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Carcass Nitrogen as a Predictor of Protein Requirement for Mature Female Rats

Department of Human Nutrition and Foods, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

The minimal level of dietary protein required for maintenance of nitrogen equilibrium was investigated using female Sprague-Dawley rats, 12 months of age. Rats were fed casein, supplemented with methionine, at levels ranging from 0.84 to 5.18% of the diet for 4, 8 or 12 weeks and were compared to a control group fed a similar diet containing 9.90% protein or to a baseline group fed a stock diet. The effect of dietary protein levels on total serum protein and on liver and carcass composition was assessed. Protein requirement levels with 95% confidence intervals were predicted by linear regression. Dietary protein levels of 2.14% or lower were not adequate to maintain body weight and serum protein levels, and animals fed these diets had elevated levels of liver lipid. Protein nutriture of rats fed 3.20% protein was generally intermediate between those fed 2.14% or less and those fed 3.62% or greater. Compared to carcass nitrogen, carcass water was not as reliable a parameter for determination of protein requirements. Using the baseline group for comparison and carcass nitrogen as the dependent variable, a dietary protein requirement was predicted for these animals of 3.91–4.19% with confidence intervals of 3.50–4.56% and 3.13–5.48%, respectively. This level is comparable to the maintenance protein requirement of 4% indicated by the National Research Council for adult rats.

KEY WORDS: • protein requirements • carcass nitrogen

Manuscript received 18 August 1980.

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