3-Methylhistidine as an Index of Rate of Muscle Protein Breakdown
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Skeletal muscle constitutes the major tissue of the body. Using 15N-labeled amino acids, the total daily turnover of body protein in a 70 kg man approximates 300 g (5). Based on the estimated release of amino acids into the plasma by the total skeletal musculature of a fasting man, muscle protein turnover accounts for at least 75 g daily (7). However, this must be an underestimate, since amino acids released within the muscle cell can be used again for protein synthesis. This recycling would represent breakdown without release into the blood-stream. On the other hand, the amount released is the net balance which can vary from negative during fasting to positive after meals. As will be seen later, our data suggest that the true daily turnover of muscle protein is about twice the net release of amino acids into the blood of the fasting subject.
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