The short-term effects of protein intake on 3-methylhistidine excretion.

The urinary excretion of 3-methylhistidine is used as a convenient index of muscle protein catabolism. Histidine is methylated in peptide linkage within muscle protein, and is quantitatively excreted when the protein is catabolized. 3-Methylhistidine would be expected to be present in ingested muscle protein, and unless altered in the digestive process, to be excreted as a function of intake. We studied its excretion at four levels of mixed protein intake, in separate groups of obese subjects, on the 1st day of altered input, with and without concurrent energy provision. A significant (P less than 0.001) linear relationship between protein intake and 3-methylhistidine excretion was observed, with a mean excretion of 198 mumoles/day at 0 intake, and an increment of 1.34 mumoles/g of ingested protein. There was no relationship between its excretion and the concurrent nitrogen balance. Thus, estimates of muscle protein catabolism with this method require that muscle protein be absent from the diet or its contribution quantified.