Longitudinal determination of total body creatine pool size and skeletal muscle mass in rats by D ₃ ‐creatine dilution

We recently described a new method to determine total body creatine pool size and skeletal muscle mass based on dilution of an oral dose of D 3 ‐creatine in urinary creatinine. Enrichment of D 3 ‐ creatinine was measured by isotope ratio mass spectrometry. Here we adapt the method to LC‐MS/MS, and investigate the potential for repeated application over time. Rats were given an oral tracer dose of D 3 ‐creatine (1.0 mg/kg body weight) at 10 weeks of age. Urine D 3 ‐creatinine enrichment was determined 72 hr after tracer administration and creatine pool size was calculated. Lean body mass (LBM) measured by quantitative magnetic resonance correlated with creatine pool size ( r = 0.92, P = 0.0002). The rate constant for decline in urinary D 3 ‐creatinine enrichment was slow and linear (2.73 ± 0.06 %/day). Subtracting background urinary D 3 ‐creatinine enrichment from the enrichment following a second dose of D 3 ‐creatine at 17 weeks permitted repeat calculations of creatine pool size. As at 10 weeks, 17 week LBM correlated with creatine pool size ( r = 0.98, P < 0.0001). In addition, the change in creatine pool size was correlated with the change in LBM during the 7 weeks of somatic growth between measurements ( r = 0.96, P < 0.0001). Thus, the LC‐MS/MS‐based D 3 ‐creatine dilution method can be applied repeatedly to measure skeletal muscle mass change in longitudinal studies. These studies were funded by GlaxoSmithKline.