Adaptation to underfeeding in growing rats. Effect of energy restriction at two dietary protein levels on growth, feed efficiency, basal metabolism and body composition.

Adaptation to a 50% restriction in energy at two levels of protein intake was studied in growing male and female rats. Growth was proportional to diet intake. The basal metabolic rate (BMR) (VO2/kilogram 0.75) was unaltered, whereas the percent fat deposited was reduced in diet-restricted rats at both 10% and 20% levels of protein. The percent of the total energy intake for tissue deposition was reduced in food-restricted groups to such an extent that the amount of energy available over what is required for tissue deposition was the same in all groups. The relationship between energy available for maintenance and that for BMR was low in diet-restricted groups. These observations suggest that growing rats adapt to low energy intake by reducing tissue deposition and by better utilization of energy for maintenance without reducing BMR. By lowering protein level in the diet feed efficiency is reduced in both diet-restricted groups and groups fed ad libitum.