Energy intake and utilization vary during development in rats

Energy intake, utilization, and partitioning were determined in male Wistar rats from 25 to 180 d of age. Serum free triiodothyronine, leptin, and fre e fatty acid concentrations were also measured. Energy balance measurements allowed us to identify a period from 25 to 90 d, characterized by a rapid body growth rate and another from 90 to 180 d, during which body growth rat e slowed. From 25 to 180 d, we found decreases in daily energy intake and e xpenditure, which were faster before 90 d. The first period was characteriz ed by storage of lipid and protein. In the second period, protein depositio n approached zero and the excess of ingested energy was entirely stored as fat, so that age-associated obesity began to develop. The inability of rats to maintain a stable body weight after the cessation of growth of lean bod y mass is not due to decreased resting metabolism but rather to a partial l eptin resistance.