Role of activity-induced thermogenesis in twenty-four hour energy expenditure of lean and genetically obese (ob/ob) mice.

The contribution made by spontaneous physical activity to 24 h energy expenditure has been assessed in lean and ob/ob mice living at 28 degrees C. Measurements were made in young animals when the ob/ob mice were the same body weight as their lean littermates, and again when they were adult and the ob/ob mice were twice as heavy as their littermates. In the young ob/ob mice, not only was 24 h heat production lower than in their lean littermates (P less than 0.005) but so also was motor activity (P less than 0.01). In adults, total 24 h heat production was similar in lean and obese animals, and thus on the basis of metabolic body size it was considerably lower in the obese adults (P less than 0.005). The obese adults were also significantly less active than their lean littermates (P less than 0.05). Differences between genotypes were particularly marked during the night. Partition of 24 h energy expenditure into the two components of rest and activity showed that in the young ob/ob mice the lower heat production occurred because they expended 31% less energy on rest and 47% less on activity than did their lean littermates. In the obese adults, the lower heat production per unit metabolic body size was accounted for by 16% less energy being expended on rest and 74% less on activity than in the lean mice. It is concluded that differences in motor activity between lean and ob/ob mice could account, at least in part, for the development and maintenance of obesity in these animals.