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. Can it help exercise performance? Specifically high intensity exercise performance?
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.
It is estimated that only 1% of ingested arginine is available
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.
What did they find?
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.
7 days of citrulline (6g/day), but not arginine supplementation can improve blood pressure and exercise performance