L-Arginine vs. L-Citrulline: Which Actually Raises Nitric Oxide?
Both are sold as nitric oxide boosters. One reaches the bloodstream far more efficiently, and both were studied at doses most capsules cannot hold.
Two amino acids, one destination
Arginine and citrulline sit on the same short loop of chemistry. Nitric oxide synthase turns arginine into nitric oxide and, as a by-product, citrulline. The kidneys and other tissues can turn citrulline back into arginine. So both are sold as nitric oxide boosters, and many products, including some memory capsules, contain both.
The question that matters for anyone swallowing them is which one actually raises arginine in the blood, and by how much.
Why oral arginine underdelivers
Arginine taken by mouth meets arginase, an enzyme in the gut wall and liver that breaks it down to ornithine and urea. Much of a dose never reaches the general circulation[1]. In a Stanford study of healthy volunteers, the absolute bioavailability of a single 10 g oral dose was about 20%[2].
There is a second limit. In healthy cells, nitric oxide synthase is already well supplied with arginine, so extra substrate often changes little. Pharmacologists call this the arginine paradox; supplements tend to help mainly where nitric oxide production is impaired[3].
Why citrulline does better
Citrulline is not a preferred target for gut arginase, so it is absorbed largely intact and converted to arginine by the kidneys. A double-blind crossover study in 20 healthy volunteers compared six regimens. Citrulline raised plasma arginine more effectively than arginine, in a dose-dependent way, and at 3 g twice daily improved the ratio of arginine to its natural inhibitor ADMA and raised urinary markers of nitric oxide signaling[4].
A review of citrulline and cardiometabolic health reaches a similar conclusion: citrulline is the more efficient way to raise arginine, with effects described at gram doses[5]. In rats and rabbits, giving half doses of each together raised plasma arginine faster than either alone[6], which is the rationale for combining them.
The doses that were studied
| Use | Arginine | Citrulline |
|---|---|---|
| Raising plasma arginine | 1-2 g/day (less effective) | 1.5-6 g/day |
| Blood pressure | 4-24 g/day[7] | gram doses in smaller trials |
| A typical single memory capsule | hundreds of mg | about 150 mg |
The difference between gram and milligram doses is the most important thing on any nitric oxide label. A product can contain the right ingredient and still contain too little of it to reproduce the studies.
So which is better?
For raising arginine, citrulline. For cost per gram, plain arginine is often cheaper, but you lose much of it to the gut. For practical use, research doses of either need a powder rather than a single capsule. People with heart disease, low blood pressure or on nitrate or sildenafil-type medicines should ask a doctor before using either at any dose.
Quick answers
Is L-citrulline better than L-arginine?
For raising blood arginine, yes. Citrulline avoids gut breakdown and was more effective than arginine in a controlled crossover study.
How much citrulline raises nitric oxide?
Effects on plasma arginine were clear at 1.5 to 6 g a day in study conditions. Milligram doses have not been tested.
Sources behind this page
Each link opens the original abstract or agency page. The trials gave single ingredients, usually in grams rather than milligrams; no study has tested the HoneyPezil capsule itself.
- 1Wu G, Morris SM Jr. Arginine metabolism: nitric oxide and beyond. Biochem J. 1998;336(Pt 1):1-17. pubmed.ncbi.nlm.nih.gov/9806879/
- 2Tangphao O, Grossmann M, Chalon S, Hoffman BB, Blaschke TF. Pharmacokinetics of intravenous and oral L-arginine in normal volunteers. Br J Clin Pharmacol. 1999;47(3):261-266. pubmed.ncbi.nlm.nih.gov/10215749/
- 3Böger RH. The pharmacodynamics of L-arginine. J Nutr. 2007;137(6 Suppl 2):1650S-1655S. pubmed.ncbi.nlm.nih.gov/17513442/
- 4Schwedhelm E, Maas R, Freese R, et al. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. Br J Clin Pharmacol. 2008;65(1):51-59. pubmed.ncbi.nlm.nih.gov/17662090/
- 5Allerton TD, Proctor DN, Stephens JM, et al. l-Citrulline supplementation: impact on cardiometabolic health. Nutrients. 2018;10(7):921. pubmed.ncbi.nlm.nih.gov/30029482/
- 6Morita M, Hayashi T, Ochiai M, et al. Oral supplementation with a combination of L-citrulline and L-arginine rapidly increases plasma L-arginine concentration and enhances NO bioavailability. Biochem Biophys Res Commun. 2014;454(1):53-57. pubmed.ncbi.nlm.nih.gov/25445598/
- 7Dong JY, Qin LQ, Zhang Z, et al. Effect of oral L-arginine supplementation on blood pressure: a meta-analysis of randomized, double-blind, placebo-controlled trials. Am Heart J. 2011;162(6):959-965. pubmed.ncbi.nlm.nih.gov/22137067/
Run a fair 60-day test of Honey Pezil
Payment happens on the seller's BuyGoods checkout. Take one capsule a day exactly as the label says, write down two or three memory moments you want to see change, and if nothing has shifted by about day 50, email the seller while the refund window is still open.