Key Terms

The following definitions are provided for quick reference. Full context for each appears in the relevant section of the guide.

Autophagy

A cellular process in which cells break down and recycle their own damaged components. Genuinely triggered by fasting, but the strongest evidence for a meaningful effect comes from animal studies and from longer human fasts than a typical daily time-restricted eating window — see Section 9. For the cellular-aging mechanism behind this, see the Longevity guide.

Bioavailability

How much of a nutrient in food or a supplement the body can actually absorb and use, as opposed to the total amount present. Affected by the chemical form of the nutrient, what it's consumed alongside, and individual gut function — a key reason why two products claiming the same dose are not always equivalent in practice.

BMR (Basal Metabolic Rate)

The energy burned at complete rest simply maintaining basic physiological function — breathing, circulation, cell repair — measured under controlled conditions with no recent food or activity. The largest single component of most people's total daily energy expenditure.

Energy Density

The number of calories per gram of a food. Water and fibre lower energy density; fat raises it sharply, since it supplies more than double the calories per gram of protein or carbohydrate. Lower energy-density foods generally support satiety at a given calorie level better than higher energy-density foods.

FODMAPs

Fermentable Oligo-, Di-, Mono-saccharides And Polyols — a group of short-chain carbohydrates poorly absorbed in the small intestine, which can cause bloating and discomfort in sensitive individuals when they ferment in the colon. Often the actual trigger behind self-reported "gluten sensitivity," since high-FODMAP foods (wheat included) and gluten-containing foods overlap heavily — see Section 13.

Fortification

The deliberate addition of a nutrient to a food during processing, typically to address a known population-level deficiency — for example, iodine added to salt, or vitamin D added to milk. Distinct from a food's naturally occurring nutrient content.

GLP-1 (Glucagon-Like Peptide-1)

A hormone naturally released by the gut after eating, which slows gastric emptying and signals satiety to the brain. GLP-1 receptor agonist medications (semaglutide, liraglutide, tirzepatide) mimic this hormone at doses well above natural levels, producing substantial appetite suppression and weight loss — see Section 5.

Glycaemic Index (GI)

A ranking of carbohydrate-containing foods by how quickly they raise blood glucose relative to a reference food. Genuinely useful for diabetes management, but a weaker predictor of body composition outcomes in the general population than total calorie intake and fibre content.

Glycogen

The storage form of glucose, held mainly in muscle and liver tissue. A limited reserve — roughly 400–500g in a typical adult — that is depleted during prolonged or intense exercise and replenished through subsequent carbohydrate intake.

Hyper-Palatable Food

A food engineered or formulated to combine fat, sugar, salt, and/or carbohydrate in ratios rarely found together in whole foods, which independently scoring systems associate with a greater tendency to be overeaten. A more precise term than "junk food" for the mechanism behind why ultra-processed foods are so easy to overconsume — see Section 1.

Hyponatraemia

A dangerously low blood sodium concentration, most often caused in an exercise context by replacing heavy sweat losses with plain water alone over a prolonged period rather than including sodium. Rare, but a genuine risk in ultra-endurance events specifically.

IARC Carcinogenicity Classification

A four-tier system (Group 1 "carcinogenic," Group 2A "probably carcinogenic," Group 2B "possibly carcinogenic," Group 3 "not classifiable") used by the International Agency for Research on Cancer to rate how confident the evidence is that something can cause cancer at all — not how big the real-world risk is at typical exposure. A Group 1 rating (processed meat, alcohol) simply means the evidence is very solid, regardless of how large or small the actual increase in risk turns out to be; Group 2B (aspartame, among many other everyday substances) means only limited evidence exists either way. See Sections 3 and 4.

NOVA Classification

A food classification system that groups foods by the extent and purpose of industrial processing, from unprocessed/minimally processed (Group 1) to ultra-processed (Group 4) — formulations made mostly from extracted substances (oils, starches, isolated proteins, sugars) plus additives, designed for hyper-palatability and shelf life. Widely used in ultra-processed food research, though it remains debated whether "processing" itself or the nutrient profile it tends to produce is the more active variable — see Section 1.

Probiotic

A live microorganism which, when administered in an adequate amount and a specific strain, has been shown in controlled trials to confer a specific health benefit. A precise scientific term used loosely on packaging — most commercial products have not been tested in the strain, dose, and population stated on the label — see Section 7.

RDA / RNI

Recommended Daily Allowance (the US/most international term) and Reference Nutrient Intake (the UK term) describe the same concept: an official, government-set intake target for a nutrient, generally set high enough to meet the needs of nearly all healthy people in a population rather than representing an exact individual requirement. This guide uses RDA throughout as the more internationally recognised term — treat it as interchangeable with RNI or your own country's equivalent body's figure.

RED-S (Relative Energy Deficiency in Sport)

A syndrome caused by chronic under-eating relative to training demands, which can disrupt reproductive hormone function, bone density, and metabolic rate. In women it frequently presents first as a missed or irregular menstrual cycle, which should be treated as a physiological warning sign rather than a side effect of being lean.

TDEE (Total Daily Energy Expenditure)

The total amount of energy a person burns in a day, combining BMR, the thermic effect of food, non-exercise activity, and structured exercise. The practical starting point for setting a calorie target, typically estimated from a predictive equation and then refined against measured weight trend.

Thermic Effect of Food (TEF)

The energy the body spends digesting, absorbing, and processing food itself. Protein has the highest thermic effect of the three macronutrients — roughly 20–30% of its calories, compared with 5–10% for carbohydrate and fat — which is one reason higher-protein diets modestly support fat loss.

Time-Restricted Eating

An eating pattern that confines food intake to a defined daily window (commonly 6–10 hours) rather than restricting which foods or how many calories are eaten within it. Produces fat loss outcomes broadly comparable to standard calorie restriction when total weekly intake is matched, with some preliminary evidence of additional metabolic benefit specific to earlier eating windows.