Reproducibility of urinary 3-methylhistidine excretion in human subjects consuming freely selected diets

Eight males and six females consumed their normal diet for 9 weeks, except for weeks 3 and 9 when they followed a self-selected diet free of 3-methylhistidine (3MH) for five subsequent weekdays (days 1 to 5). Timed 24-hours urine collections were obtained on days 3, 4, and 5 of these 3MH-free dietary periods and on one day during weeks 1 and 7. At the group level, no differences in 3MH excretion (P greater than .05) between days 3, 4, and 5 in weeks 3 and 9 and between these weeks were observed. Reproducibility at the individual level was indicated by the within-subject coefficient of variation (CVw). CVw within weeks 3 and 9 was 2.5% and 5.1% for men and 4.8 and 8.0% for women. CVw between these weeks was highest when more than one measurement per week was performed. Relating 3MH to creatinine decreased the CVw between weeks. If muscle protein breakdown is to be measured at the individual level, a reasonably large biologic variation is to be taken into account and repeated measurements should be done to decrease the CVw.