Application of saturation kinetics to model effects of energy restriction on protein utilization.

Diets were formulated to contain varying concentrations of lactalbumin (3, 6, 9, 12, 15 and 18%), with lactalbumin increased at the expense of the carbohydrate source (Formulation 1) or with the nonprotein composition held constant (Formulation 2). Diets were fed for 21 days to male weanling rats of the Sprague-Dawley strain at three levels of intake (7, 9 or 11 g/day). For each diet formulation, maximum body weight gain (Rmax) was predicted for each intake level by relating body weight gain and protein intake using the saturation kinetics model. Rmax and asymptotic standard errors at intake levels 1, 2 and 3 were 10 +/- 3, 50 +/- 16 and 80 +/- 12 g, respectively, for Formulation 1, and 12 +/- 3, 40 +/- 3 and 69 +/- 4 g, respectively, for Formulation 2. Combining diet formulations, Rmax expressed as percentages of Rmax at intake level 3 were approximately 15 and 60% at intake levels 1 and 2, respectively. Since intakes at levels 1 and 2 were approximately 64 and 82%, respectively, of that at level 3, the results of this study show that the percentage decrease in Rmax is much greater than the percentage decrease in intake. This study has demonstrated that saturation kinetics can be used to model the effects of energy restriction on protein utilization.