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Estimators of Body Nitrogen in Growing Rats Fed Varying Levels and Qualities of Protein¹

Department of Food Science, University of Georgia Experiment Station, Experiment, GA 30212

The relationships between total body nitrogen and body weight, carcass water and carcass [body minus gastrointestinal (GI) tract] nitrogen were investigated with Sprague-Dawley rats fed diets containing casein, peanut flour or wheat gluten as protein sources. Twelve diets containing 2–31% of each protein were fed to groups of six (12 for protein-free diet) weanling rats for 4 weeks. The animals were killed by sodium pentobarbital injection and weighed. GI tracts were removed and washed free of their contents. Carcass water was determined by oven drying; carcass and GI tract nitrogen were determined by Kjeldahl analysis. Linear regression equations for each relationship and protein closely fit the data (r² > 0.9). In each relationship the equations for casein and peanut protein were not significantly different (P < 0.05), but both were different from the equation for gluten. Despite high r² values, plots of deviations from regression showed a distinct pattern in several cases. Quadratic equations relating body nitrogen to the other parameters gave improved fit of the data in most instances. Quadratic equations for a given relationship were not significantly different for the three proteins. Net protein utilization (NPU) values based on body nitrogen estimated by linear equations were significantly different from those based on determined values in several cases. When quadratic estimating equations were used, the values were different only in one case.

KEY WORDS: • estimated body nitrogen • protein quality/quantity • net protein utilization

¹ Presented in part at the 63rd Annual Meeting of the Federation of American Societies for Experimental Biology, Dallas, TX, 15 April 1979.

Manuscript received 7 December 1979.