Experimental Variables in Predicting Protein Minima for Rats

Abstract

Data for predicting protein minima for 150-gm albino rats were obtained from feeding diets with 0 to 10% dried egg and 4 or 12% lard in a cross-over type of experiment. Adjustment to the metabolic stress imposed by protein intakes which did not meet normal protein requirements was reflected in diminished nitrogen excretion of rats on diets containing 0 to 4% dried whole egg. Scatter diagrams of data used for predicting minimum nitrogen intakes for maintenance of nitrogen equilibrium and body weight showed that points obtained in the second period of a cross-over experiment fell between those obtained in the first and third periods. This occurred regardless of the sequence in which the diets were fed. Thus, predicted minimum protein requirements of the rats reflected changes in protein stores during the course of the experiment. Linear and multiple regressions of caloric intake, nitrogen intake, nitrogen retention and weight change indicated that for these rats, weight changes in the area of predominately negative nitrogen balance were dependent on the ability of individual rats to retain nitrogen on a given intake, and nitrogen retention in this area appeared to depend on nitrogen intake. Linear regressions for predicting protein minima were reproduced in parallel experiments but were altered by varying the caloric value of diets. A 4% egg diet was completely adequate for maintaining weight and nitrogen equilibrium in the rats observed; however, nitrogen utilization was almost as efficient in the area above maintenance as in that below maintenance. It is recommended that the problem of estimating protein requirements from regression curves in the area of diminishing efficiency of utilization be investigated further.