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Energetic Efficiency of Rats Fed Low or High Protein Diets and Grown at Controlled Rates From 80 to 205 Grams¹

Department of Animal Science, Cornell University, Ithaca, NY 14853-4801

Energetic efficiency of protein and lipid deposition was estimated based on the data from 53 rats fed purified diets. Intercepts of equations relating energy gain to metabolizable energy intake were different (P < 0.002) for rats fed diets that contained 10 and 20% casein, so that metabolizable energy for maintenance was 118 and 108 kJ/day, respectively. Surprisingly, in view of the supposedly greater efficiency of lipid deposition, energetic efficiency above maintenance was constant at 0.38 ± 0.02. Part of the reason is that composition of body gain was much less rigidly tied to daily energy intake—even across the wide range of data in this experiment—than is necessary to quantify reliably such an effect. Two methods of quantifying partial energetic efficiencies of protein and lipid deposition gave results that were comparable to previous estimates of about one-third and two-thirds, respectively. One of the methods used a more biological model but failed to fit data for rats fed the 10% casein diet. Because traditional statistical methods are unlikely to be extended much further to account for the many unsolved problems associated with estimating partial efficiencies, systems based at tissue and cellular levels are necessary to model detailed differences in energy utilization.

KEY WORDS: • rats • energy • growth • efficiency • composition

¹ Supported in part by Cornell University Agricultural Experiment Station Hatch Project 426 and Northeast Regional Hatch Project 148, U.S. Department of Agriculture.

² To whom correspondence should be addressed.

Manuscript received 18 July 1990. Revision accepted 13 February 1991.