Fasting comes with a lot of jargon because it touches on metabolism, hormones, nutrition, and religious traditions. This glossary defines the most common terms in plain language, with a short note on what each one means in practice for someone who fasts. Where research is limited or uncertain, we say so.
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. During fasting, the body increases lipolysis (release of fatty acids from adipose tissue), which supplies fuel and the raw material for ketone production.
Adrenaline (epinephrine)
A hormone released by the adrenal glands that helps mobilize glucose and fatty acids. Levels can rise modestly during fasting, which may contribute to the alertness some people report, although responses vary.
Autophagy
Literally “self-eating.” The cellular process by which damaged proteins, organelles, and other components are broken down and recycled. Fasting increases autophagy in animal and cell studies. In humans it is hard to measure, and there is no established timeline for how many hours of fasting are needed. The 2016 Nobel Prize in Physiology or Medicine was awarded for research on the mechanisms of autophagy. 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 about 0.5 mmol/L are usually described as nutritional ketosis; levels tend to rise further the longer a fast lasts.
Blood glucose
The concentration of glucose in the blood. In healthy people, blood glucose may drift down somewhat during a fast and then stays within a fairly narrow range because the liver releases stored glucose and makes new glucose (gluconeogenesis). A normal fasting glucose is generally considered to be about 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 tends to decrease somewhat with prolonged calorie restriction and weight loss. Short fasts do not appear to lower it meaningfully, and some studies have reported small, temporary increases.
Bone broth
Broth made from animal bones, often slow-cooked. Contains amino acids and minerals. Technically breaks a strict fast because it contains protein and calories, but some people use it during longer fasts for comfort and sodium. See our bone broth guide.
C
Calorie deficit
Eating fewer calories than you expend. The mechanism behind weight loss in any dietary approach, including fasting. Fasting often leads to a calorie deficit by limiting eating time, but eating more during the eating window can cancel it out.
Catecholamines
A group of stress hormones including adrenaline (epinephrine) and noradrenaline (norepinephrine). They can rise modestly during fasting and help release stored fat for fuel.
Circadian rhythm
The body’s ~24-hour internal clock. Affects insulin sensitivity, hormone secretion, body temperature, and digestion. Some research suggests that eating earlier in the day rather than late at night may be better for blood sugar control, although evidence is still developing. See our fasting and sleep guide.
Cortisol
A glucocorticoid hormone released by the adrenal glands. It helps maintain blood glucose, among many other roles, and follows a daily rhythm (highest in the morning). Short-term fasting may change cortisol modestly; the effects of long-term fasting on cortisol in humans are not well established. See our cortisol and fasting page.
D
Dirty fasting
Informal term for fasting with small caloric “cheats” — coffee with cream, bone broth, MCT oil, etc. Whether small amounts of calories meaningfully change the effects of a fast has not been well studied; for a strict fast, avoid them. Contrast with “clean” or “strict” fasting.
E
Eat Stop Eat
An approach that involves one or two 24-hour fasts per week, with normal eating on the other days. 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). Some small studies have found better blood sugar or insulin measures with early windows than with later ones, but results are mixed.
Electrolytes
Minerals that carry an electrical charge in body fluids — sodium, potassium, magnesium, calcium, chloride, phosphate. Especially important to maintain during longer fasts. See our electrolyte guide.
Extended fast
Usually any fast longer than about 36–48 hours, such as multi-day water fasts. Extended fasts carry greater risks, including electrolyte problems and, in some cases, refeeding syndrome. They should only be done with medical supervision, and they are not appropriate for many people.
F
Fasted state
The state the body enters once a meal has been digested and absorbed, when insulin is low and the body draws on stored energy (first mainly liver glycogen, then increasingly fat). It is often described as starting several hours after the last meal, depending on meal size and composition. Contrast with fed state.
Fat adaptation
An informal term for the idea that the body gets better at using fat for fuel after weeks of low-carbohydrate eating or regular fasting. Many people report that fasting feels easier over time, though the term is not precisely defined in research.
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. It was developed in university research, is sold commercially as a packaged program, and has been studied in a limited number of trials. See our fasting-mimicking diet guide.
G
Ghrelin
The “hunger hormone.” Released by the stomach in anticipation of meals. It tends to rise around habitual mealtimes and then fall again, which helps explain why hunger often comes in waves during a fast. Ghrelin appears to adjust to regular meal times, which may be why hunger often eases once a new schedule becomes routine.
Glucagon
A hormone released by the pancreas when blood glucose drops. Triggers gluconeogenesis and lipolysis. Rises during fasting and generally acts opposite to 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 largely used up after roughly a day of fasting (sooner with exercise); muscle glycogen is used mainly by the muscle itself and is not released into the bloodstream as glucose.
Growth hormone (GH, somatotropin)
A pituitary hormone that promotes tissue maintenance and lipolysis. Levels can rise substantially during fasts lasting a day or more. This may help preserve protein during fasting, although the practical significance for muscle is not fully understood.
H
HbA1c
Glycated haemoglobin. A blood marker reflecting average glucose levels over roughly 2–3 months. Used to diagnose and monitor diabetes. It can improve with weight loss and other lifestyle changes; anyone with diabetes should discuss fasting with their healthcare provider.
HIIT (High-Intensity Interval Training)
Short bursts of hard effort separated by recovery. Many people perform better when they have eaten beforehand; fasted HIIT can feel very demanding. See our exercise guide.
Hypoglycaemia
Abnormally low blood glucose. Symptoms include shaking, sweating, confusion, weakness. Uncommon in healthy adults during short fasts because the liver maintains glucose, but a real risk for people taking 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. Low insulin allows fat to be released from fat cells for fuel.
Insulin resistance
A condition in which cells respond poorly to insulin, requiring higher levels to maintain blood glucose. Closely linked to type 2 diabetes and metabolic syndrome. It can improve with weight loss and regular physical activity.
Insulin sensitivity
The opposite of insulin resistance — how well cells respond to insulin. Some studies of intermittent fasting have found improvements, often alongside weight loss; results vary.
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 alternate-day fasting. See fasting methods.
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 longer fasts (often after about a day or more, depending on the person and prior diet) or through a very-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. Fluid and sodium losses are thought to contribute, and adequate fluids and electrolytes may help. Symptoms usually pass within days. See our electrolyte guide.
L
Leptin
A hormone produced by fat cells that signals satiety and energy sufficiency. Falls during caloric restriction and fasting. Lower leptin is one reason hunger can increase during sustained calorie restriction.
Lipolysis
The breakdown of stored triglycerides in fat cells, releasing fatty acids and glycerol into circulation. Suppressed by insulin and increased when insulin is low. The first step in using stored fat for fuel.
M
MCT oil
Medium-chain triglycerides. Absorbed quickly and readily converted to ketones in the liver. Sometimes added to coffee. Contains calories and breaks a strict fast.
Metabolic flexibility
The ability to switch efficiently between burning carbohydrate and burning fat. Physical activity and weight loss are thought to improve it; whether fasting has a specific effect is less clear. Reduced metabolic flexibility has been described in people with insulin resistance.
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). Activated mTOR supports muscle building; reduced mTOR activity is linked to autophagy and, in animal studies, longer lifespan. Whether this applies to people is not known.
N
Noradrenaline (norepinephrine)
A catecholamine that can rise during fasting and helps release stored fat for fuel.
Nutrient density
The concentration of vitamins, minerals, and beneficial compounds per calorie of food. Especially important when eating windows are short, because you have fewer meals to meet your nutrient needs. 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. One of the more demanding daily schedules. 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 tendency to reduce protein breakdown as a fast continues and it relies more on fat and ketones. It limits, but does not prevent, loss of lean tissue during prolonged fasting. Eating enough protein and doing resistance training help preserve muscle.
Protocol
Generic term for a specific fasting schedule (16:8, 5:2, OMAD, etc.).
R
Refeeding
The process of resuming eating after a fast. After extended fasts it should be done gradually, and under medical supervision where there is a risk of refeeding syndrome. See breaking a fast.
Refeeding syndrome
A potentially fatal complication that can occur when food, particularly carbohydrate, is reintroduced after prolonged fasting or malnutrition. It involves rapid shifts of electrolytes such as phosphate, potassium, and magnesium into cells. It needs medical management. See our safety guide.
Resistance training
Strength training using weights, body weight, or resistance bands. Important for preserving muscle mass, whether or not you fast.
S
Snake juice
An informal name for a homemade electrolyte drink (water, salt, potassium chloride, and sometimes magnesium) associated with the extreme “Snake Diet.” Getting the amounts wrong, especially of potassium, can be dangerous. See our Snake Diet safety page.
Sulfonylureas
A class of diabetes medications (gliclazide, glipizide, glimepiride, glyburide) that stimulate insulin release. They carry a high risk of hypoglycaemia during fasting, and any fasting plan needs medical supervision and possible dose changes.
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 often fall with weight loss, including weight loss achieved through fasting.
W
Water fasting
Consuming nothing but water (often plus electrolytes and zero-calorie drinks) for 24+ hours. Demanding and not suitable for everyone; fasts beyond about a day or two should only be done with medical supervision. 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.