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Effects of Dietary Protein Content and Ratio of Fat to Carbohydrate Calories on Energy Metabolism and Body Composition of Growing Rats¹

Nutritional Laboratories, Department of Animal Science, The Pennsylvania State University, University Park, Pennsylvania 16802

Relationships among dietary protein, fat and carbohydrate were investigated in a modeling experiment using male rats and isocaloric diets factorially arranged to describe outcome responses of diets varying in protein content from 6 to 70% and in fat to carbohydrate calories ("ratio") from 0.2 to 1.4. Regression of body weight, weight gain and carcass dry matter, nitrogen and ash gain on protein and "ratio" was described by linear and quadratic coefficients for protein. Energy gain was optimal at 37.3% protein, Maximum quantities of heat of basal metabolism occurred at a dietary composition of 38.5% protein, 42.6% carbohydrate and 12.9% fat. Fecal and urinary nitrogen increased as dietary protein increased, but decreased as "ratio" increased. Body nitrogen gain, unaffected by "ratio," was maximal at 44% dietary protein. Digested energy and metabolized energy were unaffected by "ratio" at 40% protein, but at low and high protein, the effect of "ratio" was negative and positive, respectively. Heat production was constant at a "ratio" of 1.4; below and above 36% protein it was increased and decreased, respectively, by lower "ratios." Heat increment, unaffected by "ratio," was minimal at 46% dietary protein. Significance of this model to human and animal nutrition is discussed.

KEY WORDS: • energy • protein • nitrogen • fat • carbohydrate

¹ Authorized for publication January 12, 1971 as Journal Series no. 8909.

Manuscript received 6 May 1971.

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