There are three kinds of fat tissue. White adipose tissue (WAT) stores energy and in excess leads to obesity. Brown adipose tissue (BAT) consumes energy and produces heat for thermo-regulation. Beige adipose tissue (BeAT) emerges within WAT during cold exposure and manifests thermogenic function comparable to BAT. Animals with high BAT/BeAT status are protected against diabetes and obesity. Recent re-discovery of thermogenic BAT in humans has brought the relation between ambient temperature, thermogenesis and systemic energy and substrate metabolism to the forefront (1).
Humans maintain core temperature through a complex neuroendocrine circuitry, coupling environmental thermal and nutritional cues to heat-producing and dissipating mechanisms. Up to 40% of resting energy expenditure contributes to thermal homeostasis maintenance. The dynamic interplay between BAT, BeAT and WAT modulates systemic energy homeostasis and highlights the presence of a previously under-appreciated thermogenic adipose axis in humans.
In addition to well-known pituitary-thyroid-adrenal axis, recently identified endocrine signals, such as FGF21 and irisin (2), orchestrate crosstalk between WAT, BAT and muscle, tuning non-shivering and shivering thermogenesis responses. Cold-activated BAT modulates systemic metabolic and endocrine milieu, and cold-induced hormones cause bioenergetics transformation sufficient to impact whole body energy and substrate balance (3), suggesting BAT may serve important physiologic functions beyond thermoregulation in humans.