...

Autophagy is the cell's built-in recycling process, in which damaged proteins, worn-out organelles, and other cellular debris are broken down so their building blocks can...

The number of calories your body burns at rest to maintain its basic functions.

BCAAs break a fast. Branched-chain amino acids, leucine, isoleucine, and valine, are protein building blocks, which means they carry calories, so taking them as a...

A plant compound extracted from several shrubs and long used in traditional medicine, now widely studied for its effects on glucose and lipid metabolism. It...

The most abundant ketone body, produced by the liver from fat during fasting and used as fuel.

A compound also known as trimethylglycine (TMG), found in beets and produced by the body from choline. It plays two well-established roles. As an osmolyte...

The amount of glucose circulating in your blood, your body's main quick-access fuel. Eating raises it and prompts insulin to store the excess, while fasting...

Bone broth breaks a fast. It contains protein, fat, and calories, which trigger digestion and a metabolic response, so by any strict definition it ends...

Caffeine does not break a fast. On its own, and in black coffee or plain tea, caffeine contains essentially no calories and does not provoke...

Reducing overall calorie intake without necessarily timing meals. It's distinct from fasting, which is defined by when you eat.

The breakdown of stored molecules (fat, glycogen, and at times protein) to release energy. Catabolic processes increase during fasting.

A small release of insulin triggered by the sight, smell, or taste of food before anything is digested. It is why an intensely sweet taste...

An essential trace mineral involved in how the body handles carbohydrates and fats, chiefly by supporting the action of insulin at the cellular level. The...

The body's roughly 24-hour internal clock, which influences metabolism and the timing of meals.

Clean fasting is the practice of consuming nothing but water and other strictly calorie-free drinks that do not trigger a metabolic response during the fasting...

Collagen breaks a fast. Collagen powder and collagen peptides are protein, which means they contain calories and provide amino acids that trigger a metabolic response,...

A hormone involved in the stress response and blood-sugar regulation that can rise modestly during a fast.

A compound stored in muscle that helps regenerate ATP, the body's fast-access energy currency, during short bursts of effort. It is one of the most...

A gentler approach in which you fast on two or three non-consecutive days a week rather than daily. It's sometimes suggested as an entry point...

Dirty fasting is the practice of allowing small amounts of calories during the fasting window, on the view that staying close to a fasted state...

A neurotransmitter central to motivation, focus, and reward. The body builds it from the amino acid L-tyrosine, which is one reason tyrosine is studied for...

Abstaining from both food and water. An advanced, higher-risk practice that isn't recommended for most people.

A protocol built around one or two full 24-hour fasts per week, with normal eating on the other days. Popularized by Brad Pilon.

The set period each day during which you eat in time-restricted eating, for example the 8-hour window in 16:8.

Electrolytes do not break a fast. Sodium, potassium, and magnesium are minerals, not a source of calories, so taking them as plain electrolytes does not...

Exogenous ketones break a fast. Whether taken as a ketone ester, ketone salt, or ready-to-drink ketone product, they contain calories, so by a strict definition...

Any fast lasting longer than about 24 hours. Longer fasts carry more risk and are best done with medical guidance.

The post-absorptive state in which the body has finished processing the last meal and runs on stored energy.

Fasting goals are the different outcomes people fast to achieve, and they matter because what counts as breaking a fast depends on which goal you...

A low-calorie, low-protein, plant-based eating plan (typically five days at a time) designed to trigger fasting-like effects while you still eat. Developed by researcher Valter...

The process by which the body becomes more efficient at burning fat and ketones for fuel.

The absorptive state after eating, when insulin is elevated and the body stores and uses incoming nutrients.

Fat molecules released from adipose tissue into the bloodstream to be used for energy during fasting.

The brain's main calming, or inhibitory, neurotransmitter, which quiets nervous-system activity. L-theanine, an amino acid from tea, is thought to support a calm but alert...

The “hunger hormone,” which rises before habitual meal times and signals appetite.

A hormone that raises blood glucose by prompting the liver to release stored sugar. It rises during fasting.

The process by which the body makes new glucose from non-carbohydrate sources, such as amino acids, during fasting.

The body's stored form of carbohydrate, held in the liver and muscles and depleted in the early hours of a fast.

A hormone that supports fat metabolism and helps preserve lean tissue. Its secretion rises during fasting, though the rise is transient and its muscle-building effect...

The temporary surge of hunger, often late morning, that typically passes within a short time as the body shifts to using stored fuel.

Blood sugar that drops below its normal range, which can cause shakiness, sweating, irritability, or trouble concentrating. Mild cases are common in the first days...

A growth-promoting hormone stimulated by protein and calories. It falls during fasting and protein restriction, and that reduction is a focus of longevity research.

A state in which cells respond poorly to insulin, so the body makes more of it and blood sugar tends to run high. It's the...

Insulin sensitivity describes how readily the body's cells respond to insulin, the hormone that signals cells to take up glucose from the blood. When cells...

The hormone that stores nutrients and lowers blood sugar after eating. It falls during fasting, which enables fat burning.

Intermittent fasting is an eating pattern that cycles between periods of eating and periods of voluntary abstention from food. Rather than specifying which foods to...

A cluster of transient symptoms (headache, fatigue, brain fog) common when starting a fast or low-carb diet, largely driven by electrolyte loss.

A dangerous state of extremely high ketones and acidic blood, distinct from nutritional ketosis and mainly a risk in type 1 diabetes. It is a...

The metabolic process by which the liver produces ketones from fat, the machinery behind ketosis.

Fuel molecules (BHB, acetoacetate, and acetone) the liver makes from fat when carbohydrate is scarce.

Ketosis is a metabolic state in which the body, running low on glucose, shifts to breaking down fat for fuel and produces molecules called ketones...

An amino acid found almost exclusively in tea leaves and structurally related to glutamate. It crosses the blood-brain barrier, and research associates it with increased...

An amino acid the body uses as the raw material to build the catecholamine neurotransmitters: dopamine, norepinephrine, and epinephrine. Supplementing it enlarges that precursor pool,...

A hormone released by fat cells that signals fullness and helps regulate energy balance.

Leucine breaks a fast. Not only does it carry calories, but it also warrants more caution than other nutrients and amino acids in a fasting...

The breakdown of stored fat into fatty acids that the body can use for energy.

An electrolyte and enzyme cofactor involved in hundreds of reactions, including energy production, muscle relaxation, and sleep. Fasting raises how much magnesium the kidneys excrete,...

MCT oil breaks a fast. It is pure fat at around 115 calories a tablespoon, so like any caloric food it ends a strict fast...

Metabolic flexibility is the body's ability to switch efficiently between its two main fuel sources, glucose and fat, depending on what is available. A metabolically...

The metabolic switch is the point during a fast when the body shifts its main fuel source from glucose to fat. After a meal, cells...

The creation of new mitochondria, the structures that produce energy inside cells. Fasting is studied for its potential to stimulate it.

A targeted form of autophagy that clears out old or damaged mitochondria, the structures that produce cellular energy. By recycling worn-out mitochondria and making room...

mTOR, short for mechanistic target of rapamycin, is a nutrient-sensing enzyme that acts as one of the cell's main growth switches. When nutrients are abundant,...

A coenzyme found in every cell that is essential for energy production and for switching on sirtuins, a family of proteins involved in cellular repair...

The increased excretion of sodium and water by the kidneys in the first days of a fast, driven by falling insulin. This rapid mineral loss...

Non-caloric sweeteners do not break a fast in the strict sense of adding calories, but they are not all equally neutral, and the outcome depends...

A form of time-restricted eating in which all of the day's calories are eaten in a single meal.

An imbalance between damaging free radicals and the body's defenses, which fasting is studied to help modulate.

An electrolyte that works alongside sodium to regulate heartbeat, fluid balance, and muscle contraction. It sits mostly inside your cells, and longer fasts can deplete...

The body's tendency to preserve muscle protein by relying on fat and ketones for fuel during a fast.

A dangerous shift in fluids and electrolytes that can occur when reintroducing food after prolonged fasting. It's best managed with medical supervision.

The process of reintroducing food after a fast, ideally done gradually.

An adaptogenic herb whose root has long been used in cold, high-altitude regions to counter fatigue. Its activity is attributed mainly to two compound groups,...

The dried red stigmas of the Crocus sativus flower, rich in the compounds crocin, safranal, and picrocrocin. Saffron extracts are studied in two distinct directions:...

A family of NAD+-dependent proteins involved in cellular repair and metabolism. They become more active when energy is low, as during fasting, and are widely...

The main electrolyte in the fluid outside your cells, responsible for blood volume, blood pressure, and nerve signaling. When insulin falls during a fast, the...

A water-soluble B vitamin (B1) that the body converts into the coenzyme thiamine pyrophosphate, a required cofactor for the enzymes that turn carbohydrates into usable...

Time-restricted eating is a form of intermittent fasting in which all of the day's food is eaten within a set window, with the remaining hours...

Fat stored deep in the abdomen around the organs, distinct from the fat just under the skin. It is more metabolically active and more closely...

A form of time-restricted eating with a 20-hour undereating period and a 4-hour evening eating window. Created by Ori Hofmekler, it's more restrictive than most...

A fast during which only water is consumed. Extended water fasts should be medically supervised.