Studies on energy metabolism in lines of mice selected for different growth parameters

In six experiments on young male mice, selected over more than 50 generatio ns for body weight (Line DU-6), protein amount in the carcass (DU-6P) and a n index combining body weight and treadmill performance (DU-6+LB) or nonsel ected (DU-Ks), energy balances were measured by means of indirect calorimet ry through carbon and nitrogen balance. The environmental temperatures were 22, 15 or 30 degrees C. Furthermore, in two experiments 6 h measurements o f heat production and physical activity in the fasted and the postprandial stare were conducted at 22 degrees C to estimate the resting metabolic rate (RMR). The DU-6 mice realized significantly higher daily ME intakes and en ergy gains than DU-6P and DU-Ks mice in most of the experiments at 22 degre es C. At the end of the energy balance periods the protein content of DU-6P mice was about 0.5 to 2 percentage-units higher and the fat content 4 to 7 percentage-units lower than those of DU-6 mice. The values of maintenance energy requirements (MEm, kJ/kg BW0.75 per day), derived from the energy ba lance data, were similar at the temperature near thermoneutrality (30 degre es C), but were higher by 6 to 22% for 30 to 50-day-old DU-6P mice compared with DU-6 mice at 22 degrees C. However, no differences could be observed in RMR between DU-6P mice and DU-6 mice at 22 degrees C. The MEm values of the DU-6+LB mice were intermediate between the two other selected lines. Th e differences in MEm estimated at 22 degrees C did not remain evident at 15 degrees C suggesting no differences in thermoregulatory hear production be tween the selected lines. The results suggest that the differences in MEm b etween DU-6P and DU-6 mice observed at 22 degrees C are mainly due to diffe rences in physical activity.