Citrulline

Citrulline is an unusual amino acid: your body makes it and uses it, but it is never actually built into your proteins. Instead, it works quietly behind the scenes as a stepping-stone in the chemistry that keeps your blood vessels flexible and your circulation flowing. Its claim to fame is a bit of a paradox — the best way to raise the level of a different amino acid in your blood, arginine, turns out to be to swallow citrulline instead. That quirk is the reason citrulline has become popular in blood-pressure research, pre-workout supplements, and studies of erectile function. It is also the reason a slice of watermelon has a mild claim to being a circulation food. This page explains what citrulline is, the elegant pathway it feeds, and — honestly — what the science does and does not support.

What Citrulline Is
The Citrulline–Arginine–Nitric Oxide Pathway
Why Citrulline Beats Arginine
Food Sources & the Watermelon Connection
What the Evidence Shows
L-Citrulline vs Citrulline Malate
How Much, and Which Form
Safety & Cautions
Research Papers
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What Citrulline Is

Citrulline is a non-proteinogenic amino acid. That mouthful simply means it is a genuine amino acid — built on the same nitrogen-carrying blueprint as the amino acids in your food — but it is not one of the twenty building blocks that your cells string together to make proteins. You will never find citrulline embedded in a muscle fiber or an enzyme the way you find leucine or lysine. It circulates as a free molecule, doing its work in solution rather than as a brick in a wall.

The name is a clue to where it was first found. Citrulline was isolated from watermelon in the early twentieth century, and it takes its name from the melon's botanical genus, Citrullus (watermelon is Citrullus lanatus). So unlike carnitine (from carnis, "flesh") or asparagine (from asparagus), citrulline's name points straight at a fruit.

Your body is not dependent on food for citrulline — it manufactures its own. Most of it is produced in the small intestine, by the lining cells called enterocytes, which convert the amino acids glutamine and glutamate (and some arginine) into citrulline. The kidneys then play the complementary role: they pull citrulline out of the bloodstream and convert much of it back into arginine. This intestine-to-kidney relay is a normal, continuous part of how the body manages its nitrogen and its arginine supply. Because citrulline is made almost exclusively by healthy gut lining, doctors even use the blood citrulline level as a marker of how much functioning small-intestine tissue a person has.

So citrulline sits at a crossroads: made in the gut, recycled in the kidney, and — as we will see — a surprisingly effective way to feed the body's production of nitric oxide.

The Citrulline–Arginine–Nitric Oxide Pathway

To understand why anyone would take citrulline, you have to follow a short chemical relay that ends in one of the most important signaling molecules in your body: nitric oxide (NO).

Nitric oxide is a tiny gas that the cells lining your blood vessels release to tell the surrounding muscle to relax. When those vessel walls relax, the vessel widens — a process called vasodilation — and blood flows more easily at lower pressure. Nitric oxide is central to healthy circulation, blood-pressure regulation, and the blood flow involved in exercise and erectile function.

Here is the relay, step by step:

The body makes nitric oxide from the amino acid arginine. An enzyme called nitric oxide synthase takes arginine and splits off nitric oxide, leaving citrulline behind as the by-product.
That leftover citrulline is not wasted. Cells (and especially the kidneys) can recycle citrulline back into arginine, through a two-step conversion sometimes called the "citrulline–NO cycle."
The freshly regenerated arginine can then be used to make more nitric oxide, and the loop continues.

This is the key insight: citrulline is both the raw material for arginine and the recyclable product of nitric-oxide production. Give the body more citrulline, and you top up the arginine pool that fuels nitric oxide. That is the entire rationale behind citrulline supplements — they are, in effect, a slow and efficient way of feeding your nitric-oxide machinery.

It is worth being clear-eyed here: raising nitric-oxide availability is a plausible mechanism, and it is why researchers keep testing citrulline for circulation-related outcomes. But a sensible mechanism is a reason to run the experiment, not proof that the supplement works. The rest of this page is about what those experiments actually found.

Why Citrulline Beats Arginine

If arginine is what actually makes nitric oxide, the obvious question is: why not just take arginine? Strangely, the answer is that oral citrulline raises your blood arginine level more effectively than swallowing arginine itself does. This is one of the genuinely counterintuitive findings in supplement science, and it comes down to geography inside the body.

When you swallow arginine, it faces a gauntlet before it ever reaches your general circulation. First, bacteria and enzymes in the gut break a large share of it down. Then, whatever survives has to pass through the liver, which contains an enzyme (arginase) that is very good at chewing up arginine. This double toll — in the gut and again in the liver — is called first-pass metabolism. The upshot is that a big oral dose of arginine produces only a modest, short-lived bump in blood arginine, and larger doses tend to cause stomach upset and diarrhea before they cause much benefit.

Citrulline takes a smarter route. It is largely ignored by the gut and the liver — it slips past that first-pass gauntlet almost untouched — and travels intact to the kidneys, which then convert it efficiently into arginine and release that arginine into the bloodstream. Because the conversion happens downstream of the liver, the arginine it produces is not immediately destroyed.

The practical result, measured directly in pharmacokinetic studies (see Research Papers), is striking: gram for gram, oral citrulline produces a higher and more sustained rise in plasma arginine than oral arginine does, and it does so with far less digestive distress. In effect, citrulline is a well-tolerated "pro-drug" for arginine. That is the single most important fact about citrulline, and it explains why modern research has largely shifted from arginine to citrulline for anything involving nitric oxide.

Food Sources & the Watermelon Connection

Citrulline is not abundant in the diet the way, say, the protein amino acids are — because it is not a protein amino acid, most foods contain very little of it. The one famous exception is watermelon.

Watermelon — by far the standout dietary source, which is fitting since citrulline was named after it. The amount varies by variety and ripeness, but a typical serving supplies a meaningful (though sub-supplemental) amount.
Watermelon rind — here is a genuinely surprising detail: the pale rind (the part most people throw away) is richer in citrulline than the sweet red flesh. Some people juice or blend the rind specifically to capture it.
Other cucurbits — relatives of watermelon such as cucumber, various melons, pumpkin, and gourds contain smaller amounts.

An important reality check: the citrulline in food, watermelon included, comes in relatively small quantities compared with the doses used in the studies below. Research typically uses several grams of citrulline per day, and you would have to eat a large amount of watermelon to match that. So watermelon is a pleasant, hydrating, genuinely citrulline-containing food — not a substitute for the doses tested in trials. Think of it as a nice bonus of eating the fruit, not a therapy.

Your own body remains the largest source of citrulline regardless of diet, through the gut-and-kidney production described above.

What the Evidence Shows

Citrulline has been studied mainly for outcomes tied to blood flow and nitric oxide: blood pressure, exercise, and erectile function. The honest summary is that the mechanism is sound and some results are encouraging, but the studies are generally small, short, and mixed, and citrulline is nowhere near a proven treatment for any of these conditions. Here is the balanced picture.

Blood pressure and vascular stiffness

This is probably citrulline's most plausible area. Several small trials — some using watermelon extract, some using pure citrulline — have reported modest reductions in blood pressure, especially in people with prehypertension or with stiffer arteries, and improvements in measures of how well arteries relax (see Research Papers). The effects are real in these studies but generally small, the trials are short and involve few participants, and results are not perfectly consistent from study to study. It is reasonable to say citrulline may gently support healthy blood pressure and vascular function; it is not reasonable to call it a blood-pressure medication or a replacement for one.

Blood flow, exercise, and soreness

Because more nitric oxide could mean more blood flow to working muscle, citrulline (often as citrulline malate) is a staple of pre-workout supplements. The evidence is genuinely mixed. Some trials report benefits — more repetitions before fatigue in resistance exercise, better performance in cycling time trials, or reduced muscle soreness in the day or two after hard training. Other well-run trials find little or no effect on strength or endurance. A fair reading is that citrulline may offer a small edge for training volume and recovery in some people and some tasks, but it is not a reliable or dramatic performance enhancer, and the picture is far from settled.

Erectile function

Since erections depend on nitric-oxide-driven blood flow — the same pathway targeted by well-known ED drugs — researchers have tested citrulline here too. A small study in men with mild erectile dysfunction found that L-citrulline improved erection hardness in a portion of participants (see Research Papers). The signal is interesting and biologically logical, but the evidence base is very thin: few studies, small numbers, and short durations. Citrulline is clearly weaker than prescription ED medications, and it should be viewed as a mild, unproven option rather than a treatment.

Other uses

Citrulline has been explored as an add-on in some medical settings — for example, small heart-failure studies and its established use as a blood marker of gut health — and in specific inherited urea-cycle disorders it has a legitimate clinical role prescribed by specialists. These are distinct from the over-the-counter supplement claims and belong in the hands of physicians.

L-Citrulline vs Citrulline Malate

Walk down a supplement aisle and you will see two main forms, and the difference trips up a lot of people trying to compare doses.

L-citrulline — this is pure citrulline, the free amino acid. It is the "active ingredient" and the form used in most blood-pressure and pharmacokinetic research.
Citrulline malate — this is L-citrulline bonded to malate (malic acid), a compound involved in your cells' energy cycle. It was originally popularized in exercise research, and the malate portion is sometimes claimed to add its own benefit for energy metabolism, though that extra effect is not well established.

The crucial practical point is about labeling and dose comparison. Citrulline malate is only partly citrulline by weight — roughly half to a bit more, depending on the ratio (products are often sold in a "2:1" citrulline-to-malate ratio). That means a gram of citrulline malate delivers noticeably less actual citrulline than a gram of pure L-citrulline. This is exactly why the citrulline-malate doses in studies look larger (commonly around 8 grams) than the pure L-citrulline doses (commonly around 3–6 grams): a chunk of that 8 grams is malate, not citrulline. When comparing products, always check whether the label lists the dose as pure L-citrulline or as citrulline malate, or you can accidentally under- or over-dose the ingredient you actually care about.

How Much, and Which Form

Dosing depends on which form you choose and, loosely, on your goal. Note that citrulline is a supplement, not a nutrient with an official requirement — there is no Recommended Dietary Allowance for it, and (like all amino acids) no FDA Daily Value, so you will not see a "% Daily Value" for it anywhere. The figures below simply reflect the doses commonly used in research.

Pure L-citrulline — studies for blood pressure and general nitric-oxide support have typically used about 3 to 6 grams per day. Blood-pressure trials often sit around 3 g/day; some circulation and exercise studies go toward 6 g.
Citrulline malate — exercise studies most often use about 8 grams, usually taken 40–60 minutes before a workout. Remember that because of the malate, this is not the same as 8 grams of pure citrulline.

Citrulline is usually taken as a powder mixed into water (it has a mild taste) or in capsules. Because its effect on arginine builds and is sustained, some blood-pressure protocols use it daily over weeks rather than as a single dose. For exercise, timing it before training is the norm.

None of this is a prescription. These are the amounts researchers have found tolerable and sometimes effective — a starting point for a conversation, not medical advice.

Safety & Cautions

By supplement standards, citrulline has a reassuring safety profile. This is actually one of its selling points over arginine: because it bypasses the gut-and-liver processing that makes high-dose arginine so hard on the stomach, citrulline is much better tolerated, and most people take the common doses without trouble.

Still, a few sensible cautions apply:

Digestive upset at high doses — while gentler than arginine, very large amounts can still cause mild stomach discomfort, bloating, or loose stools in some people. Starting low and increasing gradually helps.
Blood-pressure and heart medications — because citrulline can modestly lower blood pressure, combining it with blood-pressure drugs (antihypertensives) could, in theory, push pressure too low. Anyone on such medication should talk to their doctor and monitor for lightheadedness.
Erectile-dysfunction and other nitric-oxide-related drugs — this is the most important caution. Both citrulline and drugs like PDE5 inhibitors (the common ED medications) as well as nitrate heart medications act on the same nitric-oxide/vasodilation pathway. Stacking them could amplify the blood-pressure-lowering effect. Do not combine citrulline with these medications without medical guidance.
Pregnancy, breastfeeding, and children — there is not enough safety data in these groups; supplementation is best avoided unless a doctor advises otherwise.
Kidney considerations — since the kidneys do the work of converting citrulline to arginine, people with kidney disease should check with their physician before supplementing.

As always, choose reputable products, look for independent quality testing where you can, and treat citrulline as what it is: a generally well-tolerated supplement with modest, still-emerging evidence — helpful to some people for circulation-related goals, but not a substitute for a healthy diet, exercise, or any medication your doctor has prescribed.

Research Papers

Schwedhelm E, Maas R, Freese R, et al. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. British Journal of Clinical Pharmacology. 2008;65(1):51–59. doi:10.1111/j.1365-2125.2007.02990.x — The pivotal comparison showing oral L-citrulline raises plasma arginine more effectively than L-arginine itself, the key fact behind citrulline supplementation.
Moinard C, Nicolis I, Neveux N, et al. Dose-ranging effects of citrulline administration on plasma amino acids and hormonal patterns in healthy subjects: the Citrudose pharmacokinetic study. British Journal of Nutrition. 2008;99(4):855–862. doi:10.1017/S0007114507841110 — A dose-ranging study mapping how oral citrulline is absorbed and converted to arginine, informing practical dosing.
Figueroa A, Sanchez-Gonzalez MA, Perkins-Veazie PM, Arjmandi BH. Effects of watermelon supplementation on aortic blood pressure and wave reflection in individuals with prehypertension: a pilot study. American Journal of Hypertension. 2011;24(1):40–44. doi:10.1038/ajh.2010.142 — A small trial linking watermelon-derived citrulline to modestly lower aortic blood pressure in prehypertensive adults.
Ochiai M, Hayashi T, Morita M, et al. Short-term effects of L-citrulline supplementation on arterial stiffness in middle-aged men. International Journal of Cardiology. 2012;155(2):257–261. doi:10.1016/j.ijcard.2010.10.004 — L-citrulline was associated with reduced arterial stiffness, supporting a plausible vascular benefit.
Allerton TD, Proctor DN, Stephens JM, Dugas TR, Spielmann G, Irving BA. l-Citrulline supplementation: impact on cardiometabolic health. Nutrients. 2018;10(7):921. doi:10.3390/nu10070921 — A comprehensive review weighing citrulline's evidence for blood pressure, vascular function, and metabolism, with appropriate caution.
Bailey SJ, Blackwell JR, Lord T, Vanhatalo A, Winyard PG, Jones AM. l-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans. Journal of Applied Physiology. 2015;119(4):385–395. doi:10.1152/japplphysiol.00192.2014 — A tightly controlled study reporting improved oxygen-uptake kinetics and high-intensity performance with L-citrulline.
Suzuki T, Morita M, Kobayashi Y, Kamimura A. Oral L-citrulline supplementation enhances cycling time trial performance in healthy trained men: a double-blind randomized placebo-controlled 2-way crossover study. Journal of the International Society of Sports Nutrition. 2016;13:6. doi:10.1186/s12970-016-0117-z — A crossover trial showing a small improvement in cycling time-trial performance.
Pérez-Guisado J, Jakeman PM. Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. Journal of Strength and Conditioning Research. 2010;24(5):1215–1222. doi:10.1519/JSC.0b013e3181cb28e0 — An often-cited study reporting more repetitions to fatigue and less post-exercise soreness with citrulline malate — part of the mixed exercise evidence.
Wax B, Kavazis AN, Weldon K, Sperlak J. Effects of supplemental citrulline malate ingestion during repeated bouts of lower-body exercise in advanced weightlifters. Journal of Strength and Conditioning Research. 2015;29(3):786–792. doi:10.1519/JSC.0000000000000670 — Reported increased training volume with citrulline malate in trained lifters, illustrating the resistance-exercise findings.
Sureda A, Córdova A, Ferrer MD, Pérez G, Tur JA, Pons A. L-citrulline-malate influence over branched chain amino acid utilization during exercise. European Journal of Applied Physiology. 2010;110(2):341–351. doi:10.1007/s00421-010-1509-4 — Examines how citrulline malate affects amino-acid and nitric-oxide-related metabolism during cycling.
Cormio L, De Siati M, Lorusso F, et al. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction. Urology. 2011;77(1):119–122. doi:10.1016/j.urology.2010.08.028 — A small study reporting improved erection hardness with L-citrulline — a promising but very preliminary result.
Gonzalez AM, Trexler ET. Effects of citrulline supplementation on exercise performance in humans: a review of the current literature. Journal of Strength and Conditioning Research. 2020;34(5):1480–1495. doi:10.1519/JSC.0000000000003426 — A balanced review concluding the exercise evidence is inconsistent and effects, where present, are modest.

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