Evaluating Protein Quality by Estimating Energy Deposition Coefficients

Abstract

Concepts of nutritional energetics were applied in this study to evaluate protein quality. Isonitrogenous purified diets containing lactalbumin, commercially prepared non-defatted corn germ or unheated isolated soybean protein were fed for 3 weeks at different levels of intake to male weanling rats. The metabolizable energy intake (MEI) that main tained total carcass energy (carcass energy maintenance coefficient) and the efficiency with which MEI was utilized to promote an increase in car cass energy (carcass energy deposition coefficient) were determined for each diet. Changes in total carcass energy were partitioned between changes in the energy of carcass fat and carcass lean (fat-free carcass). The MEI that maintained the energy of carcass fat and carcass lean (fat and lean energy maintenance coefficients) were determined. The rates of retention of MEI as fat and as lean as intake of the diet increased (fat and lean energy deposition coefficients) were estimated for each diet. Regardless of diet, there were no significant differences among energy maintenance coefficients for the total carcass, carcass fat, or carcass lean. Energy deposition coefficients for both total carcass and carcass fat were significantly higher for the lactal bumin diet than for the corn germ or soybean diets, with no significant difference between the latter two. The energy deposition coefficient for carcass lean was significantly higher for the lactalbumin diet than for the corn germ diet which in turn was significantly higher than for the soybean diet. The observed differences among the energy deposition coefficients for carcass lean were attributed to differences with which growth of lean tissue was promoted by the three dietary proteins. The advantages of using energy deposition coefficients for evaluating protein quality are discussed. J. Nutr. 108: 1555-1565, 1978.