L-Citrulline Benefits: What the Research Says

In short: L-citrulline is an amino acid your body turns into arginine and then nitric oxide, a molecule that helps blood vessels relax. The best-supported L-citrulline benefits are small: a few extra reps in hard lifting sessions and a little less next-day muscle soreness. It doesn’t seem to help endurance exercise, and the evidence that it boosts growth hormone is weak.

L-citrulline shows up in almost every “pump” or pre-workout supplement, often with big promises attached. So what are the real L-citrulline benefits once you look at the studies? Here’s a plain-English look at what the research actually shows, how much people take, and where the evidence is still thin.

What is L-citrulline?

L-citrulline is a “nonessential” amino acid, which simply means your body can make its own. It’s part of the urea cycle, the process your liver uses to clear ammonia (a waste product of breaking down protein), alongside two related amino acids, arginine and ornithine.

Its name comes from watermelon (Citrullus), one of the richest known food sources [1]. US Department of Agriculture researchers measured roughly 1–2 mg of citrulline per gram of fresh watermelon flesh or rind [1]. Even at the top of that range, you’d need to eat around 7 pounds (about 3 kg) of watermelon to match a typical 6-gram supplement dose, and red-fleshed watermelon, the most common kind, had less citrulline than yellow or orange types [1].

Supplements come in two main forms:

• L-citrulline: the amino acid on its own.
• Citrulline malate: citrulline bound to malate (malic acid), a compound involved in energy production. Products are usually sold as 2:1 or 1:1 blends, so 8 grams of citrulline malate gives you only about 4–5.3 grams of actual citrulline by weight. Read more about malic acid.

How L-citrulline works: the arginine shortcut

Your body uses arginine to make nitric oxide (NO), which signals blood vessels to widen. You might think taking arginine directly would be the obvious move, but much of an arginine pill gets broken down in your gut before it reaches your bloodstream. Citrulline is readily absorbed, and your body then converts at least part of it into arginine [2].

In a double-blind, placebo-controlled crossover study in 20 healthy volunteers (each person tried every option for a week), oral citrulline raised blood arginine levels more effectively than arginine supplements did [2]. Another small study of 8 healthy men found citrulline is a potent source of arginine, though the conversion seems to level off at very high doses of 10–15 grams [3]. This is why citrulline is often seen as a better way to raise arginine than taking L-arginine supplements.

One important catch: higher arginine and nitric oxide markers in the blood don’t automatically mean better blood flow to your muscles. A 2020 review found the evidence for an actual “pump” or improved muscle blood flow after citrulline is scarce and inconsistent [4].

L-citrulline benefits for strength training

This is where the evidence is strongest, though the effects are modest.

Some early studies looked impressive. In a 2010 crossover trial, 41 men took 8 grams of citrulline malate or a placebo before a chest workout. With citrulline malate they managed more bench press reps in the later sets and reported about 40% less muscle soreness over the next two days [5]. Studies in 12 advanced male weightlifters doing leg exercises [6] and in 15 resistance-trained women doing bench and leg presses [7] also found more reps to failure with citrulline malate.

When researchers pool all the studies together, the picture is more modest:

• A 2019 meta-analysis (a study that combines the results of many trials) of 12 studies and 198 people found a small, borderline benefit for high-intensity strength and power exercise [8].
• A 2021 meta-analysis of 8 studies found that 6–8 grams of citrulline malate taken 40–60 minutes before training increased reps to failure by about 3 reps on average (roughly 6%) [9].
• A 2022 meta-analysis of 4 trials found no effect on maximal muscle strength in trained adults [10].

Put simply, citrulline malate may help you squeeze out a couple more reps when you train close to failure, but it doesn’t seem to make you stronger on a one-off max lift. The individual studies are also small, usually 10 to 40 people.

Muscle soreness and how hard exercise feels

A 2020 meta-analysis of 13 trials (206 people) found that citrulline lowered “rating of perceived exertion” (how hard a workout feels) and reduced muscle soreness 24 hours after exercise. It didn’t lower blood lactate, and there was no clear effect on soreness at 48 or 72 hours [11].

Does citrulline help endurance exercise?

Probably not. A 2022 meta-analysis of 10 studies found no meaningful benefit of citrulline for aerobic (endurance) performance, oxygen use, or perceived effort [12]. A 2021 review of nitric-oxide-boosting foods also found no performance benefit from citrulline-rich foods like watermelon, though it included only three such studies [13].

Even for repeated sprints, results are mixed. In one study, 10 well-trained men took a large 12-gram dose of citrulline malate and saw no improvement in cycling sprints or time to exhaustion [14]. An older, much-quoted study did report less fatigue and better muscle energy production, but it had no placebo group and involved 18 men who complained of fatigue doing finger exercises inside a scanner after 15 days of 6 grams a day [15], which is a long way from a real workout.

Does L-citrulline boost growth hormone?

This claim is often repeated, but it rests on thin evidence. In one study of 17 young male cyclists, growth hormone rose in everyone after a 137 km (85-mile) race stage, but it rose more in those who took 6 grams of citrulline malate 2 hours before [16]. But that was a short-lived spike during extreme exercise, and it doesn’t show any effect on muscle growth.

At rest, a study giving healthy men 2 to 15 grams of citrulline found no change in growth hormone or insulin at all [3]. So citrulline isn’t a reliable growth hormone booster.

Muscle building

Citrulline has some interesting effects on protein metabolism, mostly in people who aren’t eating enough protein. In a small pilot study of 8 healthy adults on a low-protein diet for three days, citrulline increased muscle protein synthesis (the process of building new muscle protein) compared with other amino acids [17]. But in 12 well-fed healthy adults, a week of high-dose citrulline didn’t change whole-body protein metabolism [18].

A 3-week trial in 24 malnourished older adults (mostly women aged 80 to 92) found no effect on protein synthesis overall, although in a subgroup analysis the women taking 10 grams of citrulline a day gained some lean mass compared with their starting point [19]. These are early, small studies in specific groups, so they don’t tell us much about whether citrulline helps healthy gym-goers build muscle.

Other L-citrulline benefits: blood pressure, blood vessels and erections

Blood pressure

The results here are mixed. One 2019 meta-analysis of 8 small trials found citrulline (3–9 grams a day) lowered systolic blood pressure (the top number) by about 4 mmHg, with diastolic blood pressure dropping only at doses of 6 grams or more [20]. Another meta-analysis of 5 trials found no significant effect at all [21]. So if there is an effect, it’s probably small, and citrulline isn’t a substitute for blood pressure treatment.

Blood vessel function

A 2022 meta-analysis of randomized trials found that taking citrulline for several weeks slightly improved “flow-mediated dilation,” a measure of how well the arteries widen. Single doses had no effect [22]. That’s a promising sign for blood vessel health, but it doesn’t yet show any reduction in heart attacks or strokes.

Erectile dysfunction

In a small single-blind study, 24 men with mild erectile dysfunction took a placebo for a month and then 1.5 grams of L-citrulline a day for a month. Erection hardness improved in half of them on citrulline, compared with 8% on placebo [23]. Because the placebo always came first and the men weren’t randomized, this is weak evidence. The authors noted it was less effective than prescription medicines such as sildenafil (Viagra). It’s an interesting early result, but larger studies are needed.

How much L-citrulline do people take?

• For workouts: most positive studies used 6–8 grams of citrulline malate about an hour before training [9].
• As plain L-citrulline: because there’s no malate, smaller amounts (around 3–6 grams) supply a similar amount of citrulline.
• For blood pressure or blood vessel studies: researchers typically used 3–9 grams a day for several weeks [20].

Stacking citrulline with other supplements

Citrulline is often paired with nitrate sources like beetroot, antioxidants, or branched-chain amino acids. A 2020 review found the evidence for these combinations is still limited [4]. One study combining 2 grams of citrulline with glutathione (an antioxidant your body makes) raised nitric oxide markers more than placebo at one time point after a workout, but it wasn’t shown to beat citrulline alone [24]. The ingredients were supplied by, and one author worked for, the company that makes both, and the study measured blood markers rather than performance.

Side effects and safety

Citrulline seems well tolerated in short-term studies. Healthy men who took single doses of up to 15 grams reported no side effects [3]. The most common complaint is stomach discomfort, reported by about 15% of people in one study using 8 grams of citrulline malate [5].

Most studies lasted only days to a few weeks, so we know less about long-term use. Because citrulline may slightly lower blood pressure, it could in theory add to the effects of some medicines, so it’s worth checking with your doctor first if you take blood pressure medicine, nitrates for chest pain, or erectile dysfunction medicine, or if you’re pregnant or breastfeeding.

The bottom line

L-citrulline does a good job of raising arginine levels in the blood. The best-supported L-citrulline benefits are a small boost in reps during hard, high-rep strength training and a bit less soreness the next day. It doesn’t appear to help endurance exercise, it isn’t a meaningful growth hormone booster, and its effects on blood pressure and erections are promising but not yet firmly established.

This article is for general information only and isn’t medical advice. Talk to your doctor or another qualified health professional before starting a new supplement. See our medical disclaimer.

References

1. Rimando AM, Perkins-Veazie PM. Determination of citrulline in watermelon rind. Journal of Chromatography A. 2005;1078(1-2):196-200. doi:10.1016/j.chroma.2005.05.009
2. Schwedhelm E, et al. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. British Journal of Clinical Pharmacology. 2008;65(1):51-59. doi:10.1111/j.1365-2125.2007.02990.x
3. Moinard C, et al. Dose-ranging effects of citrulline administration on plasma amino acids and hormonal patterns in healthy subjects: the Citrudose pharmacokinetic study. British Journal of Nutrition. 2008;99(4):855-862. doi:10.1017/S0007114507841110
4. Gonzalez AM, Trexler ET. Effects of citrulline supplementation on exercise performance in humans: a review of the current literature. Journal of Strength and Conditioning Research. 2020;34(5):1480-1495. doi:10.1519/JSC.0000000000003426
5. Pérez-Guisado J, Jakeman PM. Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. Journal of Strength and Conditioning Research. 2010;24(5):1215-1222. doi:10.1519/JSC.0b013e3181cb28e0
6. Wax B, et al. Effects of supplemental citrulline malate ingestion during repeated bouts of lower-body exercise in advanced weightlifters. Journal of Strength and Conditioning Research. 2015;29(3):786-792. doi:10.1519/JSC.0000000000000670
7. Glenn JM, et al. Acute citrulline malate supplementation improves upper- and lower-body submaximal weightlifting exercise performance in resistance-trained females. European Journal of Nutrition. 2017;56(2):775-784. doi:10.1007/s00394-015-1124-6
8. Trexler ET, et al. Acute effects of citrulline supplementation on high-intensity strength and power performance: a systematic review and meta-analysis. Sports Medicine. 2019;49(5):707-718. doi:10.1007/s40279-019-01091-z
9. Vårvik FT, Bjørnsen T, Gonzalez AM. Acute effect of citrulline malate on repetition performance during strength training: a systematic review and meta-analysis. International Journal of Sport Nutrition and Exercise Metabolism. 2021;31(4):350-358. doi:10.1123/ijsnem.2020-0295
10. Aguiar AF, Casonatto J. Effects of citrulline malate supplementation on muscle strength in resistance-trained adults: a systematic review and meta-analysis of randomized controlled trials. Journal of Dietary Supplements. 2022;19(6):772-790. doi:10.1080/19390211.2021.1939473
11. Rhim HC, et al. Effect of citrulline on post-exercise rating of perceived exertion, muscle soreness, and blood lactate levels: a systematic review and meta-analysis. Journal of Sport and Health Science. 2020;9(6):553-561. doi:10.1016/j.jshs.2020.02.003
12. Viribay A, et al. Effects of citrulline supplementation on different aerobic exercise performance outcomes: a systematic review and meta-analysis. Nutrients. 2022;14(17):3479. doi:10.3390/nu14173479
13. d’Unienville NMA, et al. Effect of food sources of nitrate, polyphenols, L-arginine and L-citrulline on endurance exercise performance: a systematic review and meta-analysis of randomised controlled trials. Journal of the International Society of Sports Nutrition. 2021;18(1):76. doi:10.1186/s12970-021-00472-y
14. Cunniffe B, et al. Acute citrulline-malate supplementation and high-intensity cycling performance. Journal of Strength and Conditioning Research. 2016;30(9):2638-2647. doi:10.1519/JSC.0000000000001338
15. Bendahan D, et al. Citrulline/malate promotes aerobic energy production in human exercising muscle. British Journal of Sports Medicine. 2002;36(4):282-289. doi:10.1136/bjsm.36.4.282
16. Sureda A, et al. L-citrulline-malate influence over branched chain amino acid utilization during exercise. European Journal of Applied Physiology. 2010;110(2):341-351. doi:10.1007/s00421-010-1509-4
17. Jourdan M, et al. Citrulline stimulates muscle protein synthesis in the post-absorptive state in healthy people fed a low-protein diet – a pilot study. Clinical Nutrition. 2015;34(3):449-456. doi:10.1016/j.clnu.2014.04.019
18. Thibault R, et al. Oral citrulline does not affect whole body protein metabolism in healthy human volunteers: results of a prospective, randomized, double-blind, cross-over study. Clinical Nutrition. 2011;30(6):807-811. doi:10.1016/j.clnu.2011.06.005
19. Bouillanne O, et al. Impact of 3-week citrulline supplementation on postprandial protein metabolism in malnourished older patients: the Ciproage randomized controlled trial. Clinical Nutrition. 2019;38(2):564-574. doi:10.1016/j.clnu.2018.02.017
20. Barkhidarian B, et al. Effects of L-citrulline supplementation on blood pressure: a systematic review and meta-analysis. Avicenna Journal of Phytomedicine. 2019;9(1):10-20. PubMed 30788274
21. Mirenayat MS, et al. Effect of L-citrulline supplementation on blood pressure: a systematic review and meta-analysis of clinical trials. Current Hypertension Reports. 2018;20(11):98. doi:10.1007/s11906-018-0898-3
22. Smeets ETHC, Mensink RP, Joris PJ. Effects of L-citrulline supplementation and watermelon consumption on longer-term and postprandial vascular function and cardiometabolic risk markers: a meta-analysis of randomised controlled trials in adults. British Journal of Nutrition. 2022;128(9):1758-1770. doi:10.1017/S0007114521004803
23. Cormio L, et al. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction. Urology. 2011;77(1):119-122. doi:10.1016/j.urology.2010.08.028
24. McKinley-Barnard S, et al. Combined L-citrulline and glutathione supplementation increases the concentration of markers indicative of nitric oxide synthesis. Journal of the International Society of Sports Nutrition. 2015;12:27. doi:10.1186/s12970-015-0086-7