Independent supplement ratingsUS and Canada · Updated weekly
A ratings desk for a category that has spent thirty years grading its own homework.
A ratings desk for a category that has spent thirty years grading its own homework.
Take arginine by mouth and your gut destroys most of it before it reaches your blood. Citrulline goes round the problem — and raises arginine better than arginine does.
The short version: if you want more arginine in your blood, arginine is the wrong thing to swallow. An enzyme in your gut wall breaks down most of an oral dose before it reaches circulation. L-citrulline is absorbed intact and converted to arginine afterwards — and in a head-to-head trial it raised plasma arginine more effectively than arginine itself.
Arginine is the raw material your body uses to make nitric oxide, which relaxes blood vessels. That is the whole pitch behind every "nitric oxide" and "pump" product on the shelf, and the biochemistry is sound.
The delivery is not. Oral arginine runs into arginase, an enzyme active in the intestinal wall that breaks arginine down on the way through. Researchers describe it as extensive presystemic elimination: a large part of the dose is gone before it ever reaches your bloodstream. (Schwedhelm et al., 2008)
Swallowing more does not fix it neatly either, because the gut discomfort arrives before the useful blood levels do.
Citrulline is not a substrate for arginase. It passes through the gut intact, and the kidneys convert a good part of it into arginine — arriving in the circulation rather than being destroyed on the way in.
The comparison has been run directly. In a double-blind, randomised, placebo-controlled crossover study, 20 healthy volunteers worked through six different dosing regimes of placebo, citrulline and arginine. Citrulline raised both the peak and the total exposure of plasma arginine dose-dependently and more effectively than arginine did (p < 0.01).
At the top dose — 3 g twice a day — the ratio of arginine to ADMA, the body's own brake on nitric oxide production, improved from 186 ± 8 at baseline to 278 ± 14 (p < 0.01), with urinary nitrate rising alongside it. (Schwedhelm et al., 2008)
So the supplement sold as the arginine booster is beaten, on its own measure, by the one most labels treat as an alternative.
Raising a molecule in your blood and changing what your body does are different claims, and this is where the category gets ahead of itself.
A 2020 review in the Journal of Strength and Conditioning Research went through the human evidence on citrulline and exercise. Its verdict on the mechanism everyone assumes: evidence that supplementation acutely improves vasodilation and muscle tissue perfusion is "scarce and inconsistent". Its verdict on outcomes: several studies do report improved performance and recovery, and the mechanism behind that remains to be worked out. (Gonzalez & Trexler, 2020)
Read together: the delivery problem is settled, the payoff is not. That is a more useful thing to know than either "it works" or "it does nothing", and it is roughly where we place citrulline in our ingredient index.
The trial above used 3 g twice daily for a week. Across the performance literature the studied range sits at roughly 3,000–6,000 mg a day of L-citrulline.
Hold that against what the category sells. In our men's sexual health ranking, one powder delivers a full 3,000 mg of citrulline per scoop, a second about 2,000 mg, a third 1,500 mg — and a capsule product sold on the same mechanism delivers 250 mg.
Two hundred and fifty milligrams is not a small dose of the studied amount. It is under a tenth of it, in a product sold on exactly this mechanism.
Most powders are sold as citrulline malate 2:1 — two parts citrulline to one part malic acid. A 3 g serving of citrulline malate 2:1 delivers roughly 2 g of citrulline, not 3.
It is the same arithmetic trap as elemental magnesium, and it catches people the same way. If a label gives you the citrulline malate figure, take two thirds of it to find what you are actually getting, and check it against the 3,000–6,000 mg range rather than against the number on the tub.
The Label Dose Checker runs that comparison on the products we rate, and the category ranking shows which ones clear the bar and which cannot even be checked.
Educational only, not medical advice. Nitric-oxide supplements can interact with blood pressure medication and with nitrates prescribed for chest pain — that combination matters, so ask a clinician first.