Termogeneesi

Termogeneesi, or thermogenesis, is the production of heat as a result of energy expenditure in living organisms. It helps maintain core body temperature and contributes to long-term energy balance. In humans and other mammals, thermogenesis occurs through several mechanisms that convert chemical energy from nutrients into heat rather than useful work.

There are two broad categories: obligatory thermogenesis and facultative thermogenesis. Obligatory thermogenesis refers to energy expended to sustain basal metabolic processes, including postprandial heat production after meals. Facultative thermogenesis, also called non-shivering thermogenesis, is mainly mediated by brown adipose tissue (BAT) and beige adipocytes. Cold exposure and sympathetic nervous system activity activate BAT, with norepinephrine binding to beta-adrenergic receptors, increasing intracellular cAMP and upregulating heat-producing pathways.

At the mitochondrial level, the heat is generated by the protein thermogenin, also known as UCP1, which uncouples oxidative phosphorylation from ATP synthesis, releasing energy as heat. Beige adipocytes can arise within white adipose tissue under stimuli such as prolonged cold exposure or certain hormonal signals, contributing to thermogenesis as well.

Key regulators include thyroid hormones (T3 and T4), catecholamines, and nutritional state. Diet-induced thermogenesis describes the energy cost of processing and storing food and typically accounts for a modest portion of daily energy expenditure. Shivering thermogenesis is another rapid heat-producing response that uses skeletal muscle contractions when non-shivering mechanisms are insufficient.

Clinical and research relevance: thermogenesis influences total energy expenditure and body fat, with variations in BAT activity linked to obesity and metabolic health. Assessment methods include indirect calorimetry to estimate energy expenditure and imaging techniques such as PET-CT to detect active BAT. Cold exposure studies help characterize individual thermogenic capacity. Brown adipose tissue is most abundant in newborns and persists in small depots in adults, where it can be activated by cold exposure.