[Utilization of proteins in rats with different fat and saccharide intakes]

For 14-day periods, 30-day-old and 90-day-old male rats were fed ad libitum the diets with a constant protein content (casein) of 10 p. c. and with an increased fat content (margarine) of 10, 20. 30, 40 and 50 p.c. (first experiment) and then the diets with a constant protein content (casein) of 10 p. c. and with a constant fat content (margarine) of 30 p. c. (30-day rats) and of 11 p. c. (90-day rats) and with an increasing saccharide content of 31, 36, 41, 46 and 51 p. c. (90-day rats were further fed by 56, 61 and 66 p. c.) composed of rice starch, sugar and potato starch at a ratio of 6.4 : 1.2 :1 (second experiment). Net protein utilization (NPU) was determined on the basis of body nitrogen content and protein intake. The animal liver was examined for the gluconeogenesis by measuring the specific activity of phosphoenolpyruvate carboxykinase (PEPCK). The highest NPU value can be achieved as follows: in 30-day animals by administration of the feeds containing 30 p. c. of fat, 36 p. c. of saccharides; in 90-day animals by 10 p. c. of fat and 51 p. c. of saccharides. Under the optimum nutrition conditions, both age groups utilize protein in the same way. The extreme nutrition conditions are better tolerated by the just weaned animals than by 90-day animals. The results of this biological method were also proved by the course of gluconeogenesis (activated at 40 and 50 p. c. fat content and 31 p. c. saccharide content in the diet fed to 30-day animals; in 90-day animals the activation occurred beginning 20 p. c. fat content and at 31 to 46 p. c. saccharide content). This paper contributes to the determination of optimum physiological nutrient rates by biological and biochemical methods.