Evaluation of protein in foods. XII. Effects of calorie restriction.

Abstract

The effects of calorie restriction on net protein utilization (N.P.U.) were studied with proteins of different nutritional value. When protein intake remained constant, reduction in the intake from 32 to 16 kcal per day resulted in a rapid decline in N.P.U. values. Increases in body weight were directly proportional to calorie intake per kg0.73 body weight. Even under conditions of marked caloric restriction, only 70–75% of the ingested protein was used for energy purposes, the rest evidently being used in essential anabolic activities. Studies on carcass composition showed that animals on restricted calorie intakes tried to adapt through shifts in metabolism, and utilized fat in preference to protein for caloric requirements. RNA metabolism in liver and muscle was influenced by energy and protein intakes.When the percentage of calories from protein remained constant, N.P.U. values were not markedly altered until the calorie intake reached the maintenance energy requirement of 150 kcal per kg0.73 body weight. Below this point, N.P.U. values dropped sharply, the rate of decrease being greater for proteins of higher nutritive value. Differences in the nutritive value of proteins, as shown by their ability to satisfy protein requirements in protein-depleted rats, still existed even at restricted intakes of calories.