Table 1 Operational definitions of terms

Adipose tissue expansion

The process of net deposition of fat in adipose tissue. Excessive fat mass occurs via increased adipocyte size (hypertrophy) and increased number of adipocytes (hyperplasia).

Energy (im)balance

The difference between net energy intake and whole-body energy expenditure over a specified period of time.

Energy density (of food)

The amount of metabolisable or bioavailable energy per unit of total (wet) weight for a food or a beverage (and sometimes for the whole diet), expressed in joules or kilocalories per gram.

Energy expenditure

The energy derived from oxidation of fat, carbohydrate, protein and occasionally alcohol, spent for basic physiological functions (basal metabolism), for processing and storing ingested nutrients (thermic effect of food), for supporting all body movements and physical activities (voluntary and involuntary) and, in some cases, for thermoregulation. Measured at the whole-body level.

Energy intake, gross

The amount of chemical energy released during complete combustion (in a bomb calorimeter) of ingested foods and drinks.

Energy intake, net (metabolisable energy intake)

The difference between ingested gross energy and energy lost in faeces and urine (or vomiting in some circumstances), also accounting for the energy made available to the host via the action of the gut microbiome on undigested food material.

Food palatability

The pleasantness of a specific food, determined by its sensory properties (for example, smell, taste, texture, visual appearance). It is measured by the relative preference to other foods and/or subjective hedonic ratings and may be innate or modified by prior experience or physiological state (for example, fasted or fed).

Food reward

The multifactorial process integrating sensory information with interoception resulting in reinforcement of food seeking and consummatory behaviour, often below conscious awareness.

Fuel partitioning

The processes of routing energy-yielding substrates toward deposition into or mobilisation from body stores, partitioning metabolic fuels among different tissues of the body (for example, adipose versus lean tissues) and channelling them between storage or oxidation within cells.

Glycaemic index

The ability of a carbohydrate-containing food to raise blood glucose concentrations relative to a reference food (usually, glucose or white bread). Typically, it is calculated as the incremental area under the curve for glucose over 2 h after consuming a standard amount of bioavailable carbohydrate (usually, 50 g) from the test food relative to the area after the reference food.

Glycaemic load

The product of the glycaemic index of a carbohydrate-containing food and the amount of that food carbohydrate that is consumed.

Hunger

A motivational ‘need state’ related to physiological cues of lack of energy or lack of a specific nutrient.

Hyperphagia (overeating)

A substantial increase in food intake relative to a ‘normal’ baseline or control condition or group. Over a sustained period of time, overeating is substantiated by net fat accumulation in the body. Accordingly, the term is equivalent to weight gain and cannot inform on the causal direction.

Metabolic flexibility

The ability of cells, tissues or the whole body to align the mixture of substrates being oxidized for energy with the substrates that are available, particularly in response to dynamic changes in nutrient availability (induced by meal ingestion, dietary changes, physical activity, hormonal and neural inputs). It has both magnitude (that is, ‘how much’) and temporal (that is, ‘how fast’) dimensions.

Obesity (phenotype)

A state of the body characterized by increased or excessive fat mass in a way that adversely affects health and is often resistant to reduction and prone to relapse if such a reduction occurs by eating less and exercising more.

Obesogenic environment

An environment that hypothetically promotes weight gain and obesity because of a variety of food-related factors (for example, composition, price, convenience) and non-food-related factors (for example, built environment, sociocultural context) that favour increased food intake and decreased energy expenditure.

Postabsorptive (fasted) state

The metabolic state that occurs typically from ~8 to ~14 h after eating (that is, after an overnight fast). It is characterized by accelerated catabolic pathways, a greater contribution of fat oxidation to meet energy demands and a relative steady state in the concentrations of hormones and metabolic fuels in the bloodstream.

Postprandial (fed) state

The metabolic state that occurs immediately after consuming a meal and typically lasts for ~3 up to ~8 h depending on the caloric content and macronutrient composition of the meal. It is characterized by accelerated anabolic pathways, a greater contribution of carbohydrate oxidation to meet energy demands and dynamic changes in the concentrations of hormones and metabolic fuels in the bloodstream.

Predisposition (to obesity)

A set of genetic and/or behavioural traits of some individuals in a population associated with increased ease of gaining weight and developing obesity.

Susceptibility (to obesity)

The tendency of some individuals to gain weight and develop obesity under the influence of specific environmental exposures.

Ultraprocessed food

Food resulting from extensive industrial processing, using relatively inexpensive ingredients, additives and preservatives rarely found in home kitchens, formulated to enhance palatability and appearance and prolong shelf life and maximize convenience.

Weight gain, dynamic phase

A period of typically slow, progressive weight gain that characterizes obesity development for most people.

Weight gain, stable/static phase

The period after obesity has been developed, during which weight gain slows or plateaus.