The effectiveness of proper blood flow and gas exchange is especially important for athletes. During exercise, muscles consume oxygen and produce heat. The body receives these cues and responds by increasing blood flow to muscle groups and the skin. The former process enables the continuation of exercise (though may require other forms of energy) and the latter process dissipates heat.
At a fundamental level, the underlying feature of the vascular system that enables this distribution of blood flow is the ability to vasodilate or vasoconstrict. As you may guess, vasodilation means the vessels get wider and vasoconstriction means the vessels get narrower. Generally speaking, the body uses vasodilation to deliver more oxygen to critical areas with less resistance.
The body’s ability to execute this mechanism at a high level enables greater intensity and duration of muscular output. For athletes, this translates to more shots on goal through longer time on task or greater intensity during said task. [1,2]
Many processes can lead to vasodilation, but one pathway that has gained significant research interest is the nitric oxide (NO) pathway. The body can synthesize NO through the oxidation of the amino acid L-arginine, and several studies have demonstrated that increases in nitric oxide improves exercise performance and exercise economy.
Unfortunately, oral supplementation of L-arginine is inefficient and has had limited success in human models due to the body’s tendency to eliminate over 50% of available L-arginine after bacterial consumption and liver filtration. Some researchers have found that only 1% of an administered L-arginine dose is available for nitric oxide synthase, the molecule that converts the amino acid to nitric oxide. [1,2]
Interestingly, L-citrulline (an amino acid byproduct of the arginine/nitric oxide conversion process) is efficiently recycled back into L-arginine and is available for another round of nitric oxide production. Further, L-citrulline is not filtered by the liver and does not bind intestinal bacteria. After processing by the kidneys, L-citrulline is converted into L-arginine. Therefore, the majority of an orally administered L-citrulline dose is available for the desired target as arginine.[1,2]
Several studies have examined the effects of both L-citrulline and L-arginine on exercise performance. In one such study, L-citrulline supplementation led to a series of statistically significant findings. These included decreases in blood pressure, faster V02 kinetics, a 9% increase in peak power, a 7% increase in total work, and a 12% increase in exercise tolerance (time to exhaustion) over placebo. Overall, the study found that there was a 21% increase in the biomarker for plasma nitric oxide, and concluded that many of the improvements in exercise performance were due to improved oxygen delivery to muscle microvasculature. In the same study, there were no such significant differences when the athletes consumed L-arginine. 
In a separate study that examined the effects of L-citrulline supplementation on cycling performance tests, researchers found that the amino acid significantly increased plasma L-arginine levels and subsequently reduced the cyclists’ time to completion. 
While the link is not directly related to the production of nitric oxide, numerous researchers are now investigating the effects of plasma L-arginine levels on the availability of growth hormone. Though much of the research is still ongoing, one 2017 study found that L-arginine promotes the synthesis and secretion of growth hormone in certain cell types . In 2011, other researchers found L-arginine treatment led to increases in serum growth hormone, growth hormone mRNA, and growth plate width on long bones. 
For athletes, these findings support the use of L-citrulline as a way to supplement your exercise performance and overall health. That’s why we made sure to include 5 g of third-party tested L-citrulline in our stack, along with 5+ other key, science-backed supplements.
- Suzuki, Takashi. Oral L-citrulline supplementation enhances cycling time trial performance in healthy trained men: Double-blind randomized placebo controlled 2-way crossover study. Journal of the International Society of Sports Nutrition. 2016.
- Bailey, Stephen J. L-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans. Journal of Applied Physiology. 2015.
- Oh, Hyun-Seok. Effects of L-arginine on growth hormone and insulin-like growth factor 1. Food Science and Biotechnology. 2017.
- Ming-Yu, Jiang. Oral arginine improves linear growth of long bones and the neuroendocrine mechanism. Neuroscience Bulletin. 2011.