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The Protein Requirements of the Albino Mouse

(From the Department of Biochemistry, School of Medicine, Western Reserve University, Cleveland.)

Young mice were fed complete diets of the "synthetic" type in which the protein (casein) levels were, respectively, 0.95, 1.96, 4.0, 7.8, 15.6, 23.2 and 49.7 per cent of the total calories ingested. The food consumption and changes in body weight were measured between the ages of three and seven weeks. All the animals ate approximately equal quantities of food, except those fed the highest and lowest protein levels, respectively, where the intakes were less, though perhaps not significantly so. The mice fed 0.95 per cent casein lost weight, those fed 1.96 per cent maintained weight, and those fed 4.0 per cent grew at a definitely sub-normal rate. The mice fed 15.6, 23.2 and 49.7 per cent casein grew at nearly identical rates and attained almost the weight of animals fed upon a stock diet. Mice fed upon 7.8 per cent casein grew at a slightly sub-normal rate. The reduced glutathione concentration in the muscles of the mice was about the same in all animals that grew, even though the growth might be stunted. The mice fed 7.8 per cent casein had about half as much glutathione in the liver as mice fed higher amounts of casein, indicating that their growth, which was also the most efficient in terms of gain in weight per gram of ingested casein, was under a condition of nutritional stress. It is concluded that, contrary to what has been believed, the protein requirements of the mouse are certainly fulfilled by diets containing 15.6 per cent casein, and probably somewhat less would also be satisfactory. These are about the same as the requirements of the rat, the differences due to metabolic rates becoming apparent when the gain in weight per gram of ingested protein is measured.