Fasting attracts more jargon than most diet practices, partly because it touches on metabolism, endocrinology, and ancient religious traditions all at once. This glossary defines the terms you’ll encounter, with practical context for what each one actually means for someone fasting.
A
ADF (Alternate-Day Fasting)
A protocol where fasting days alternate with normal eating days. Strict ADF means zero calories on fast days; modified ADF allows around 500 calories. See our ADF guide.
Adipose tissue
Body fat. Specifically, the tissue where energy is stored as triglycerides. Fasting promotes lipolysis — release of fatty acids from adipose tissue — which is the basis for fat loss and ketone production.
Adrenaline (epinephrine)
A stress hormone that rises during fasting (modestly) to mobilise glucose and fatty acids. The mild adrenaline elevation is part of the alertness many fasters report.
Autophagy
Literally “self-eating.” The cellular process by which damaged proteins, organelles, and other components are broken down and recycled. Upregulated during fasting, particularly after 18–24+ hours. The 2016 Nobel Prize in Physiology was awarded for autophagy research. See our autophagy guide.
B
BHB (Beta-hydroxybutyrate)
The most abundant ketone body produced during fasting and ketosis. Measured in blood ketone meters. Levels above 0.5 mmol/L indicate nutritional ketosis; values during fasting commonly reach 1–4 mmol/L.
Blood glucose
The concentration of glucose in the blood. Fasting blood glucose typically falls during the first 24 hours, then stabilises as the liver produces glucose via gluconeogenesis. Healthy fasting glucose is roughly 70–99 mg/dL (3.9–5.5 mmol/L).
BMR (Basal Metabolic Rate)
The energy your body uses at rest to maintain basic functions. BMR adapts modestly downward during prolonged caloric restriction, but short-term fasting (under ~72 hours) does not lower BMR meaningfully — and may slightly increase it via noradrenaline elevation.
Bone broth
Broth made from animal bones, often slow-cooked. Contains amino acids and minerals. Technically breaks a strict fast (it has calories and triggers some insulin response) but is widely used for general fasting comfort and electrolytes.
C
Calorie deficit
Eating fewer calories than you expend. The mechanism behind weight loss in any dietary approach, including fasting. Fasting tends to produce a calorie deficit naturally by limiting eating windows, but it isn’t magic — overeating in the eating window erases the deficit.
Catecholamines
A group of stress hormones including adrenaline (epinephrine) and noradrenaline (norepinephrine). Rise modestly during fasting; partially responsible for the alertness, slight metabolic increase, and lipolytic effect.
Circadian rhythm
The body’s ~24-hour internal clock. Affects insulin sensitivity, hormone secretion, body temperature, and digestion. Eating in alignment with circadian rhythm — earlier in the day rather than late at night — appears to improve metabolic outcomes. See our fasting and sleep guide.
Cortisol
A glucocorticoid hormone released by the adrenal glands. Rises during fasting to maintain blood glucose via gluconeogenesis. Modest rises are normal and adaptive; chronically elevated cortisol from over-aggressive fasting can cause sleep disruption and other issues.
D
Dirty fasting
Informal term for fasting with small caloric “cheats” — coffee with cream, bone broth, MCT oil, etc. May still produce most of the metabolic benefits of fasting (lower insulin, fat oxidation) but is not strict for purposes of autophagy or extended water fasting. Contrast with “clean” or “strict” fasting.
E
Eat Stop Eat
A specific protocol popularised by Brad Pilon: one or two 24-hour fasts per week, normal eating otherwise. See our Eat Stop Eat guide.
eTRE (early Time-Restricted Eating)
Time-restricted eating with the eating window placed earlier in the day, typically morning to mid-afternoon (e.g., 8 AM – 4 PM). Generally outperforms late eating windows on metabolic markers in research.
Electrolytes
Minerals that carry an electrical charge in body fluids — sodium, potassium, magnesium, calcium, chloride, phosphate. Critical to maintain during longer fasts. See our electrolyte guide.
Extended fast
Generally any fast longer than 36–48 hours, including 3-, 5-, 7-day water fasts. Carry meaningful refeeding-syndrome risk and benefit from medical supervision past 5 days.
F
Fasted state
The metabolic state several hours after eating, when insulin is low and the body is mobilising stored energy. Generally begins around 4–6 hours after the last meal as glycogen depletes. Contrast with fed state.
Fat adaptation
The metabolic shift to efficient fat burning over weeks of repeated fasting or low-carbohydrate eating. The transition reduces “hangry” symptoms and improves the comfort of longer fasts.
FMD (Fasting-Mimicking Diet)
A 5-day low-calorie, low-protein, plant-based eating protocol designed to produce some of the metabolic effects of water fasting while still consuming food. Developed by Valter Longo’s lab. Most often refers to the commercial “ProLon” product, but the principle can be replicated independently.
G
Ghrelin
The “hunger hormone.” Released by the stomach in anticipation of meals. Rises and falls in waves rather than continuously, which is why hunger comes and goes during a fast rather than building monotonically. Ghrelin patterns adapt within 1–2 weeks of a new meal schedule.
Glucagon
A hormone released by the pancreas when blood glucose drops. Triggers gluconeogenesis and lipolysis. Rises during fasting; the inverse of insulin.
Gluconeogenesis
The liver’s production of new glucose from non-carbohydrate sources — primarily amino acids (from protein) and glycerol (from fat breakdown). Maintains blood glucose during fasting once glycogen is depleted.
Glycogen
The storage form of glucose, held primarily in the liver (~100 g) and muscles (~400 g). Liver glycogen is depleted within 24 hours of fasting; muscle glycogen lasts longer because it can’t be released back into the bloodstream.
Growth hormone (GH, somatotropin)
A pituitary hormone that promotes tissue maintenance and lipolysis. Rises significantly during fasting — sometimes 5-fold or more. Part of the muscle-sparing mechanism that prevents fasting from causing rapid muscle loss in healthy adults.
H
HbA1c
Glycated haemoglobin. A blood marker reflecting average glucose levels over roughly 2–3 months. Used to diagnose and monitor diabetes. Often improves with sustained fasting practice in people with elevated baseline.
HIIT (High-Intensity Interval Training)
Short bursts of all-out effort separated by recovery. Often performed in fasted state. Tolerated well by adapted fasters; demanding for beginners. See our exercise guide.
Hypoglycaemia
Abnormally low blood glucose. Symptoms include shaking, sweating, confusion, weakness. Rare in healthy adults during fasting (the liver maintains glucose). A genuine risk for people on insulin or sulfonylureas. See our medications guide.
I
Insulin
The hormone that lowers blood glucose by signalling cells to take it up. Rises after meals, particularly carbohydrate-rich ones. Falls during fasting. The drop in insulin is the principal switch that enables fat-burning and autophagy.
Insulin resistance
A condition in which cells respond poorly to insulin, requiring higher levels to maintain blood glucose. Underlies type 2 diabetes and metabolic syndrome. Often improves with sustained fasting practice combined with weight loss.
Insulin sensitivity
The opposite of insulin resistance — how well cells respond to insulin. Fasting tends to improve insulin sensitivity over weeks and months.
Intermittent fasting (IF)
Any pattern of eating that cycles between defined eating and fasting periods. Includes daily time-restricted eating (16:8, 18:6, OMAD), weekly approaches (5:2, Eat Stop Eat), and longer extended fasts.
K
Ketones / Ketone bodies
Water-soluble molecules produced by the liver from fatty acids when carbohydrate is scarce. The three ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate, and acetone. Used by the brain and other tissues as fuel.
Ketosis
The metabolic state in which ketones become a significant fuel source — generally blood BHB above 0.5 mmol/L. Reached during fasting (typically 24–48 hours in) or via low-carbohydrate diet. See our keto and fasting guide.
Keto flu
The cluster of symptoms (headache, fatigue, irritability, brain fog) that hits some people in the first days of low-carb eating or extended fasting. Largely caused by sodium depletion and resolved by adequate electrolytes. See our electrolyte guide.
L
Leptin
A hormone produced by fat cells that signals satiety and energy sufficiency. Falls during caloric restriction and fasting. Persistently low leptin is part of why aggressive long-term fasting can dysregulate hunger and metabolism.
Lipolysis
The breakdown of stored triglycerides in fat cells, releasing fatty acids and glycerol into circulation. Suppressed by insulin; activated by low insulin, glucagon, and catecholamines. The first step in fat burning.
M
MCT oil
Medium-chain triglycerides. Rapidly converted to ketones in the liver. Sometimes added to coffee for a quick ketone boost. Contains calories and breaks a strict fast.
Metabolic flexibility
The ability to switch efficiently between burning carbohydrate and burning fat. Improved by repeated fasting practice. The opposite is metabolic inflexibility, common in insulin-resistant individuals, who feel poorly during the gap between meals.
mTOR (mechanistic target of rapamycin)
A cellular signalling pathway that promotes growth and protein synthesis when activated. Suppressed during fasting; activated by feeding, particularly by amino acids (especially leucine). Trade-off: high mTOR aids muscle building; low mTOR aids autophagy and longevity in animal models.
N
Noradrenaline (norepinephrine)
A catecholamine that rises during fasting and contributes to alertness, lipolysis, and modest BMR elevation.
Nutrient density
The concentration of vitamins, minerals, and beneficial compounds per calorie of food. Critical when eating windows are short — you have fewer meals to hit micronutrient targets. See our nutrient density guide.
O
OMAD (One Meal A Day)
A protocol of one meal per day, producing a roughly 23-hour fast and 1-hour eating window. Aggressive but popular. See our OMAD guide.
P
Postprandial
After a meal. Postprandial glucose, postprandial insulin — measured in the hours following food intake.
Protein sparing
The body’s mechanism for preserving lean mass during fasting, primarily via elevated growth hormone. Effective in the short term; eventually overcome in prolonged fasting, particularly with inadequate eating-window protein.
Protocol
Generic term for a specific fasting schedule (16:8, 5:2, OMAD, etc.).
R
Refeeding
The process of resuming eating after a fast. Critical to do gradually after extended fasts to avoid refeeding syndrome.
Refeeding syndrome
A potentially fatal metabolic complication that can occur when food (particularly carbohydrates) is reintroduced after prolonged fasting or malnutrition. Caused by rapid intracellular electrolyte shifts. See our safety guide.
Resistance training
Strength training using weights, body weight, or resistance bands. Essential for preserving muscle mass during sustained fasting practice.
S
Snake juice
A DIY electrolyte mix used by extended fasters: water + salt + potassium chloride + magnesium. Functional, despite the unfortunate name.
Sulfonylureas
A class of diabetes medications (gliclazide, glipizide, glimepiride, glyburide) that force insulin release. High hypoglycaemia risk during fasting; require dose adjustment under medical supervision.
T
Time-restricted eating (TRE)
The umbrella term for daily eating windows shorter than ~12 hours (16:8, 18:6, etc.). Sometimes called time-restricted feeding (TRF) — same idea.
Triglycerides
The storage form of fat, both in adipose tissue and circulating in blood. Blood triglycerides typically improve with sustained fasting practice.
W
Water fasting
Consuming nothing but water (often plus electrolytes and zero-calorie drinks) for 24+ hours. Strict, demanding, and benefits from medical supervision past a few days. See our water fasting guide.
Window (eating window)
The portion of the day during which you eat. A 16:8 protocol has an 8-hour window.