L-Citrulline — The Nitric Oxide Precursor
L-citrulline is a non-essential amino acid found in watermelon that serves as the most efficient precursor to L-arginine and nitric oxide (NO) in the vascular endothelium. Unlike direct L-arginine supplementation, citrulline bypasses first-pass hepatic metabolism and more effectively raises plasma arginine and NO levels. It is a key compound for cardiovascular health, exercise performance, and vascular aging.
Mechanism of Action
Citrulline is converted to arginine in the kidneys via the urea cycle, bypassing the intestinal and hepatic arginase that degrades oral arginine. Arginine is then converted to NO by endothelial nitric oxide synthase (eNOS). NO relaxes vascular smooth muscle (vasodilation), reduces arterial stiffness, and inhibits platelet aggregation. NO production declines with age due to reduced eNOS activity and increased arginase.
Human Trial Evidence
A 2017 Journal of Nutrition RCT showed 6 g/day L-citrulline significantly reduced arterial stiffness (pulse wave velocity) in postmenopausal women over 8 weeks. Multiple RCTs confirm citrulline malate (6–8 g/day) improves exercise performance and reduces muscle soreness. A 2018 Nutrients meta-analysis confirmed blood pressure-lowering effects.
Dosing Protocol
3–6 g/day of L-citrulline for cardiovascular benefits. 6–8 g/day of citrulline malate (2:1) for exercise performance. Best taken 60 min before exercise or with meals. L-citrulline is more bioavailable than citrulline malate on a per-gram basis. Effects on blood pressure and arterial stiffness seen after 4–8 weeks.
Safety & Contraindications
Excellent safety profile. Well-tolerated up to 15 g/day. Mild GI effects at very high doses. May cause hypotension — caution with blood pressure medications. Avoid with PDE5 inhibitors (additive hypotension). No significant drug interactions at typical doses.