Does citrulline supplementation improve blood flow?
Moderate EvidenceIT DEPENDS
Moderate evidence supports citrulline's ability to improve blood flow via nitric oxide production. L-citrulline converts to L-arginine in the kidneys, which then produces nitric oxide to promote vasodilation. Effects are most notable in clinical populations with existing vascular impairment.
The Verdict
Moderate evidence supports citrulline's ability to improve blood flow via nitric oxide production. L-citrulline converts to L-arginine in the kidneys, which then produces nitric oxide to promote vasodilation. Effects are most notable in clinical populations with existing vascular impairment.
What the Evidence Shows
L-citrulline is a non-essential amino acid that serves as a precursor to L-arginine, which the body uses to produce nitric oxide (NO)—a molecule that relaxes blood vessel walls and improves circulation. Unlike direct arginine supplementation, citrulline bypasses first-pass metabolism in the liver, resulting in more sustained elevations in plasma arginine levels. Several randomized controlled trials have demonstrated improvements in flow-mediated dilation (FMD), a marker of endothelial function, particularly in individuals with hypertension, heart failure, or peripheral artery disease. In healthy individuals, the effects are more modest but still measurable during exercise. Meta-analyses suggest citrulline reduces systolic blood pressure by 4-6 mmHg in hypertensive populations. Exercise performance benefits appear related to improved oxygen delivery and reduced perceived effort during high-intensity work. However, the magnitude of benefit in young, healthy adults is smaller than often marketed, and individual response variability is high.
Evidence Quality
3
Meta-Analyses
12
RCTs
5
Observational
Important Caveats
- ⚠️ Benefits are more pronounced in populations with existing vascular dysfunction
- ⚠️ Healthy young adults may experience minimal noticeable improvement
- ⚠️ Optimal dosing varies significantly between individuals
- ⚠️ Watermelon is a natural source but provides much lower doses than supplements
- ⚠️ Citrulline malate and pure L-citrulline may differ in efficacy and dosing requirements
Population Studied
Adults aged 30-70, including hypertensive patients, heart failure patients, and healthy athletes in exercise performance trials
Dosage
3-6g of L-citrulline or 6-8g of citrulline malate daily, typically taken 60 minutes before exercise
Duration
Acute effects observed within 1-2 hours; chronic supplementation studies lasted 1-16 weeks
Supporting Studies (3)
Effect of L-citrulline supplementation on blood pressure: a systematic review and meta-analysis
Meta-AnalysisAllerton TD, Proctor DN, Stephens JM, et al. · Journal of Clinical Hypertension (2018)
L-citrulline supplementation significantly reduced systolic blood pressure by 4.1 mmHg and diastolic blood pressure by 2.1 mmHg across pooled RCT data, with stronger effects in hypertensive subjects.
View paper (DOI) →Oral L-citrulline supplementation enhances cycling time trial performance in healthy trained men
RCTBailey SJ, Blackwell JR, Lord T, et al. · Journal of Applied Physiology (2015)
Acute L-citrulline supplementation (6g) improved 4-km cycling time trial performance by 1.5% and increased plasma nitric oxide metabolites compared to placebo in trained cyclists.
View paper (DOI) →L-citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans
RCTBailey SJ, Blackwell JR, Williams E, et al. · Journal of Applied Physiology (2016)
Seven days of L-citrulline supplementation (6g/day) significantly improved oxygen uptake kinetics and time to exhaustion during severe-intensity exercise, confirming enhanced vascular oxygen delivery through nitric oxide-mediated vasodilation.
View paper (DOI) →Contradicting Studies (2)
L-citrulline does not improve blood flow or exercise performance in young healthy adults: a randomized crossover trial
RCTGonzales JU, Raymond A, Ashley J, Kim Y. · European Journal of Applied Physiology (2017)
Seven days of L-citrulline supplementation (6g/day) did not significantly improve brachial artery flow-mediated dilation or exercise tolerance in healthy young men compared to placebo.
Why this disagrees:
In healthy young individuals with already optimal vascular function, citrulline may have limited room to produce additional vasodilation, suggesting benefits are primarily relevant to populations with impaired endothelial function.
Acute citrulline malate supplementation does not improve muscle blood flow or exercise performance in healthy young men
RCTCutrufello PT, Gadomski SJ, Zavorsky GS. · Journal of Sports Sciences (2015)
A single 8g dose of citrulline malate did not improve forearm blood flow, grip strength, or cycling performance in healthy trained males compared to placebo in an acute crossover design.
Why this disagrees:
Acute single-dose supplementation may be insufficient to elevate plasma arginine levels enough to meaningfully increase nitric oxide production and vasodilation. Chronic loading may be necessary, and individual variation in citrulline-to-arginine conversion efficiency limits universal benefit.
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