Influence of age on protein and energy utilization in adult rats

Male, Sprague-Dawley rats at 15 and 25 months of age were fed for 21 days diets containing 2, 3, or 4% casein supplemented with 0.078 g methionine/g casein at three levels of intake. Metabolizable energy intakes (MEI) required to maintain energy of total carcass and carcass protein were predicted from regression equations relating change in energy of total carcass and carcass protein to MEI. Mean MEI required to maintain energy of total carcass for 15- and 25-month-old rats, respectively, were 112 versus 88, 99 versus 76 and 94 versus 74 kcal/(kg0.75 X day) for 2, 3 and 4% casein, respectively. Mean MEI required to maintain energy of carcass protein for 2, 3 and 4% casein were 103, 98 and 85 kcal/(kg0.75 X day) for the 15-month-old animals and 93, 90 and 62 kcal/(kg0.75 X day) for the 25-month-old animals. Confidence intervals for mean maintenance energy intakes were wide and overlapped as age and casein concentration were varied. However, trends were evident. For both ages, as casein concentration increased, mean requirements decreased, and at each casein concentration, the 25-month-old animals had lower mean requirements than the 15-month-old animals.