What is L-citrulline? A plain guide

L-citrulline is a non-essential amino acid the body makes on its own and gets in smaller amounts from food, mostly watermelon. It isn't used to build muscle protein directly — its job is as an intermediate in the urea cycle and as the body's preferred route to arginine, the amino acid that feeds nitric oxide production in blood vessels.

Where it sits in the body's chemistry

Citrulline belongs to a small group of amino acids — ornithine is another — that cycle through the urea cycle rather than getting assembled into protein the way most dietary amino acids do. A 2013 pharmacology review describes it as a naturally occurring, non-essential amino acid that becomes "conditionally essential" in certain intestinal conditions, when the gut's own production of it drops below what the body needs. The same review calls it a more effective precursor of arginine and nitric oxide than arginine itself, which is the basic rationale for selling citrulline as a supplement rather than arginine directly.

Why a supplement uses citrulline instead of arginine

Oral arginine runs into a practical problem: a large share of it gets broken down in the gut and liver before it ever reaches the bloodstream, in what's called first-pass metabolism. Citrulline mostly escapes that breakdown and is converted into arginine afterward, largely by the kidneys, once it's already circulating. A 2022 food-science review of watermelon's vascular research puts it plainly: "supplementation of L-citrulline is recommended instead of L-arginine since most L-arginine is catabolized during its course to the endothelium." The arginine that results is what the body uses as raw material for nitric oxide, a signaling molecule involved in dilating blood vessels — the mechanism behind most of the research covered in L-citrulline benefits: what the research does and doesn't show.

Its natural food source

Outside a supplement bottle, L-citrulline shows up mainly in watermelon — its name comes from Citrullus, the fruit's genus — concentrated more in the rind than the flesh. The amount in the fruit is real, but reaching a dose comparable to what a research trial uses takes more than a slice: the same 2022 review estimated that more than 700 mL of watermelon juice would be needed to deliver an amount of citrulline in the range tested in supplementation studies, which is part of why watermelon-extract supplements concentrate it rather than relying on the whole fruit.

The two forms sold as a supplement

What's actually in a "citrulline" capsule

Form	What it is	What a gram of it provides
L-citrulline	The free amino acid on its own	1 g = 1 g of citrulline
L-citrulline DL-malate	Citrulline bound to malic acid, usually in a 2:1 ratio by weight	Roughly 0.7 g of citrulline per gram — the rest is the malate portion

The "DL" in citrulline DL-malate refers to the malic acid half of the molecule, not the amino acid — malic acid is commonly sold as a racemic (mixed D- and L-) form, while the citrulline itself is always the L-configuration. Because roughly 30% of a citrulline malate serving's weight is malate rather than citrulline, an 8 g dose of citrulline malate provides meaningfully less actual citrulline than an 8 g dose of plain L-citrulline — worth doing that arithmetic before comparing two labels. The one trial that put the two forms head-to-head, a 2025 six-week study, randomized 33 resistance-trained men to 8 g/day of plain L-citrulline, 12 g/day of citrulline DL-malate (chosen to provide a comparable amount of actual citrulline), or a placebo. Both citrulline groups showed similar improvements in upper-body muscular endurance and similar changes in a blood marker of nitric-oxide production compared with placebo, with no meaningful difference between the two forms themselves.

What it's used for

As a supplement, L-citrulline is marketed mainly around exercise performance, blood-vessel function, and, less often, sexual health — on the strength of its role in the arginine/nitric-oxide pathway described above. L-citrulline benefits: what the research does and doesn't show covers what human trials have and haven't found for each of those uses.

Safety at a glance

L-citrulline isn't automatically appropriate for everyone at every amount. L-citrulline side effects, who should not take L-citrulline, and L-citrulline and medications: what to check cover the specifics — worth reading before starting, particularly if you take blood pressure medication, a PDE-5 inhibitor, or have reduced kidney function.

Questions people ask

What is L-citrulline made from?

As a molecule, the body makes it from ornithine as part of the urea cycle. In food, it's concentrated in watermelon; as a manufactured supplement ingredient, it's produced and sold as the purified free amino acid or bound to malic acid as citrulline malate.

What is L-citrulline malate 2:1?

It's citrulline bound to malic acid in a ratio of roughly two parts citrulline to one part malic acid by weight — about 70% of the serving's weight is citrulline, the rest malate. See the table above for how that changes the actual citrulline amount behind a given gram figure on a label.

Which is better, L-arginine or L-citrulline?

For raising the body's own arginine level, research points to citrulline: oral arginine is heavily broken down before it reaches circulation, while citrulline mostly escapes that breakdown and converts to arginine afterward. That's a pharmacokinetic difference, not a verdict on which does more for any specific outcome — see what is L-arginine for how the two compare directly.

Can you take L-arginine and L-citrulline together?

We didn't find a trial testing that combination against citrulline on its own. Some products pair them on the theory that citrulline's conversion to arginine adds to a direct arginine dose, but that specific combination hasn't been tested head-to-head.

What does L-citrulline do for men?

Search interest in this phrase mostly points at exercise performance and sexual health, both marketed toward men more than women. L-citrulline benefits: what the research does and doesn't show covers what's actually been tested for each, and the limits of that research.

Does L-citrulline increase blood flow?

The nitric-oxide pathway citrulline feeds into is involved in blood-vessel dilation, which is the mechanism behind the vascular and blood-pressure research covered in L-citrulline benefits — that page covers what was, and wasn't, found.