[Protein retention in relation to age]

Male Norway rats of the Wistar strain, aged 30, 75, 90, 150, 180, and 360 days, were given ad libitum diets with optimum nutrient contents at every age. Determining the net protein utilization, the values of protein retention were calculated from protein intake. The energy demand per unit of protein retention was determined from the saccharide and fat intake. The energy ratios of proteins, fat, and saccharide were calculated from the optimum nutrition values; this ratio was 13:(57-29):(30-58) in the period of rapid growth (30-90 days old animals). 12:28:60 in the period of maturing (slower growth, 150 days old animals), and 10.5:28:61.5 at maturity (finished growth, 180-360 days old animals). The daily weight gains of the animals given optimum diets (5.52, 5.22, 5.09-3.73-2.11, 1.85 g daily) and the values of protein retention also corresponded to the division of the animals according to their growth (after administration of the standard Larsen diet). In the period of rapid growth, protein retention was increasing up to the age of 90 days--this suggests that the proteins are utilized for building the body and for its development. A decrease in protein retention in 150 days old animals means that the animals have entered the stage of slow growth. Finally, the low retention values at adult age suggest that the proteins are utilized only for the regeneration of worn tissues and for maintenance. It is indicated by the higher values of energy demand per unit of body protein retention at adult age that more energy (mainly the energy of saccharides--starch) is needed to secure the processes of tissue maintenance and regeneration.