Arginine
Also known as: L-arginine
Arginine is a protein-forming amino acid involved in the urea cycle, creatine synthesis, and production of nitric oxide, a signaling molecule that can relax blood vessels. The body can usually synthesize arginine, but it may become conditionally essential during growth or severe physiological stress. L-arginine is the naturally protein-forming molecular form used in foods and most supplements.
Meat, fish, dairy, legumes, nuts, and seeds contain arginine as part of dietary protein. Supplemental L-arginine is marketed in pre-workout products to increase nitric-oxide availability, blood flow, and the exercise pump. The biochemical pathway is real, but swallowing more precursor does not guarantee that nitric-oxide production, strength, or hypertrophy increases; absorption and extensive intestinal and liver metabolism limit the response.
An acute increase in muscle blood volume or perceived pump is not itself a gain in muscle tissue. Studies use different doses, training tests, participant groups, and combinations, producing mixed results. The NIH performance-supplement review concludes that arginine has little to no demonstrated effect on strength, exercise performance, or muscular recovery overall.
Some favorable conclusions have come from pooled analyses, but methods matter. A 2025 methodological critique of a prominent arginine meta-analysis showed how extraction and analysis errors could distort estimated benefits. This does not prove that every protocol is ineffective; it means broad promises and precise "optimal" dosing claims are not justified by an unstable evidence base.
Arginine and citrulline malate are related but not interchangeable supplements. Citrulline is converted to arginine and can raise circulating arginine efficiently because it bypasses some first-pass metabolism. That mechanism still does not guarantee a meaningful training outcome, and results from one ingredient cannot be assigned to the other.
Oral arginine can cause nausea, abdominal pain, bloating, or diarrhea, particularly at larger servings, and it can lower blood pressure or interact with medications. People using blood-pressure, nitrate, erectile-dysfunction, or glucose-lowering medication, as well as those with relevant cardiovascular, kidney, or other medical conditions, should obtain individualized clinical advice. Product testing also matters because multi-ingredient pump formulas may contain undeclared or overlapping substances.