The Effect of Dietary Protein and Calorie Restriction on Growth, Kidney Function, and Survival of Male Rats

Growth, kidney function, and survival were studied in male Wistar rats fed for 1 (12 animals per dietary group) or 2 (36 animals per dietary group) years low, medium, and high protein diets at 2 caloric levels and diets fed ad libitum but gradually altered in protein content during growth and development. Restricted groups (R) fed 18, 30, and 42% casein diets were provided two-thirds of the mean quantity of diet consumed by groups fed 12, 20, and 28% casein ad libitum (A), respectively, and thus consumed the same amount of protein but one-third less calories. Two other groups were fed ad libitum initially a 20% casein diet which was either decreased to a 12% casein level or increased in casein content to 28% by 1 year of age. Caloric restriction significantly decreased the mature body weight and rates of weight gain at 10 and 20 weeks of age but increased growth rate (k) as calculated from the Brody growth equation using body weights obtained at 16 points during the first year of life. While dietary protein level had no significant effect on mature body weight, it significantly affected the rate at which mature body weight was attained. High dietary protein levels were generally associated with high k values at both caloric levels, however, R animals fed a medium protein diet had the highest k value. Dietary protein level had a greater effect on rates of weight gain in A than in R groups and was positively associated with rate of weight gain during the early growth period (10 weeks of age) but during the later growing period (20 weeks of age) was negatively iv associated. Animals fed varying levels of protein ad libitum had higher mature body weights and rates of weight gain than those fed a constant level of protein and generally consumed more diet. Animals fed a constant low level of protein tended to consume a smaller quantity of diet than those fed higher dietary protein levels. Measurements made in this study for tests of renal function were urinary protein excretion and volume, in vitro transport of para-aminohippuric acid (PAH) by kidney slices, and kidney DNA content. Age and diet comparisons were made on 1 (young) and 2 (old) year animals. Age significantly increased urinary volume and protein excretion, decreased PAH transport, and had no effect on renal DNA content. Caloric restriction delayed the age-associated decline in renal function as indicated by a decrease in urinary protein excretion and an increase in PAH transport in Ras compared to A groups. Young and old animals fed restricted diets excreted larger volumes of urine than those fed ad libitum. Caloric intake had no effect on renal DNA content. Urinary protein excretion and volume significantly increased with dietary protein level in old animals. There was also a significant quadratic effect of protein and a quadratic protein-calorie interaction for urinary protein excretion in old animals, i.e., old animals fed ad libitum a medium protein diet had the highest mean protein excretion value. Dietary protein level had no significant effect on PAH transport. While there was no significant effect of protein level in the diet on kidney DNA content, there was a significant linear interaction of protein and calories in old animals; DNA values were positively associated with dietary protein level in old ad libitum fed groups but negatively V