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Kelsey McLaughlin, M.S.

L-citrulline (CIT), a nonessential amino acid that can be found in abundance in watermelon and watermelon rind, has garnered an increasing amount of attention among sport nutrition researchers for its potential benefit to sport performance, particularly in endurance events. The effects of CIT on an exercising individual are thought to be two-fold, both increasing blood flow to working muscle through the enhancement of nitric oxide (NO) production and enhancing clearance of fatigue-inducing metabolites through the urea cycle. CIT is a precursor to L-arginine (ARG), which is converted into NO by a reaction catalyzed by endothelial nitric oxide synthase (eNOS). CIT supplementation has proven more effective than ARG supplementation in increasing plasma ARG levels as orally ingested ARG is degraded by the enzyme arginase, whereas CIT is not degraded in any large amount and is converted efficiently into ARG, causing a peak of ARG in the blood around 2 hours after consumption (Cutrufello, 2015). As ARG is also an intermediate in the urea cycle, this increase in plasma ARG can improve the removal of waste, including exercise metabolites from the blood, reducing the stress on exercising tissue (Schaefer, 2002).

For this reason, CIT is frequently included in pre-workout supplements, though its direct effects are difficult to determine in such a context due to the presence of many other ergogenic aids. Previous research on the subject has yielded mixed results as to the benefit of CIT to exercise performance, though methodological differences in the dosage and timing of CIT, the exercise protocol used, and the inclusion of other chemical compounds such as malate limit the conclusions that can be drawn about the effectiveness of pure CIT supplementation (for review, see Bescós, 2012). Indeed, no studies to date have utilized performance on a race-length aerobic exercise trial to test whether CIT supplementation would confer an advantage for a trained endurance athlete such as a cyclist.

To investigate claims of performance benefits and to determine the extent to which they may be attributable to CIT itself, the Applied Exercise Science Laboratory at Texas A&M University will be comparing the effects of 7 days of CIT supplementation against placebo on aerobic performance in recreationally trained triathletes. The proposed study will look at both the impact of supplementation on race time for a simulated 40 km cycling stage as well as the maximal power output that can be attained by these athletes on the cycle ergometer shortly after completing the time trial. If CIT supplementation is effective in improving the delivery of oxygen to working muscles and removing metabolites, it would be expected that time to complete 40 km would decrease and the maximal power output after fatiguing exercise would be greater. This could be of great significance to athletes who wish to be able to perform at a high submaximal intensity for the majority of a long endurance race, and then sprint to the finish in the last 30 seconds of the race, potentially boosting their finishing position relative to other racers.

Suggested Readings:

1. Cutrufello, P.T., et al., The effect of L-citrulline and watermelon juice supplementation on anaerobic and aerobic exercise performance. J Sports Sci, 2015. 33(14): p. 81459-66.

2. Schaefer, A., et al., L-Arginine reduces exercise-induced increase in plasma lactate and ammonia. Int J Sports Med, 2002. 23: p. 403-7.

3. Bescós, R., et al., The effect of nitric-oxide-related supplements on human performance. Sports Med, 2012. 42(2): p. 99-117.

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