Interactions between the gut, the brain and brown adipose tissue function.

Brown adipose tissue (BAT) has the unique ability to oxidize fatty acids to generate heat, a process termed thermogenesis. The mitochondrial uncoupling protein 1 is predominantly expressed in BAT and controls the thermogenetic properties of this tissue. Since activated BAT dissipates energy, it is considered beneficial in controlling metabolism, i.e. by combating obesity. Indeed, humans with a higher BMI have less active BAT. Many researchers attempt to uncover regulatory pathways in BAT activity in the pursuit for novel BAT modulators to control body weight. Endocrine factors such as thyroid hormone, sex steroid hormones and glucocorticoids can modulate BAT activity. Since the intestinal tract has emerged as an endocrine organ regulating energy balance and glucose homeostasis, this review will discuss how gut-derived hormones and other intestinal tract-related factors such as bile acids modulate BAT activity. Emphasis will be put on whether these hormones regulate BAT directly or via the central nervous system. In summary, it can be globally stated that anorexigenic gut hormones stimulate BAT while orexigenic gut hormones inhibit BAT activity. How these hormones modulate BAT and whether this is via a direct and/or central effect is largely unknown. Novel insights about gut-derived factors such as bile acids suggest that they also affect BAT activity. Altogether, effects of food intake per se on BAT activity are rather complex to interpret and depend on many (hormonal) factors.