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Watermelon is widely loved for its sweet taste, yet it’s naturally low in calories (it’s 92 percent water, after all) and boosted with antioxidants. But fitness enthusiasts have another reason to prefer it: its high citrulline content.
This article will explain what citrulline is, what it does, and how it could benefit endurance, performance, and overall health.
Citrulline is a nonessential amino acid that plays a critical role in circulatory function. It is nonessential, meaning your body can produce it from other raw materials in your diet.
Citruliline acts as a vasodilator in the body. That simply means it helps open blood vessels that deliver oxygen and nutrients.
That suggests citrulline can play an important role in your health. For exercise enthusiasts, citrulline can help boost muscle-protein synthesis, improve exercise performance, and increase aerobic capacity.1 Further research confirms its role in delaying fatigue via a mechanism in the urea cycle, helping your body excrete ammonia, which is toxic to the body and contributes to muscle fatigue.1 That means more reps and a greater anabolic stimulus.
You’d better have a big appetite if you want to consume enough watermelon to equal the dosages researchers have found sufficient for marked improvements in the gym. That’s because the sweet spot in terms of dosage is about 6-8 g, the equivalent of some 18-24 cups of melon. That makes supplementing with citrulline an easier way to get your daily intake.
What does the research tell us about the health and fitness benefits of citrulline? Let’s take a closer look.
In your body, citrulline is converted to arginine, which serves as a precursor for nitric oxide production.2 You’ve probably heard about nitric oxide because research has suggested its consequential role for athletes.
According to Kiani et al (2022): “Nitric oxide (NO) is an essential component of the human body, involved in blood vessel dilation, stimulation of hormone release, signaling and regulation of neurotransmission. … Nitric oxide supplementation improves cardiac health, enhances performance during exercise, reduces high blood pressure during pregnancy, reduces erectile dysfunction, and improves healing processes and respiratory response.”3 Increasing blood flow, especially during training and competition, means an increase in oxygen and nutrient delivery to working tissue and more efficient removal of metabolic waste.
Though citrulline is converted to arginine in the body, it’s a mistake to think arginine supplementation is the best way to increase nitric oxide production.
A scientific review by Park (2023) reviewed 50 studies and concluded arginine itself was ineffective on markers of exercise performance.4 That’s likely because arginine is far less likely to make it into your bloodstream as it’s highly catabolized in the intestines and kidneys.5 Research instead shows that oral ingestion of citrulline better increases arginine levels in the bloodstream, leading to enhanced activation of nitric-oxide synthase and improvements in various biomarkers of nitric oxide.3,6-8
Citrulline is often combined with malic acid because it’s believed to improve the absorption and bioavailability of citrulline in the body. “It’s thought that this form of citrulline could have bigger effects because it potentially increases ATP – adenosine triphosphate, or the fuel for working muscles – production and availability.”1 Studies confirming this have not been definitive and further research is necessary. “It is unclear whether Malate, Citrulline, or both, are responsible for the effects observed in studies on Citrulline Malate supplementation.”9
While the Park study highlighted the failures of the amino acid arginine, it did recommend 2.4-6 g citrulline per day for 7-16 days … as it “had a positive impact, increasing NO synthesis, enhancing athletic performance indicators, and reducing feelings of exertion.”10 Other studies also support citrulline supplementation on muscular endurance. A study of trained male subjects showed that 2.4 g/day of L-citrulline for seven days increased plasma nitric-oxide metabolites, plasma arginine, and 4-kilometer time trial performance.11 Another study reported that seven days of citrulline (6 g/day) significantly increased plasma arginine and nitrite levels, and it significantly improved VO2 kinetics and exercise performance.12
In addition to aerobic exercise research, three studies examined the impact of an 8-g citrulline malate dose on various performance outcomes during resistance training. In all three, citrulline malate significantly increased performance during upper- and lower-body multiple-bout resistance exercise performance. One study evaluated the effects of the number of repetitions performed for chin-ups, reverse chin-ups, and push-ups to failure in trained males.13 Another study evaluated the effect of citrulline supplementation on the number of repetitions performed for five sequential sets (60 percent of single-rep max) to failure on the leg press, hack squat, and leg extension exercises in trained males.14 The third study evaluated the effects of citrulline supplementation on the number of repetitions performed during six sets each of bench-press and leg-press exercises to failure at 80 percent of one-rep max in trained females.15
The same mechanisms that give athletes a potential performance edge with citrulline supplementation extend beyond the gym, and we’d be remiss to skip over some important health benefits.
For example, Volino-Souza et al (2022) reported: “Considering that vascular dysfunction precedes cardiovascular disease, supplementation with precursors of NO synthesis … is warranted. … Watermelon-derived L-citrulline appears as a supplementation that can improve vascular function, including arterial stiffness and blood pressure.”16
Further, Barkhidarian (2019) found citrulline supplementation may reduce systolic blood pressure; reductions in diastolic blood pressure were observed in studies using doses greater than or equal to 6 g.17
All told a daily dosage of at least 6 g appears to be the optimal amount for achieving both anaerobic performance enhancements and potential health benefits.
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