Dietary control of protein turnover.

The balance between protein synthesis and breakdown (protein turnover) regulates whole-body protein mass. The relationships between dietary changes (amount and composition of food) and protein synthesis, protein breakdown and amino acid oxidation have been explored in order to better understand adaptations of protein and amino acid metabolism. Methods for measuring protein synthesis, especially whole-body protein synthesis, can be divided into two groups: the 15N end-product method (urea and/or ammonia), and the incorporation of labelled amino acid(s) into proteins. Assumptions and limitations of the widely used two-pool model (free amino acid and protein pools) are discussed. Results obtained with different methods and for amino acids have been compared, to assess their ability to detect changes in protein synthesis rates. Methods of measuring protein breakdown have also been described briefly. Food intake affects whole-body and tissue protein turnover throughout development of animals and humans in different ways. Protein metabolism fluctuates during the 24-hour period in response to intermittent food intake. During the post-prandial period, a net whole-body protein deposition occurs. This is essentially due to increased protein synthesis. The free amino acid pool and amino acid oxidation rates also increase. Consequently, amino acids are used to a great extent as energy substrates. In contrast, a decrease in protein breakdown could enhance protein deposition. During fasting, the rates of whole-body protein synthesis are lower than those of protein breakdown. This results in protein loss, essentially because the drop in protein synthesis rate in muscle is pronounced. N balance is controlled by the amounts and composition of the diet and by changes in protein synthesis and breakdown. Increasing food intake above levels of energy equilibrium can produce growth by enhancing both the whole-body protein synthesis and breakdown rates. Below energy equilibrium, whole-body protein loss occurs because of decreased protein synthesis which becomes lower than protein breakdown. Protein synthesis rate is the main factor controlling N balance in response to alterations in food intake. Increasing dietary protein, especially the essential amino acids, involves increased rates of whole-body protein synthesis and breakdown. The improved N balance obtained by enhancing dietary non-protein energy (carbohydrate, fat) can be brought on by reducing amino acid oxidation and slightly increasing protein synthesis. The effects of dietary protein and energy on protein turnover are apparently additive.