Effects of nitrogen and energy metabolism on body weight in later life of male Wistar rats consuming a constant amount of food.

The effects of protein and energy metabolism at the whole-body level on body weight changes in later life of male Wistar rats fed 13.3 g of 20% casein diet per day (group 80) or 8.7 g/day (group 50) throughout life from 50 d of age were examined on d 600 and 800 of age. Although the rats consumed most of the food presented, their body weight started to decrease rapidly from about d 700 of age in group 80 and gradually from d 600 in group 50. On d 800 their body weights had decreased significantly (17% in group 80 and 11% in group 50). These decreases in weight were not due to decreased digestibility of the food, and in both groups a positive nitrogen balance was maintained on d 800 and the relative proportion of body protein was not different from that on d 600. In both groups, daily energy expenditure was significantly higher on d 800 than on d 600 because of an increase in the basal metabolic rate, not in physical activity, and consequently these balances were negative on d 800. From these results, it is concluded that the primary cause of the decrease in body weight in later life of aging rats consuming a constant amount of food is increased expenditure of energy.