Kinetics of specific food intake and weight gain in rat.

We report the expanded application of a recently developed time-invariant analytic function that relates food intake, growth (weight gain), and dietary nutrient concentration in the laboratory rat. Data from seven experiments were utilized, providing analysis of three stages of rat (Sprague-Dawley) growth (weanling, adult, and pregnant) and a variety of nutrients: casein, lactalbumin, amino acid mix, thiamin, and pyridoxine. For each experiment the rats were fed graded levels of a nutrient, ranging from 0% to a percentage well above the recommended amount, in an otherwise nutritionally adequate diet. Weight-specific weight gains and food intakes were calculated for each dietary group and fitted to the Gompertz equation. The resulting rate constants for each group were identical for weight gain and food intake. The rate constants were then fitted as functions of dietary nutrient concentration by the four-parameter mathematical model for physiological responses. Kinetic rate constants could be characterized as functions of the dietary concentration of each nutrient tested. This approach offers new possibilities for the determination and optimization of dietary requirements. With this approach, one may characterize food intake and growth in a single rate constant, which varies as a function of dietary nutrient concentration.