Relationship between endogenous 3-methylhistidine excretion and body composition

Abstract

Fourteen healthy men (aged 20-30 yr) consumed 2 isocaloric, isonitrogenous diets in the sequence of a 4-day meat diet (MD) followed by a 7-day meat-free diet (MFD). Urinary 3-methylhistidine (3MH) excretion during the MD (513 .+-. 21 .mu.mol.cntdot.day-1, mean .+-. SE) was significantly higher (P < 0.01) than day 3 of the MFD (230 .+-. 10 .mu.mol.cntdot.day-1), after which the mean daily 3MH output was constant with a mean coefficient of variation of 4.5%. There was no change in fat-free body mass (FFBM) determined by densitometry at the start (62.3 .+-. 1.8 kg) and the end (62.2 .+-. 1.9 kg) of the 11-day dietary period. Mean mucsle mass (MM) calculated from measurements of total-body K and N was 23.4 .+-. 1.3 kg. Endogenous 3MH excretion was related more closely to MM (r = 0.91, P < 0.001) than to FFBM measured by densitometry (r = 0.81, P < 0.001). Only a low correlation coefficient (r = 0.33, P > 0.05) was observed between 3 MH and the nonmuscle component of FFBM. Urinary creatinine output also was correlated significantly with 3MH (r = 0.87; P < 0.001) and MM (r = 0.79; P < 0.01). Because endogenous 3MH is significantly related to MM in man, it can apparently be used as a marker to study in vivo total-body muscle protein degradation provided that the necessary dietary restrictions are observed.