The development of brown adipose tissue during experimental overnutrition in rats.

The characteristics of interscapular brown adipose tissue (IBAT) growth in normally-eating and in overnourished lean rats have been described from 3 days postweaning until 18 weeks of age. Additionally, the relationship between IBAT, diet, and body composition were investigated. The IBAT weight of normally-eating rats increased rapidly until ten weeks of age, and only gradually after that time. In overnourished rats, IBAT weight was significantly greater than in normally-eating rats after only 3 days of the over-eating regimen and increased more than twice as rapidly as in controls thereafter. Brown adipocyte diameter increased by only 30 percent in both groups during the study, while adipocyte number of overnourished rats became three-fold greater by ten weeks of age. IBAT represented 36 percent of the total brown adipose tissue in both normally-fed and in overnourished rats. Overnourished rats demonstrated a significant decrease in the efficiency of weight gain, but still became fatter than normally-fed animals. Surgical reduction of IBAT resulted in an improved efficiency of weight gain and in a greater carcass fat content in the overnourished rats, while body composition and the efficiency of weight gain were unaltered in similarly-operated normally-feeding rats. When older animals were overfed, IBAT growth was similar to that of younger rats; and, when previously overfed adult rats were returned to a stock diet for six weeks, IBAT weight was preserved. The results of these studies are consistent with the development of a nutritionally-induced hyperplasia of brown adipose tissue in the overnourished rats, similar to that which occurs in cold-adaptation, and which could enhance an animal's capacity for nonshivering thermogenesis. These data further suggest that normally-functioning brown adipose tissue may play an active role in energy expenditure in the lean rat, and thus may influence its propensity for fatness.