Total body skeletal muscle mass: estimation by creatine (methyl-d3) dilution in humans

Abstract

Current methods for clinical estimation of total body skeletal muscle mass have significant limitations. We tested the hypothesis that creatine (methyl-d3) dilution (D3-creatine) measured by enrichment of urine D3-creatinine reveals total body creatine pool size, providing an accurate estimate of total body skeletal muscle mass. Healthy subjects with different muscle masses [n = 35: 20 men (19-30 yr, 70-84 yr), 15 postmenopausal women (51-62 yr, 70-84 yr)] were housed for 5 days. Optimal tracer dose was explored with single oral doses of 30, 60, or 100 mg D3-creatine given on day 1. Serial plasma samples were collected for D3-creatine pharmacokinetics. All urine was collected through day 5. Creatine and creatinine (deuterated and unlabeled) were measured by liquid chromatography mass spectrometry. Total body creatine pool size and muscle mass were calculated from D3-creatinine enrichment in urine. Muscle mass was also measured by magnetic resonance imaging (MRI), dual-energy x-ray absorptiometry (DXA), and traditional 24-h urine creatinine. D3-creatine was rapidly absorbed and cleared with variable urinary excretion. Isotopic steady-state of D3-creatinine enrichment in the urine was achieved by 30.7 ± 11.2 h. Mean steady-state enrichment in urine provided muscle mass estimates that correlated well with MRI estimates for all subjects (r = 0.868, P < 0.0001), with less bias compared with lean body mass assessment by DXA, which overestimated muscle mass compared with MRI. The dilution of an oral D3-creatine dose determined by urine D3-creatinine enrichment provides an estimate of total body muscle mass strongly correlated with estimates from serial MRI with less bias than total lean body mass assessment by DXA.

Keywords: creatine; creatinine; lean body mass; muscle mass.

Fig. 1.

Plasma D₃-creatine concentration time profiles for all subjects in the 30-mg dose group. Each individual line represents an individual subject. D₃, deuterated.

Fig. 2.

Mean cumulative percentage D₃-creatine dose excreted in urine over 5 days. Values are means ± SE. F, female; M, male; PM, postmenopausal.

Fig. 3.

Cumulative proportion of D₃-creatine dose excreted in urine over 5 days vs. the ratio of urine unlabeled creatine to urine unlabeled creatinine (Cr/Crn) on day 4, 0–4 h. Both parameters are natural log (ln)-transformed.

Fig. 4.

Mean urine D₃-creatinine enrichment ratio vs. time (sample intervals for urine collections). Values are means ± SE.

Fig. 5.

Boxplots of muscle or lean mass estimates by method for each subgroup of subjects. Cr, creatine; DEXA, dual-energy X-ray absorptiometry; MRI, magnetic resonance imaging; UCRM, urine creatinine method.

Fig. 6.

Total muscle mass from MRI vs. muscle mass from the D₃-creatine dilution method calculated using mean steady-state D₃-creatinine enrichment (r = 0.868, P < 0.0001). r, Pearson's partial correlation coefficient adjusted for sex.

Fig. 7.

Total muscle mass from MRI vs. dual-energy X-ray absorptiometry (DXA) appendicular lean mass (ALM) (r = 0.957, P < 0.0001) and DXA total lean body mass (LBM) (r = 0.923, P < 0.0001).

Fig. 8.

Total muscle mass from MRI vs. muscle mass from 24-h urine creatinine method (r = 0.597, P = 0.0004).

Fig. 9.

DXA total lean mass vs. muscle mass from the D₃-creatine dilution method calculated using mean steady-state D₃-creatinine enrichment (r = 0.745, P < 0.0001).