The present study is unique in that it is the first double-blind study to monitor the effect of prolonged creatine supplementation at the level of the whole body, vascular compartment, and skeletal muscle. The performance data presented indicate that total time of a sprint to exhaustion at a constant power output following two hours of variable-intensity cycling is not influenced by 28 days of low-dose dietary creatine monohydrate supplementation. Sprint time, and therefore total power output, in the creatine group was not improved to a greater extent than that seen in the placebo group. Engelhardt et al.  and Vandeburie et al.  studied cyclists and triathletes consuming 6 g and 25 g creatine, respectively, per day for five days. These previous studies demonstrating an increased power output during alternating intensity, endurance exercise following creatine supplementation were different from the present study in a number of ways. In the study by Engelhardt et al., 12 triathletes cycled for 30 minutes at 3 mmol/l blood lactate followed by ten 15-second intervals at 7.5 Watts/kg interspersed with 45 seconds rest, a two-minute rest, ten more 15-second intervals, and another 30-minute cycling bout at 3 mmol/l blood lactate. The triathletes were able to generate 18% more power after than before creatine supplementation during the intervals. The subjects in the study, however, were not blinded as to treatment, with each subject undergoing the creatine cycling bout after the non-supplemented bout. Our study participants were blind to treatment or placebo, and performed a continuous sprint to exhaustion at a constant power output, rather than variable power during intervals in the study by Engelhardt et al.. In another cycling study demonstrating positive effects of creatine supplementation during timed intervals at maximal intensity, Vandeburie et al. studied twelve elite cyclists in a double-blind fashion . Vandeburie et al. allowed up to three minutes rest between a standardized 2.5 hr cycling bout and five, 10-second maximal intensity sprints that were used to gauge performance. Active recovery performed at 0.5 kg resistance was allowed for two minutes between each sprint. Although the cyclists were able to perform at 8-10% greater power outputs during the five 10-second sprints following creatine ingestion than following placebo ingestion, the three-minute recovery following the endurance ride may have influenced the results. It should also be noted that there was no difference in cycling time (approximately 10 minutes) for a cycling bout to fatigue performed at 4 mmol/l lactate threshold immediately at the end of the standardized endurance ride. A study by Rico-Sanz and Marco  also demonstrated improved performance (+6.5 minutes) in seven cyclists following creatine ingestion (20 g/day for 5 days) compared to seven cyclists consuming placebo. Performance in this study was measured as time to exhaustion (approximately 30 minutes) during alternating intensity exercise at 30% and 90% of maximal power output. The intensity and intermittent nature of the alternate-intensity cycling performance measure to exhaustion, as well as the high-dose supplementation regime in the study by Rico-Sanz and Marco was clearly different from our low-dose supplementation study with a performance measure of timed sprint to exhaustion at a constant power output. Muscle biopsy data, used to verify increases in muscle creatine phosphate content, are lacking in all of the studies described above, although blood analysis demonstrated a significantly higher plasma creatine and creatinine following supplementation in the study by Engelhardt et al. . The primary difference between the present study, demonstrating no improved performance, and past studies, demonstrating improved cycling performance, is likely the type of performance measure: sprint to exhaustion at a constant power output in the present study as compared to interval-type performance at self-paced intensity in other studies.
The lack of effect of creatine supplementation on performance in the present study is similar to the findings of Godly et al.  and Myburgh et al., published only in abstract form. Godly et al. detected no greater improvement in performance in eight cyclists consuming creatine (7 grams/day for 5 days) compared to eight cyclists who consumed placebo. Both groups were tested before and after the 5-day blinded supplementation period. The well-trained cyclists sprinted 15 seconds every four kilometers of a 25 km time trial performed in the laboratory on their own bikes . Myburgh et al.  also detected no difference in one-hour time trial after seven days of supplementation at 20 g/day. Thirteen cyclists were tested before and after the supplementation period, with seven cyclists ingesting creatine and six ingesting placebo. These data conflict with past reports of positive benefits of creatine ingestion on endurance performance, and indicate that there is no consensus as to the effect of creatine supplementation on endurance performance of continuous or variable-intensity cycling.
The potential benefits of creatine supplementation include enhanced muscle creatine phosphate and muscle glycogen content, increased plasma volume, and alterations in substrate selection and oxygen consumption. Although there were positive effects of this low-dose creatine compared to placebo supplementation with respect to resting muscle creatine phosphate and glycogen content, as well as increased plasma volume and reduced submaximal oxygen consumption during exercise, there was no greater improvement in sprint performance in the creatine than placebo group.
There have been only two studies of creatine supplementation other than the present study reporting oxygen consumption during endurance exercise. Rico-Sanz and Marco  demonstrated an increased oxygen consumption following creatine ingestion when cyclists cycled at 90% of maximal power output. In contrast, we detected an interaction of treatment (creatine and placebo) and time (pre and post supplementation) for submaximal oxygen consumption near the end of the cycling bout in the present study, indicating that creatine supplementation results in lower submaximal oxygen consumption when cycling at 60% VO2peak. Differences in intensity and duration of the protocol may account for the discrepant findings of the current study and that of Rico-Sanz and Marco. Englehardt et al.  also reported submaximal oxygen consumption data, and found no effect of creatine supplementation on oxygen consumption during cycling at 3 mmol/l blood lactate. In the present study, submaximal oxygen consumption was 8-9% lower following creatine supplementation than following placebo near the end of two hours of cycling (P < 0.05), although the cause of this reduced oxygen consumption is unknown.
No previous studies of creatine supplementation and endurance exercise have contained reports of respiratory exchange ratio. We found no effect of supplementation on respiratory exchange ratio, suggesting that creatine supplementation does not alter fuel selection. There was also no difference between creatine and placebo groups in the change in muscle glycogen during the cycling bout. There was a higher muscle glycogen concentration five minutes prior to the end of exercise in the post-creatine cycling bout compared to the post-placebo cycling bout, but this was likely due to the slightly elevated muscle glycogen content prior to the post-supplementation exercise in the creatine group.
The vast majority of previous studies of creatine supplementation have used a five to ten day supplementation at 20 g/day. Hultman et al.  demonstrated that the high loading phase of creatine is not necessary if a longer supplementation period (28 days) is used. Their protocol of three g/day for one month had not been replicated prior to the current study. We have found that 28 days of creatine supplementation at three g/day increases muscle creatine phosphate to levels above a placebo group post supplementation. The increases in muscle creatine phosphate and total creatine were of similar magnitude (approx. 10 and 20 mmol/kg, respectively) to those demonstrated by Hultman et al. . However, there also appeared to be increases, though not significant, in our placebo group of 5 mmol/kg and 10 mmol/kg and for creatine phosphate and total creatine, respectively. These data, in combination with our performance data demonstrating an increased performance that was not dependent upon the type of supplementation (creatine or placebo), highlight the importance of using a placebo group and a double-blind protocol. Although Hultman et al. included a placebo group in their study design, they did not take muscle biopsies from the control group.