Fig. 4.

Physiological relevance of brown and beige fat. Adrenergic agonism (cold exposure) is the strongest known stimulator of brown fat in rodents and humans. Multiple studies covered in the respective section indicate that adrenergically stimulated BAT exhibits significant thermogenic capacity and activity, in association with glucose and FFA uptake. This metabolic activity further contributes to significant increases in energy expenditure in rodents and to a lesser extent in humans. Beige fat, in comparison, has been shown to exhibit dramatic fold-induction in its thermogenic capacity upon adrenergic stimulation relative to its basal state (82). Glucose and FFA uptake also occurs in beige fat; however, this was not associated with any detectable metabolic activity in rodent iWAT, questioning its contribution to whole body energy expenditure (82). In humans, detectable brown fat exhibits glucose and FFA uptake, as well as significant metabolic activity on adrenergic stimulation (113, 154). Human brown fat largely expresses beige markers; however, classic brown fat signatures are also detectable in this depot (46, 74, 93, 106, 142). Note: Arrows are not drawn to reflect on the exact proportional relevance of brown and beige in rodent and human.