The magnitude of the nitrogen balance under any conditions is determined by the difference between synthesis and breakdown. In contrast, the evidence suggests that rapid growth involves a fall in the rate of protein degradation. This has been shown in muscles of young growing rats, as well as in muscles in which hypertrophy has been induced by stretch or other stimuli. Experimental studies on muscle growth in general confirm the conclusion that, at least in muscle, rapid growth is associated with rapid rates of protein breakdown as well as of synthesis. Very similar results have been obtained in rapidly growing young pigs. The resultant of these two processes is that, over and above the basal rate of protein synthesis, 1.4 grams of protein have to be synthesized for 1 gram to be laid down. At the same time there is a smaller increase in the rate of protein breakdown. As might be expected, it increases linearly with the rate of weight gain. During the phase of rapid catch-up growth the rate of protein synthesis is increased. Whole body protein turnover has been measured in children recovering from severe malnutrition. Consequently, part of this paper will be devoted to questions of methodology. It is important to bear this in mind in evaluating the results. Different studies have used different methods, and all methods for measuring protein turnover have some disadvantages and sources of error. The key question is the extent to which net gain in protein results from an increase in synthesis or a decrease in breakdown or both. Some relevant information can, however, be obtained from measurements on normal growing animals, since rehabilitation and normal growth have in common a rapid rate of net protein synthesis. Not many studies have been done on protein turnover during recovery from malnutrition.
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