The significance of beige and brown fat in humans

Promotion of brown adipose tissue (BAT) activity or browning of white adipose tissue has shown great potential as anti-obesity strategy in numerous preclinical models. The discovery of active BAT in humans and the recent advances in the understanding of human BAT biology and function have significantly propelled this field of research. Pharmacological stimulation of energy expenditure to counteract obesity has always been an intriguing therapeutic concept; with the identification of the specific molecular pathways of brown fat function, this idea has now become as realistic as ever. Two distinct strategies are currently being pursued; one is the activation of bone fide BAT, the other is the induction of BAT-like cells or beige adipocytes within white fat depots, a process called browning. Recent evidence suggests that both phenomena can occur in humans. Cold-induced promotion of BAT activity is strongly associated with enhanced thermogenesis and energy expenditure in humans and has beneficial effects on fat mass and glucose metabolism. Despite these encouraging results, a number of issues deserve additional attention including the distinct characteristics of human vs rodent BAT, the heterogeneity of human BAT depots or the identification of the adipocyte precursors that can give rise to thermogenic cells in human adipose tissue. In addition, many pharmaceutical compounds are being tested for their ability to promote a thermogenic program in human adipocytes. This review summarizes the current knowledge about the various cellular and molecular aspects of human BAT as well as the relevance for energy metabolism including its therapeutic potential for obesity.