l-citrulline

L-citrulline is a non-essential amino acid, meaning the body can produce it on its own, and it also occurs naturally in foods such as watermelon. In the body it is converted into another amino acid, L-arginine, which in turn helps make nitric oxide, a signaling molecule that helps relax and widen blood vessels. Supplements are sold either as plain L-citrulline or as "citrulline malate," a form that combines it with malic acid.

According to the NIH Office of Dietary Supplements, the research supporting citrulline or citrulline malate as a way to enhance exercise or athletic performance is limited and conflicting, and does not provide strong support for its use. The few published studies have used differing designs and short durations and have produced inconsistent results. For example, one randomized trial reported that male weightlifters taking citrulline malate before bench presses to exhaustion completed more repetitions and reported less muscle soreness in the days afterward, while other studies have found no performance benefit. The NIH indicates that whether athletes in particular sports might benefit remains to be determined, and that more research is needed.

There is no established standard supplement dose. Studies summarized by the NIH have most often used a single dose of about 8 grams of citrulline malate taken roughly an hour before exercise, with some other research using a few grams per day over several days; these figures reflect research protocols rather than proven recommendations. Talk to a clinician or pharmacist before starting L-citrulline, especially if you take any medication, have a medical condition, or are pregnant or breastfeeding.

In the studies the NIH reviewed, citrulline and citrulline malate were generally well tolerated at the oral amounts studied, without reports of serious harm. However, the NIH notes that studies have not adequately assessed the safety of citrulline, particularly when it is taken in supplement form for months at a time, so long-term safety is not well established. As with all dietary supplements, the FDA does not review these products for safety or effectiveness before they are sold. People who are pregnant or breastfeeding, who have a health condition, or who take prescription medications should consult a healthcare professional before use.

Citrulline is generally reported as well tolerated in the short-term studies the NIH reviewed, and serious side effects were not commonly reported at the doses used. Mild stomach or digestive discomfort can occur in some people; for instance, in one trial a small number of participants reported stomach discomfort after taking citrulline malate. Because rigorous long-term safety data are limited, report any unusual symptoms to a clinician.

The cited NIH exercise fact sheet does not address drug interactions for citrulline. On a mechanistic basis only, because L-citrulline is converted to L-arginine and may influence nitric oxide and blood-vessel widening, there is a theoretical possibility it could add to the effects of medications that lower blood pressure or affect blood flow (such as nitrates or erectile-dysfunction drugs like sildenafil). This is not established by the cited source, and high-quality data on interactions are limited, so anyone taking prescription medications should check with a clinician or pharmacist before use.