L-Arginine + L-Citrulline — The Synergistic Nitric Oxide Boosters

L-arginine is the direct substrate for endothelial nitric oxide synthase (eNOS), which catalyzes the production of nitric oxide (NO), a critical signaling molecule for vasodilation and vascular health. However, oral L-arginine is subject to extensive first-pass metabolism by arginase in the gut and liver. L-citrulline bypasses hepatic metabolism and is converted to L-arginine in the kidneys, providing a sustained elevation in plasma L-arginine. Co-supplementation of L-citrulline and L-arginine synergistically increases plasma L-arginine levels and NO bioavailability more rapidly and effectively than either amino acid alone, enhancing cGMP-dependent vasorelaxation and endothelial function.

Human trials demonstrate that combining L-citrulline and L-arginine increases plasma L-arginine and nitric oxide biomarkers more effectively than either amino acid alone. A 2019 randomized crossover trial in male athletes found that 7 days of supplementation (1.2 g/day of each) improved 10-minute cycling performance and reduced perceived exertion. Other small trials suggest benefits for endothelial function and mild-to-moderate erectile dysfunction, though large-scale, long-term longevity trials are lacking.

1.2–3 g of L-citrulline combined with 1.2–3 g of L-arginine per day. The combination is typically taken 1 hour before exercise or sexual activity. Doses up to 6 g/day of the combination have been used in some studies.

Generally well tolerated at standard doses. High doses of L-arginine (typically >9 g/day) can cause gastrointestinal distress, including diarrhea and nausea. The combination may lower blood pressure, so caution is advised in individuals taking antihypertensive medications, nitrates, or PDE5 inhibitors (e.g., sildenafil) due to the risk of symptomatic hypotension. Avoid in patients with a history of recent myocardial infarction or active herpes simplex infections, as arginine may promote viral replication.

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