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Effects of Protein Per Calorie Ratio and Dietary Level of Fat on Calorie and Protein Utilization¹

Department of Biochemistry, College of Agriculture, University of Wisconsin, Madison

The effects of changes in the protein per Calorie ratio of the diet and in the level of dietary fat on growth, body gain per Calorie consumed and body gain per gram of nitrogen consumed have been examined using young rats fed on diets containing casein as the source of protein.

Total gain and gain per Calorie consumed increased gradually with each increase in protein per Calorie ratio up to a level of 60 mg of protein per Calorie. The optimum protein per Calorie ratio for growth and calorie utilization is probably above this when casein is the dietary protein.

The body gain per gram of nitrogen consumed increased to a maximum at between 20 and 30 mg of casein per Calorie and then decreased. This maximum, which would be determined by the nutritive value of the protein, represents the point of optimum calorie utilization per unit of nitrogen consumed.

The evidence obtained indicated that the fat content of the diet influenced neither the percentage of ingested nitrogen retained nor the efficiency of calorie utilization. There did appear to be some transitory beneficial effect of additional fat in stimulating food consumption and growth during the first week of the experiment when the protein per Calorie ratio was held constant.

The average calorie intake per unit of body weight to the 0.87 power was almost constant regardless of the protein or fat content of the diet.

It has been concluded that calorie utilization and nitrogen utilization in the growing rat are not affected appreciably by the fat content of the diet but that protein to calorie ratio is an important factor to be considered in interpreting such measurements.

Manuscript received 6 July 1957.