The Arginine-Nitric Oxide Pathway
Most people who want to raise nitric oxide try taking L-arginine directly — but this approach is limited by poor oral bioavailability. Arginase enzymes in the gut and liver break most of it down before it reaches circulation. L-citrulline sidesteps this problem entirely: it is absorbed intact through the small intestine, transported to the kidneys, and converted to arginine there. Studies consistently show that citrulline supplementation raises plasma arginine levels more effectively than arginine supplementation at equivalent doses [1].
The resulting arginine fuels endothelial nitric oxide synthase (eNOS), the enzyme that produces nitric oxide in blood vessel walls. NO then diffuses into surrounding smooth muscle, triggering relaxation and vasodilation. This is the same pathway activated by exercise, sunlight, and dietary nitrates from beets and leafy greens — citrulline simply provides more substrate to keep it running. See our beets page for a complementary dietary route to NO support.
Blood Pressure and Vascular Health
A 2019 systematic review and meta-analysis pooling 8 randomized controlled trials found that L-citrulline supplementation significantly reduced systolic blood pressure, with effects strongest at doses of 6 g/day or more and after at least 6 weeks of use [1]. People with elevated baseline blood pressure (≥130/85 mmHg) showed the largest reductions.
A 2022 randomized trial in hypertensive postmenopausal women tested 10 g/day of L-citrulline for 4 weeks [2]. The researchers used brachial artery flow-mediated dilation — a validated imaging measure of endothelial function — alongside blood pressure monitoring. L-citrulline increased both plasma arginine and flow-mediated dilation, and reduced aortic diastolic blood pressure and mean arterial pressure compared to placebo. The findings are particularly relevant for women after menopause, when the loss of estrogen impairs eNOS activity and accelerates vascular aging.
Exercise Performance
Much of the exercise research uses citrulline malate — L-citrulline bound to malic acid in a 2:1 ratio. The malate component may contribute independently by supporting the citric acid cycle, potentially improving energy production during sustained high-intensity effort.
The most-cited trial in this area tested 8 g of citrulline malate taken before a bench press protocol in 41 men [3]. Compared to placebo in a double-blind crossover design, citrulline malate increased the number of repetitions completed across 16 sets by more than 50%. Post-exercise muscle soreness was reduced by 40% at both 24 and 48 hours. Later meta-analyses confirmed a consistent, if more modest, ergogenic effect across different training contexts.
A 2020 meta-analysis of 13 trials found that citrulline supplementation reduced perceived effort during exercise (p = 0.03) and muscle soreness at 24 hours post-exercise (p = 0.04) [4]. Blood lactate levels were not significantly affected, suggesting the anti-fatigue effect works through mechanisms beyond simple lactate clearance — possibly including enhanced blood flow to working muscles and faster removal of metabolic waste.
Practical Dosage
For blood pressure support: 3–6 g L-citrulline per day, ideally taken consistently for at least 4–6 weeks.
For exercise performance: 6–8 g citrulline malate (2:1 form), or 4–6 g pure L-citrulline, taken 40–60 minutes before training.
Food sources: Watermelon is the richest dietary source — the white rind contains more citrulline than the red flesh. Cucumber, pumpkin, squash, and bitter melon also contain meaningful amounts, though supplemental doses far exceed what food provides.
L-citrulline is well tolerated with a low side effect profile. Mild gastrointestinal discomfort is occasionally reported at high doses (8+ g). No serious adverse events have been reported in human trials to date.
Evidence Review
Blood Pressure and Endothelial Function
Figueroa et al. (2019) conducted a systematic review and meta-analysis of 8 randomized controlled trials examining L-citrulline and blood pressure, yielding 10 data sets for pooled analysis [1]. Supplementation produced a significant reduction in systolic blood pressure overall. Subgroup analyses identified dose (≥6 g/day), duration (≥6 weeks), and higher baseline blood pressure (≥130/85 mmHg) as the main moderators of effect size. Diastolic blood pressure was significantly reduced only in the higher-dose subgroup. The included trials had sample sizes ranging from 12 to 34 subjects — reasonable for mechanistic vascular studies but not large enough to draw conclusions about hard clinical endpoints like cardiovascular events.
Maharaj et al. (2022) conducted a well-controlled crossover trial in 25 hypertensive postmenopausal women, testing 10 g/day L-citrulline versus placebo over 4 weeks per condition [2]. The crossover design is valuable here because each participant serves as their own control, reducing confounding from the substantial individual variation in vascular function. Key outcomes included plasma L-arginine (confirming the citrulline-to-arginine conversion pathway was working as expected), brachial artery flow-mediated dilation measured by ultrasound (confirming improved endothelial function), and automated blood pressure measurement. Aortic diastolic BP and mean arterial pressure were significantly reduced with L-citrulline versus placebo. The authors attributed the effects to increased L-arginine availability driving eNOS activity. This trial is notable for using both mechanistic biomarkers and clinical outcomes simultaneously.
Exercise Performance
Pérez-Guisado and Jakeman (2010) established the foundational evidence for citrulline malate in resistance training [3]. Their randomized, double-blind crossover study had 41 men complete two separate pectoral training sessions — one with 8 g citrulline malate and one with placebo, separated by a washout period. The increase in repetitions to failure exceeded 50% on citrulline malate versus placebo. The 40% reduction in muscle soreness at 24 and 48 hours post-exercise was equally striking. While these effect sizes are larger than what later meta-analyses found on average, the study was important for establishing the plausibility and direction of citrulline's ergogenic effect in a real training protocol.
Vårvik, Bjørnsen, and Gonzalez (2021) pooled data from 8 double-blind, placebo-controlled trials (137 participants: 101 trained men, 26 women, 10 untrained men) in a formal meta-analysis focused on repetition performance during strength training [5]. They found a statistically significant ergogenic effect of citrulline malate (Hedges's g = 0.20), indicating a small but consistent advantage over placebo. Effect sizes were somewhat larger for upper-body exercises and in trained versus untrained populations. The authors noted high heterogeneity across studies (I² > 50%), reflecting real variation in response by exercise type, dose, and subject characteristics.
Rhim et al. (2020) analyzed the recovery side of the equation, pooling 13 trials totaling 206 participants [4]. Their meta-analysis found significant reductions in perceived exertion (standardized mean difference, p = 0.03) and muscle soreness at 24 hours (p = 0.04). The 48-hour soreness result trended toward significance. Blood lactate at multiple post-exercise time points did not show significant reductions, which argues against the simple hypothesis that citrulline works by clearing lactate. The authors suggest enhanced blood flow, improved oxygen delivery, and potentially malate's role in the citric acid cycle as more likely contributors.
Limitations
The exercise literature is complicated by the common use of citrulline malate rather than pure L-citrulline, making it difficult to attribute effects to citrulline specifically versus the malate component. Most trials are acute (single dose) rather than chronic, and the long-term effect of daily citrulline supplementation on training adaptations has not been well studied. For blood pressure, the clinical trials are generally small and short-duration; the lack of long-term outcome data means that blood pressure reduction cannot yet be confidently linked to reduced cardiovascular event rates. Nonetheless, the mechanistic plausibility — improving nitric oxide bioavailability — is well established, and the short-term evidence for both blood pressure and exercise outcomes is consistent enough to support L-citrulline as one of the better-evidenced amino acid supplements available.