The measurement of rates of protein turnover, synthesis, and breakdown in man and the effects of nutritional status and surgical injury

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Abstract
Current concepts of protein turnover, synthesis, and breakdown in man are reviewed. Emphasis is placed on the distinction between protein breakdown, by which constituent amino acids are released, and amino acid catabolism, which leads to the formation of excretory products, mainly urea and carbon dioxide. Methods of measuring overall rates of turnover, synthesis, and breakdown of body protein in man by the continuous or intermittent administration of radioactive or stable isotopes of amino acids are reviewed. Data from the published literature and from ongoing studies reveal that the overall rate of protein synthesis declines with age. There is close agreement between results obtained with different isotopes in normal adults. Malnourished infants have significantly lower rates of protein turnover, synthesis, and breakdown before than after recovery. These findings may represent another example of metabolic adaptation in the malnourished child. After surgical trauma in adults, the rate of protein synthesis is significantly reduced and neither protein turnover nor breakdown shows a significant change from preoperative values. It is conciuded from these studies that the negative nitrogen balance found after injury is accounted for by a reduction in protein synthesis and that protein breakdown remains unchanged after orthopedic operations. The relevance of these findings to the changes in nitrogen metabolism seen in infection is discussed.