Effects of dietary level of protein on the maintenance energy requirement and net energetic efficiency for growth of post-weaning rats.
The effects of dietary protein on the maintenance energy requirement (MEm) and net utilization efficiency of metabolizable energy for growth (MEg) were investigated by regression analysis of energy balance with various energy intakes. Weanling rats of the Wistar strain, weighing about 85 g, were given a diet containing 0 to 70% casein freely or in restricted amounts (equivalent to two-thirds or one-third of the intake of the ad libitum group) for 5 days. The MEm was fairly constant in rats given 10 to 50% casein diets, being about 29 kcal/100 g BW/day, but increased at higher or lower dietary protein levels, indicating inefficient energy utilization in protein-malnourished animals. From the slope of the regression line between energy balance and metabolizable energy intake, the net energetic efficiencies for growth were estimated as 68, 71, 74, 77, 82, 83, 80, 78, 77 and 74% with 0, 3, 6, 10, 20, 30, 40, 50, 60 and 70% casein diets, respectively. Weanling rats fed 20 to 30% casein diets utilized the dietary energy for growth most efficiently. At protein levels higher or lower than 20 to 30%, the efficiency was less, showing that MEg utilization depended on dietary protein. The energy necessary for 1 g body weight gain was 2.6 kcal in rats receiving 30% casein diet, but increased with an increase or decrease in the protein level. These data on the food efficiency, MEm, the net efficiency of MEg and the energy necessary for 1 g weight gain show that dietary protein affects energy utilization and that protein-malnourished animals use energy inefficiently.