Nitrogen Utilization during Caloric Restriction III. The Effect of Preceding Diet

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Abstract
Young adult rats were fed a protein-free diet or diets with varying amounts of protein for a two-week standardization period either on receipt from the supplier or after a pre-feeding period. Thus, 160 to 460 mg of nitrogen per day were supplied from a commercial stock diet, casein, egg albumin, soy globulin or Army ration. The animals were then subjected to 4 days of 50% caloric restriction on a constant intake of 160 mg of nitrogen. In one series, restriction was continued through 8 and 12 days. Nitrogen balance and liver composition were measured in all groups and, in addition, plasma protein and carcass composition were determined in the prolonged-restriction study. If standardization was begun on receipt, 46 to 48 Cal. per day were required to maintain constant body weight; after refeeding with a commercial stock diet, the requirement was 54 Cal. During restriction, body weight loss was significantly greater in animals fed the unmodified commercial stock diet than in those fed from any other source. During caloric restriction, the amount of negative nitrogen balance was directly proportional to the amount of nitrogen fed during standardization. Although this carry-over effect was greatest where egg albumin was the prior protein source, the variation due to level was smallest. Nitrogen balance was not significantly affected by protein source during restriction. Liver nitrogen losses were correlated with the initial nitrogen content of the organ which was chiefly related to tissue weight. There was no correlation between liver nitrogen loss and nitrogen balance, but carcass nitrogen changes were in good agreement with balance data.