Brown Fat in Humans: Turning up the Heat on Obesity

The looming pandemic of obesity and overweight,driven by ready access to high-calorie food andan increasingly sedentary way of life, poses asevere threat to global public health. The patho-logical accumulation of excess dysfunctional adipose tis-sue that characterizes obesity is a major risk factor for many other diseases, including type 2 diabetes, cardiovas-cular disease, hypertension, stroke, arthritis, and various types of cancer (1). A basic, but often misunderstood, concept is that weight gain is caused by a fundamental energy imbalance, when energy intake from food chroni-cally exceeds energy expended by physical activity and metabolic processes (Fig. 1). Humans have evolved effi-cient biological mechanisms to acquire and defend their energy stores. A therapy for weight loss must, therefore, involve a decrease in food intake and/or an increase in energy expenditure. In addition to the better-known white adipose tissue