The effect of variation in the calorie: protein ratio of the diet on nitrogen retention and body composition in the rat.

It is generally recognized that nitrogen balance is influenced by the energy content of the diet. Many workers have noted the sparing action of energy-rich foods on protein utilization. Bosshardt & Barnes (1946) showed that protein utilization by growing rats and mice increased with calorie intake when the animals were given isocaloric diets ad lib. Among other reports giving evidence of the protein-sparing action of calorie-rich foods are those of Swanson (1951) and Marfatia & Sreenivasan (1960) with rats, Rosenthal & Allison (1951) with dogs, and Leverton, Gram & Chaloupka (1951) with young women. In a comprehensive review of literature on the subject, Munro (1951) concluded that N retention is influenced foremost by the calorie intake of the animal. Rosenthal & Allison (1956) have suggested that there may be an optimum calorie intake for each level of dietary protein. That the food intake is determined by the calorie content of the diet has been demonstrated with chicks (Hill & Dansky, 1950, 1954; Dansky & Hill, 1951; Peterson, Grau & Peek, 1954) and with dogs (Cowgill, 1928). Ontko, Wuthier & Phillips (1957) have shown that optimum growth and optimum efficiency of food conversion are closely related to a rather sensitive balance between the amount of protein and the calorie content of the diet. Hegsted & Haffenreffer (1949), working with rats, stated that the food intake of an animal is maintained by means yet unknown at a certain level above the normal basal metabolic requirements of the animal. Sibbald, Berg & Bowland (1956), Sibbald, Bowland, Robblee & Berg (1957a, b) and Sibbald, Bowland, Berg & Robblee (1957) have demonstrated with rats that an optimum calorie level exists for each N level of the diet and showed that the ratio of calories to N in the diet controls the percentage of N retained in the body. Yoshida, Harper & Elvehjem (1957) have shown that the calorie to protein ratio is an important factor in growth and N utilization. It has also been shown that as the energy level of the diet increased the percentage of protein required for maximum growth also increased (Leong, Sunde, Bird & Elvehjem, 1955, 1959). I n addition, the ratio of available energy to protein has been shown to exert a considerable influence upon fat deposition in animals. As the ratio of energy to protein in the diet was increased, the calorie intake and deposition of carcass fat increased (Donaldson, Combs & Romoser, 1956). Other studies with chicks(Hil1 & Dansky, 1954; Peterson et al. 1954) have shown that as the non-metabolizable fibrous bulk increased