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Influence of Dietary Carbohydrate-to-Fat Ratio on Whole Body Nitrogen Retention and Body Composition in Adult Rats¹

Linda J. McCargar^*,2, Vickie E. Baracos and M. Thomas Clandinin^*,,3,4

^* Nutrition and Metabolism Research Group ^* Department of Foods & Nutrition, Faculty of Home Economics Department of Medicine & Division of Gastroenterology, Faculty of Medicine Department of Animal Science, Faculty of Agriculture and Forestry, University of Alberta, Edmonton, Alberta, Canada T6G 2P5

Experiments have shown that the amount and source of dietary energy may alter protein metabolism. A high fat diet has resulted in greater nitrogen retention than a high carbohydrate (CHO) diet. To examine this question further, adult rats were fed diets providing ratios of CHO:FAT as a percentage of energy of 0.5, 1.0, 1.5, 2.0, 2.5 or 3.0 for 6 wk. Mean energy and protein intakes were 93.0 ± 0.8 kcal/d and 5.3 ± 0.1 g/d, respectively. Final body weight was lower in rats fed the high fat diet (CHO:FAT, 0.5) than in rats fed the high carbohydrate diet (CHO:FAT, 3.0) (P < 0.05), and a linear response was observed over the entire range of treatments (r = 0.92). Rats fed the high fat diet had the highest nitrogen balance; values were significantly (P < 0.05) different from those of rats fed high carbohydrate diets (CHO:FAT, 2.0 or 2.5) when expressed as mg nitrogen/kcal energy gain. Rats fed the high fat diet had the highest protein gain and the lowest fat gain as a function of energy gain. It is concluded that alterations in nonprotein energy source result in metabolic changes, which may be related to adaptations in energy expenditure and/or protein deposition.

KEY WORDS: • carbohydrate • fat • nitrogen balance • body composition • rat

¹ The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

² L. McCargar was the recipient of an Alberta Heritage Foundation for Medical Research Studentship.

³ M. T. Clandinin is a Scholar of the Alberta Heritage Foundation for Medical Research.

⁴ To whom reprint requests should be addressed.

Manuscript received 14 February 1989. Revision accepted 27 April 1989.