Nitric oxide (NO) is a very small but very important molecule in the human body. It plays a crucial role in many processes in the body. For example it is known that nitric oxide plays a role in the regulation of blood flow and blood pressure amongst many other functions.
Nitric oxide is made in the body from nitrites and nitrites can be made out of nitrate (from beetroot juice for example). Nitric oxide is also made from the amino acid arginine. It is often suggested that arginine supplementation can help NO production. The problem is that most of the arginine is broken down or converted before it reaches tissues. It is estimated that only 1% of ingested arginine is available. Another way to increase arginine in the body could be to ingest citrulline. This is not broken down but is converted in the body to arginine.
Studies with beetroot juice (a dietary source of nitrate) have suggested beneficial effects for athletes. Studies have shown that dietary nitrate supplementation can increase NO biomarkers (suggesting NO has indeed increased), reduce blood pressure and improve exercise economy/efficiency and exercise tolerance in healthy adults. There is however, a continuing search for other ingredients that can stimulate NO synthesis.
A recent study at the University of Exeter led by Dr Stephen Bailey investigated the possible role of citrulline and arginine.
Ten healthy adult males completed moderate- and severe-intensity cycling exercise on days 6 and 7 of a 7 day supplementation period with placebo, arginine or citrulline.
The order of the trials was randomised, the study included a placebo and all subjects performed all trials (cross-over study). Blood pressure was measured as well as blood arginine and nitrite concentrations. The performance test was a test that consisted of 10 min all-out exercise with a sprint in the last 60 seconds. Performance was measured as the total amount of work performed in the test as well as the ability to sprint at the end.
The first notable finding was that both arginine and citrulline resulted in elevated arginine concentrations in the blood. Blood pressure was only lower with citrulline supplementation but not with arginine. Citrulline also improved tolerance to severe-intensity exercise and increased the total amount of work completed in the exercise performance test. Arginine supplementation had no effects on blood pressure or performance.
The authors concluded that these results suggest that 7 days of citrulline (6g/day), but not arginine supplementation can improve blood pressure and
exercise performance in healthy adults.
I asked Dr Stephen Bailey what his advice would be for athletes. He replied: “Based on the findings from this study, consuming 6g of pure L-citrulline powder per day for 6-7 days prior to competition appears to be effective at improving endurance performance, at least in recreationally-active athletes. The last dose should be taken about 90 minutes prior to exercise to coincide with peak blood L-arginine concentration. However, oral L-citrulline dosHees of 15g have been well tolerated (Moinard et al., 2008) so it is unclear whether greater performance effects could be achieved with a larger dose or whether trained endurance athletes can benefit from L-citrulline supplementation”.
“The easiest way to ingest L-citrulline is in the form of powders of capsules. It is known that watermelon is a rich source of L-citrulline and that ingesting watermelon juice can increase plasma L-arginine concentration. However, based on reports of about 2.3 g of L-citrulline per L of unpasturised watermelon juice, an athlete would need to drink about 2.5 L of watermelon juice to take in the same L-citrulline dose administered in this study. This is unlikely to be a viable option prior to competition so use of L-citrulline powders or capsules is recommended.”
Summary of findings
The important new findings from this study are that short-term supplementation with citrulline:
increased the nitric oxide synthase substrate L-arginine
tended to increase plasma nitrite concentration, a marker of nitric oxide production
improved high intensity exercise performance
improved muscle oxygenation
resulted in faster oxygen uptake kinetics
SJ. Bailey, JR Blackwell, T Lord, A Vanhatalo, PG Winyard and AM Jones L-citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans. J Appl Physiol 2015 May 28:jap.00192.2014. doi: 10.1152/japplphysiol.00192.2014.