We explored the washout period of creatine (Cr) after repeated ingestions of high doses of exogenous Cr. Ten athletes ingested daily, in a randomized double-blind study design, 30 g of exoge- nous Cr (n = 5, Cr-group) or a placebo (n = 5, Pl-group). Serum and urine samples were collected 1) before supplementation (BEFO- RE), 2) after one week Cr supplementation (AFTER), and 3) one week later without supplementation (LATER). The Cr and crea- tinine (Crn) concentrations in serum (sCr, sCrn) and in multiple spots urine (uCr, uCrn) were measured. We observed a significant rise (p < 0.01) in sCr, uCr and sCrn between BEFORE and AFTER supplementation in Cr-group, as well as a significant difference between Cr-group and Pl-group. Body weight increased signifi- cantly (+1.5 kg), but relative body fat (%fat) was unchanged. After the washout period in LATER Cr-group, sCr and uCr decreased to low residual values. No loss of body weight occurred during thisperiod. In contrast, sCrn and uCrn returned to baseline values. In conclusion, regular uptake of high doses of exogenous Cr affects both Cr and Crn concentrations in serum (sCr: 14 folds; sCrn: 1.2 folds) and urine (uCr: 140 folds; uCrn: 1.5 folds). An abuse of Cr is therefore mostly spilled over in urine. Surprise drug tests, such as doping controls, happening during the period of Cr supplementa- tion can reveal an important increase in Cr and Crn concentrations, although subjects stopped suddenly Cr loading. The discernible effect of Cr supplementation on these values disappeared within one week
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