Benefits
Improved Exercise Performance
L-Citrulline increases nitric oxide production, which enhances blood flow, oxygen delivery, and nutrient uptake in muscles. This may boost endurance, reduce fatigue, and improve performance during high-intensity exercise. The trial evidence is mixed: some resistance and endurance studies show small benefits while others show no effect.
Enhanced Muscle Recovery
By reducing ammonia buildup and muscle soreness, L-Citrulline may speed up recovery post-exercise, allowing for more consistent training.
Cardiovascular Health
L-Citrulline may modestly blunt blood-pressure and arterial-stiffness responses to stress by promoting vasodilation, as seen in small crossover trials. A systematic review of five trials found no significant effect on resting blood pressure, so any benefit is small and inconsistent, and it should not replace antihypertensive treatment.
Erectile Dysfunction (ED)
By boosting nitric oxide and blood flow, L-Citrulline may modestly help mild erectile dysfunction. One small single-blind trial in 24 men with mild ED (Cormio 2011) improved erection-hardness scores versus placebo. The evidence is preliminary and limited to mild cases: PDE5 inhibitors (sildenafil, tadalafil, vardenafil) remain first-line, and citrulline is at most an adjunct.
Immune Function (Not Established)
There is no established human evidence that oral L-Citrulline enhances immune function or reduces inflammation in healthy people. A randomized trial in critically ill septic-shock patients (IMMUNOCITRE 2023) found no benefit on organ-failure (SOFA) scores. This use is unestablished.
Muscle Growth (Unestablished)
There is no direct human evidence that L-Citrulline increases muscle mass. Any effect on muscle protein synthesis is indirect and unproven, so this benefit should be considered speculative.
Mechanism of action
Nitric Oxide (NO) Production
L-Citrulline is converted to L-arginine in the kidneys, which is then used by endothelial nitric oxide synthase (eNOS) to produce NO. This promotes vasodilation, improving blood flow to muscles, heart, and other tissues.
Ammonia Detoxification
L-Citrulline participates in the urea cycle, clearing ammonia (a fatigue-inducing byproduct) from the body, which enhances exercise endurance and recovery.
Antioxidant Effects
By increasing NO and improving blood flow, L-Citrulline reduces oxidative stress and supports nutrient delivery to tissues, aiding muscle repair and cardiovascular health.
Clinical trials
Randomized, double-blind, placebo-controlled trial (NCT02221843) in 22 trained male cyclists receiving 2.4 g L-citrulline vs placebo for 7 days, with 4-km cycling time trial. (J Int Soc Sports Nutr)
22 trained male cyclists. 7-day loading.
L-citrulline reduced 4-km time trial completion time by ~1.5% vs placebo and reduced subjective fatigue. Mechanism: increased plasma arginine → NO → vasodilation → improved muscle blood flow. Note: 1.5% performance improvement is meaningful in elite endurance contexts but small in absolute terms.
Systematic review and meta-analysis of five human clinical trials evaluating L-citrulline supplementation and blood pressure. (Curr Hypertens Rep, 2018)
Pooled across 8 clinical trials.
The pooled analysis of five trials found no significant effect of L-citrulline on brachial systolic (change 0.28 mmHg) or diastolic (change -1.56 mmHg) blood pressure, nor on aortic pressures. A separate small crossover trial (Figueroa 2016) showed modest blunting of blood-pressure and arterial-stiffness responses during cold and metaboreflex stress. Overall the resting blood-pressure evidence is weak and inconsistent.
Multicenter clinical trial (NCT02864017) in 120 mechanically ventilated ICU patients with septic shock receiving enteral citrulline vs placebo. Outcomes: SOFA score Day 7. (Crit Care)
120 critically ill ICU patients with septic shock.
Primary endpoint negative: enteral citrulline did not improve SOFA score at Day 7 vs placebo. Important negative finding — extrapolating cardiovascular benefits to acute critical illness was not supported. Critical illness is metabolically very different from chronic CV applications.
Randomized controlled trial in 47 boys with Duchenne muscular dystrophy receiving L-citrulline + metformin vs placebo. Outcomes: motor function, dystrophin-related markers. (JAMA Netw Open)
47 boys with DMD.
Modest improvements in some motor function measures. Note: small trial; DMD treatment landscape has been transformed by gene therapies (delandistrogene moxeparvovec / Elevidys) and exon-skipping therapies (eteplirsen, golodirsen, viltolarsen) — supplemental citrulline + metformin has no role as primary therapy.
Randomized controlled trial in 24 men with mild ED (Erection Hardness Score 3) receiving L-citrulline (1.5 g/day) vs placebo. Outcomes: EHS, frequency of intercourse. (Urology)
24 men with mild ED. Small trial.
L-citrulline improved EHS and frequency of intercourse vs placebo. Critical caveat: very small trial (n=24); applies to mild ED only. PDE5 inhibitors remain first-line; citrulline at most adjunctive for mild cases.