A Comparison of the Biological Values of Dietary Protein Incorporated in High- and Low-Fat Diets

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Abstract
The effect of fat as compared to isocaloric amounts of carbohydrate on nitrogen utilization in the severely protein-depleted and the growing rat was investigated using the Thomas-Mitchell procedure. The low-fat and the high-fat diets contained, respectively, 5 and 33% fat, mainly lard. Casein was the dietary protein tested. The rats were fed the test diets so that they received equal amounts of protein, crude fiber, vitamins, minerals and gross energy, the main difference between the two groups being in the fat intake. When fed in isocaloric amounts, dietary fat and carbohydrate have essentially the same effect on protein utilization in both the protein-depleted rat and the growing rat. Neither the apparent digestibility nor the true digestibility of casein is improved by the higher dietary fat level. Fat does not seem to spare endogenous nitrogen under conditions of satisfactory and equal caloric intake. Higher dietary fat is not found to improve the biological value of casein. In fact, the indications in one experiment are that a high-carbohydrate diet tends to improve the biological value of casein in the protein-depleted rat. The metabolic fecal nitrogen per gram food intake increases significantly when the dietary fat is increased because of a concomitant reduction in the food intake due to isocaloric feeding. This value, however, remains constant irrespective of the fat content of the diet if the rats are fed equal amounts of the low- and the high-fat diets.