The Metabolizable Energy of College Women on Low Protein Diets

The influence of dietary protein on the efficiency of energy utilization was investigated in a human metabolic balance experiment consisting of a six-day preliminary adjustment period followed by five six-day experimental periods during which two groups of four college women each consumed low-protein diets. The protein was largely from plant sources with one group receiving 36 g. daily (Diet A) and the other 47 g. (Diet B). Other nutrients were provided at recommended levels.

The Parr Adiabatic Bomb Calorimeter was used to determine the energy content of dried food, fecal, and urine samples collected in Periods 1, 3, and 5. The basal Diets A and B provided 2300 and 2303 Cal./day respectively, and energy supplements were required to maintain subjects' weights.

Nitrogen balance data indicated that the protein intake of 36 g. daily was borderline, and that 47 g. was adequate after a short adjustment period. Analyzed energy content of urine and feces revealed that all subjects excreted 2 per cent of the gross energy intake in the urine and approximately 4 per cent in the feces. Calorie excretions were the same for all subjects, and calories were utilized equally well by subjects at the two protein levels.

No relationship was found between urinary calories and either dietary or urinary nitrogen or between fecal energy and either dietary or fecal nitrogen. As dry matter of the feces increased, fecal calories increased. Metabolizable energy calculated by subtracting energy in the excreta from total intake increased as calorie intake increased. Basal energy expenditure of the subjects did not change during the study.

The lack of an influence of nitrogen intake or excretion on calorie excretion and the fact that metabolizable energy of subjects on both diets was the same and supplied from 1.2 to 2.2 times basal needs indicated that dietary protein did not influence calorie utilization under the conditions of this experiment. The data do not exclude the possibility that calorie utilization might be affected when protein intakes differ widely.