Protein-energy interrelationships during dietary restriction: effects on tissue nitrogen and protein turnover

Young adult female rats were fed diets containing either 50 or 0% lactalbumin at levels of 2, 4 or 6 g diet/day for 2, 8, and 16 days. There was no other protein in the diet. Tissue nitrogen and loss of radioactivity from tissues labeled with 14C- and 3H-glutamate were measured. In a second study, similar rats were fed graded levels of lactalbumin at food intake levels of 3, 5, or 7 g/day. Change in tissue nitrogen varied with the tissue, the time of observation, and the severity of the food restriction. After 8 days, animals fed high levels of protein at the most severe food restriction showed increases in gastrocnemius nitrogen and losses in liver nitrogen, while after 16 days both tissues had marked nitrogen losses. Nitrogen losses at the lowest level of food intake increased with the dietary protein level, whereas dietary protein was protective of tissue nitrogen at higher food intakes. Severity of energy restriction had no effect on loss of tissue radioactivity and the apparently longer tissue protein half-lives from animals fed protein-free diets are attributed to increased amino acid recycling. Such results indicate that short-term studies and overall nitrogen balance experiments will fail to identify changes occurring in different tissues and may yield misleading results.