Optimal protein intake estimated by the resistance to streptococcal infection and the nutritional indices in mice

Effects of dietary protein levels on the resistance against a bacterial infection and on the nutritional status were studied in mice to obtain basic data for the estimation of an optimal protein intake. Female DDY strain weanling mice were fed 5, 7, 10, 20, 30, or 40% casein diet. At 2 or 4 weeks on the diets, the mice were injected intraperitoneally with 5×103 or 5×104 group B streptococci/g body weight and their survival rates were observed for the following 10 days. Nutritional indices and cell numbers of thymus and spleen were also measured. The survival rate was higher in the order of 7, 10, 20, 30, 5, 40% casein diet group. Significant differences were observed between the 7% group and the 30, 5, or 40 group, and between the 10% group and the 5 or 40% group. The nutritional indices and cell numbers of the thymus and spleen were similar among the 20, 30, and 40% casein diet groups and decreased in the order of 10, 7, 500 casein diet groups. From the results, protein levels were categorized into 4 groups: severe protein deficiency with low resistance (5% casein diet), moderate protein deficiency with high resistance (7 and 1000 casein diets), normal protein intake with normal resistance (20% casein diet), and high protein intake with low resistance (30 and 40% casein diets). This grouping suggests that when the relation between immunocompetence and nutritional status is considered, both high and low protein intakes are undesirable and the optimal level of dietary pro-tein will be limited to a narrow range.