Can Citrulline Really Enhance Athletic Performance?

L-citrulline, a non-protein amino acid, is made by the body and used as a supplement for exercise performance because of cardiovascular effects optimizing blood flow. In the body, citrulline is converted to L-arginine (another amino acid), which produces nitric oxide (NO), a small molecule that dilates blood vessels to allow working muscle to receive more oxygen and nutrients, and remove metabolic waste. This optimized blood flow may help performance and accelerate recovery.

Citrulline is supplied as a dietary L-citrulline malate supplement (a powdered capsule) or from watermelon, the only food source high in citrulline (with variable citrulline content depending on watermelon species and maturity).

Studies suggest citrulline may benefit exercise. However, there’s no recommendation for citrulline intake¹ , which means the appropriate amount of citrulline (g/day) to take and when to take it (before exercise or in the short term) is unclear. Therefore, studies investigating citrulline and exercise use different dosages and supplementation procedures.

In this article, I will discuss:

• Effects of citrulline and arginine in the body
• Studies investigating the various benefits of citrulline supplementation on exercise (while emphasizing dosage)
• Difficulties applying the science

Citrulline and Arginine

Citrulline converts to arginine in the kidneys. Arginine is an essential precursor for many important biological molecules, including NO and creatine. NO optimizes blood flow, which allows nutrients and oxygen to enter muscle and ammonia to be removed during recovery from exhaustive exercise.² Arginine is the main precursor of one of the two pathways to NO synthesis, and arginine blood levels are the limiting factor for NO synthesis in endothelial cells.³

Therefore, citrulline intake leads to more arginine and increased production of NO. There must be enough citrulline intake to increase arginine levels so that NO synthesis can occur. Citrulline blood levels (plasma) are the main barriers in the conversion of citrulline to arginine results.⁴

Citrulline intake leads to more arginine and increased production of nitric oxide. Share on X

Why not supplement with arginine? Arginine supplementation increases plasma arginine, but citrulline supplementation increases plasma arginine for a longer period of time. In fact, citrulline is more efficient at increasing arginine levels than arginine itself because arginine is mostly used by the liver, and citrulline is available for the whole body. Also, arginine supplementation of at least 10g can lead to diarrhea. Currently, citrulline doesn’t have any side effects, so it’s a preferred source over arginine.

Citrulline also is part of the urea cycle; an important pathway that removes ammonia from muscle and the liver.

Plasma Citrulline and Arginine Levels

The role of citrulline and arginine in stimulating NO synthesis was investigated in moderate- and severe-intensity cycling exercise on days six and seven during a seven-day supplementation period using 6g/day of arginine and 6g/day of citrulline.⁵ The study did not use g/kg/day, but based on the average weight of the participants, it was approximately 0.08g/kg.

The results were:

• Plasma arginine increased to a similar extent with arginine and citrulline supplementation, but plasma citrulline only increased with citrulline supplementation.
• Plasma nitrite (another substrate for NO) increased with arginine and increased with citrulline.
• Blood pressure was only lower with citrulline supplementation.
• Citrulline improved tolerance to severe-intensity exercise and the total amount of work completed.
• Arginine did not affect blood pressure or performance.

The study concluded that short-term citrulline supplementation (6g/day for seven days) may improve blood pressure and exercise performance.

Another study found improved blood flow after increasing NO, following 5.6g/day of citrulline for seven days. The improved blood flow correlated with increases in arginine levels.⁶

Further support of a dose of 6g citrulline (0.08g/kg) given to pre-professional male cyclists two hours before exercise found a 173% increase in plasma citrulline, a 123% increase in plasma arginine, and a significant increase in NO production.⁷ Creatinine also increased, which suggested that increased arginine may lead to creatine synthesis. This same dose (0.08g/kg) and a similar increase in citrulline and arginine levels were also found in 17 male professional cyclists.⁸

However, two other studies used a slightly higher amount of citrulline, 0.18g/kg (~12g of citrulline for a 150lb person). One study gave five equal dosages of citrulline at three-hour intervals within 12 hours to increase circulating levels of citrulline, which increased citrulline six times its original amount and doubled plasma arginine levels.⁹ The other study found an increase in plasma arginine and an elevenfold increase in plasma citrulline.¹⁰ Therefore, citrulline supplementation can be used to increase citrulline and arginine availability in the body.

Effect on Exercise

A study investigated the effect of different citrulline dosages and loading protocols on incremental treadmill tests to exhaustion after citrulline supplementation (9g 24 hours before test or 3g three hours before test).¹¹ Citrulline ingestion over 24 hours reduced time to exhaustion by 0.8% compared to citrulline supplementation three hours prior.

Citrulline supplementation of 2.4g/day for seven days and 2.4g of citrulline one hour before a 4-km cycling time trial reduced completion time by 1.5% in trained males.¹1 Citrulline also improved feelings of muscle fatigue and concentration immediately post-exercise.¹1

An 8g dose of citrulline has shown to increase resistance exercise performance in males,^12,13 , but due to physiological difference between sexes, the results could not be applied to females. Therefore, two recent studies have investigated citrulline in females.

One study found an increase in upper and lower body resistance exercise performance and lowered rating of perceived exertion during upper-body exercise when 8g of citrulline was taken one hour before.¹⁴ Another study found that one hour after consumption of 8g of citrulline, there was an increase in maximal and average grip strength and peak and explosive power in female master-level tennis players, which was suggested to improve tennis-match play.< sup>15

Effect on Muscle Soreness and Fatigue

Muscle soreness from an anaerobic weightlifting exercise was reduced by ~40% (24 hours after) and ~42% (48 hours after) for those taking 8g of citrulline.¹⁶ Citrulline delayed fatigue by enhancing more reps performed per set for each set (except the first two sets).

Another study providing 6g/day of citrulline for 22 days found reduced reports of fatigue from men who usually complained of fatigue.¹⁷

When Does Citrulline Stop Being Effective?

A dose-response study investigated citrulline loading (2g, 5g, 10g, and 15g) and found that higher amounts of citrulline increased plasma citrulline (tenfold at 2g and a hundredfold at 15g).¹⁸ After citrulline supplementation, plasma citrulline increased to a maximum and then decreased to baseline levels within three to five hours. Also, 15g of citrulline did not cause GI problems like arginine does at 10g.

The study concluded that saturation of citrulline begins to occur at 15g, and 10g may be an appropriate amount to use. This lower amount supports another study that found 12g of citrulline taken one hour before a time-to-exhaustion test on the bike did not provide ergogenic benefits in well-trained males.¹⁹

Difficulties With Applying the Science

Citrulline may provide various exercise benefits. However, the data is slightly inconclusive for us to apply the conclusions of these studies.

The dosage of citrulline in these studies ranged from: 2.4g to 15g.

It’s unclear how much citrulline we need. Even though there’s no recommendation on citrulline intake, it’s suggested that a 70kg person should be able to tolerate a daily dose of arginine from a normal diet (4-6g/day).²⁰ Therefore, the dose of 6g of citrulline could be the same as the daily intake of arginine.

Moreover, maybe the amount should be provided in grams per kilogram of body weight (similar to how protein requirement is determined to take into account larger and smaller athletes). If so, it appears that citrulline supplementation may be 0.08-18 g/kg/day.

The amount needed must also consider citrulline bioavailability, which may be limited by intestinal absorption.²¹ Citrulline bioavailability is higher when contained in its natural form (e.g., watermelon), but further information on this is out of the scope of this article.

It’s also unclear when to supplement with citrulline (e.g., 1-2 hours before exercise or days prior to exercise).

Ultimately, there’s no conclusive evidence that citrulline supplementation enhances exercise performance.²² We need more science—especially using athletes—on the minimal amount of citrulline needed and when to take it, to better understand how we can effectively use citrulline to enhance exercise performance.

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References

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