It is here that the paper by Ouellet et al. (2) further advances our understanding. What these authors found is that brown fat activation in the cold is not only associated with increased blood flow, but that it is actually also associated with a higher metabolism in the tissue, i.e., the burning of fat. Ouellet et al. exposed six men to controlled cold (a water-cooled suit). A bolus of radioactive acetate was then given to the subjects. The acetate in the blood will distribute to all tissues, in principle in proportion to the blood flow through the tissue. Already in this respect, brown adipose tissue is remarkable in that the uptake — when the subjects were cold — was some four times higher than that seen in muscle, an indication of a high blood flow to the brown adipose tissue. With the acetate technique, the successive loss of radioactivity from a tissue can be seen as an indication of active metabolism (Figure (Figure2).2). In cold subjects, Ouellet et al. found that the radioactivity disappeared from the brown adipose tissue with a half-life of just minutes, while the radioactivity remained in the tissue in warm subjects. Thus, the brown adipose tissue really is metabolically active.