The mechanisms controlling body weight or, to be more specific, that component of body mass that consists of adipose tissue is an active area of scientific research. Two stimuli can be discerned that give a sense of urgency to this research. The first is the data, from many sources, confirming an inexorable upward trend in the prevalence of overweight and obesity in developed countries. The picture in the emerging nations is unclear because of both a lack of appropriate survey data and the continued scourge of under- nourishment among their poor. It is likely, however, that, throughout the...