ROLE OF ACTIVITY‐INDUCED THERMOGENESIS IN TWENTY‐FOUR HOUR ENERGY EXPENDITURE OF LEAN AND GENETICALLY OBESE (OB/OB) MICE

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Abstract
The contribution made by spontaneous physical activity to 24 h energy expenditure has been assessed in lean and ob/ob mice living at 28.degree. 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 < 0.005) but so also was motor activity (P < 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 < 0.005). The obese adults were also significantly less active than their lean littermates (P < 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.