Oral creatine supplementation decreases plasma markers of adenine nucleotide degradation during a 1-h cycle test

We investigated the effect of oral creatine supplementation (20 g d(-1) for 7 days) on metabolism during a 1-h cycling performance trial. Twenty endur ance-trained cyclists participated in this double-blind placebo controlled study. Five days after familiarization with the exercise test, the subjects underwent a baseline muscle biopsy. Thereafter, a cannula was inserted int o a forearm vein before performing the baseline maximal 1-h cycle (test 1 ( T1)). Blood samples were drawn at regular intervals during exercise and rec overy. After creatine (Cr) loading, the muscle biopsy, 1-h cycling test (te st 2 (T2)) and blood sampling were repeated. Resting muscle total creatine (TCr), measured by high performance liquid chromatography, was increased (P < 0.001) in the creatine group from 123.0 +/- 3.8 - 159.8 +/- 7.9 mmol kg( -1) dry wt, but was unchanged in the placebo group (126.7 +/- 4.7 - 127.5 /- 3.6 mmol kg(-1) dry wt). The extent of Cr loading was unrelated to basel ine Cr levels (r=0.33, not significant). Supplementation did not significan tly improve exercise performance (Cr group: 39.1 +/- 0.9 vs. 39.8 +/- 0.8 k m and placebo group: 39.3 +/- 0.8 vs. 39.2 +/- 1.1 km) or change plasma lac tate concentrations. Plasma concentrations of ammonia (NH3) (P < 0.05) and hypoxanthine (Hx) (P < 0.01) were lower in the Cr group from T1 to T2. Our results indicate that Cr supplementation alters the metabolic response duri ng sustained high-intensity submaximal exercise. Plasma data suggest that n ett intramuscular adenine nucleotide degradation may be decreased in the pr esence of enhanced intramuscular TCr concentration even during submaximal e xercise.