Single- and multiple-dose pharmacokinetics of oral creatine.

Supplementation with exogenous creatine (Cr) has shown physiological benefits in humans, but little is known about the pharmacokinetics of Cr in humans. Six healthy males completed an open-label study consisting of a full pharmacokinetic analysis following a single oral dose of Cr monohydrate (71 mg kg-1) and at steady-state after 6 days of Cr administration (71 mg kg-1 qid). After the single oral dose, the clearance (CL/F) was 0.20 +/- 0.066 L h-1 kg-1, tmax was 1.9 +/- 0.88 hours, and Cmax = 102.1 +/- 11.2 mg h L-1. At steady-state, CL/F decreased to 0.12 +/- 0.016 L h-1 kg-1, tmax did not change, and Cmax increased to 162.2 +/- 30.0 mg L-1. Penetration (AUCMUSCLE/AUCPLASMA) of Cr into the interstitial muscle space, as determined by microdialysis, was 0.47 +/- 0.09 and 0.37 +/- 0.27 for the single dose and at steady-state, respectively. Plasma and muscle data were simultaneously fitted with a model incorporating a saturable absorption and first-order elimination process. In conclusion, repeated dosing of Cr caused a reduction in clearance that could result from saturation of the skeletal muscle pool of Cr.