Etiology and pathogenesis of obesity.

Obesity results from a greater consumption of energy than is used by the body. As this energy is stored, fat cells enlarge, producing the characteristic pathology of obesity. The pathologic enlargement of fat cells, in turn, produces altered levels of many peptide and nutrient signals that are responsible for the disease we call "obesity." The genetic makeup of human beings, which reflects a long history of relative scarcity of foodstuffs, has run into an age of surfeit, and many people cannot readily adapt. Thus, the increased intake of food does not signal satiety, and there is a gradual increase in energy stores as intake of energy outpaces need as we grow older. Against this background of struggle between nature and nurture, it is possible to identify an increasing number of defects or etiologies that produce obesity. For most patients, however, it is not possible to connect obesity to a specific cause. Leptin deficiency and defects in the leptin receptor both produce human obesity. Defects in the pro-opiomelanocortin receptor system, the peroxisome proliferator-activated receptor-gamma, the agouti-related peptide, and a few other rare genetic syndromes are also associated with human obesity. Of the genetic causes, Prader-Willi syndrome is the most common. Hypothalamic injury following craniopharyngioma is the most common neuroendocrine cause. Endocrine disorders such as Cushing's disease, polycystic ovary disease, and growth-hormone deficiency can lead to increased body fat. In the modern world, exposure to a high-fat diet predisposes many people to obesity, and this problem is compounded by the low levels of activity now required for daily living. Treatment strategies must be developed against this background.