L-citrulline, a non-essential amino acid found in abundance in watermelon flesh, has become the go-to nitric oxide (NO) precursor in sports nutrition, often replacing arginine itself. A number of studies have repeatedly turned up a counterintuitive finding: taking citrulline raises blood arginine levels more than taking arginine directly does. That paradox has driven fast-growing demand among people chasing better blood flow, vascular health, or exercise performance.
Nitric oxide is one of the primary signaling molecules released by the endothelium, the single-cell lining of blood vessels. It relaxes vascular smooth muscle, which increases blood flow and helps regulate blood pressure. Endothelial NO production tends to decline with age and with lower activity levels, and that decline is what is driving interest in supplementing the pathway nutritionally.
This article walks through how citrulline gets converted to nitric oxide in the body, the dosing and timing that the research actually supports, what kind of changes are realistic to expect, and what to keep in mind if you are pairing citrulline with a near-infrared wellness routine aimed at blood flow.
How Citrulline Builds Nitric Oxide
The Citrulline to Arginine to Nitric Oxide Pathway
Nitric oxide is produced in the vascular endothelium when the enzyme NOS (nitric oxide synthase) uses L-arginine as its substrate. The catch is that most orally ingested arginine gets broken down by arginase in the liver and intestinal mucosa before it ever reaches systemic circulation in meaningful amounts. This is the first-pass effect, and it is the main reason arginine supplements tend to underdeliver on raising blood arginine levels.
L-citrulline, by contrast, largely bypasses hepatic first-pass metabolism. It is absorbed in the small intestine and then converted back to L-arginine, primarily in the kidneys, via the enzymes argininosuccinate synthase (ASS1) and argininosuccinate lyase (ASL), a route commonly called the citrulline-NO cycle. Schwedhelm and colleagues (2008, British Journal of Clinical Pharmacology) found that on a mol-for-mol basis, citrulline supplementation raised plasma arginine levels more than direct arginine supplementation did, and it also improved the arginine-to-ADMA ratio (ADMA, or asymmetric dimethylarginine, is a metabolite that inhibits NOS).
Watermelon extract research
Among natural sources, watermelon flesh and rind are particularly rich in citrulline. The amino acid actually takes its name from the watermelon's genus, Citrullus. In a randomized controlled trial, Figueroa et al. (2017, Nutrients) gave prehypertensive adults watermelon extract, equivalent to roughly 4-6g of L-citrulline, for four weeks and found significant improvements in aortic blood pressure and carotid-femoral pulse wave velocity, a marker of arterial stiffness and dilation responsiveness. The same research group had reported a similar trend in an earlier pilot study (Figueroa et al., 2013), where watermelon extract improved brachial-ankle pulse wave velocity (baPWV).
The performance connection
Bailey et al. (2015, European Journal of Applied Physiology) reported that a single 6g dose of L-citrulline improved VO2 kinetics during high-intensity cycling and extended time to exhaustion. The proposed mechanism is straightforward: nitric oxide-driven vasodilation improves oxygen and nutrient delivery to working muscle. Citrulline also participates in the urea cycle, and some researchers hypothesize it may help clear ammonia and blunt the buildup of fatigue-related metabolites during exercise.
Citrulline versus the nitrate pathway
There is another well-documented way to raise nitric oxide: the nitrate-nitrite-NO pathway, driven by the inorganic nitrate found in beet juice and other sources. That route bypasses the NOS enzyme entirely, so it works through a different mechanism than citrulline does. Researchers have floated the idea that the two pathways could complement each other, but evidence for an actual synergistic effect when combined is still early-stage.
A brief history
Citrulline was first isolated and named from watermelon by a Japanese researcher in the 1930s, and it earned its place as a core amino acid in metabolic biochemistry once its role in the urea cycle was worked out. It did not get real traction in sports nutrition until the 2000s, when citrulline malate started showing up in chronic fatigue research and its connection to exercise recovery came into focus. The randomized controlled trials on vasodilation and exercise performance that form today's evidence base mostly accumulated through the 2010s.
| Source | Estimated Citrulline Content | Notes |
|---|---|---|
| Watermelon flesh (100g) | ~150-370mg | Varies widely by variety and ripeness |
| Watermelon rind, white part (100g) | Often higher than the flesh | Not typically eaten |
| L-citrulline supplement | 3-8g per dose | Range commonly used in research |
| Citrulline malate | 6-8g per dose (~3.5-4.5g citrulline) | Bound to malic acid; common in sports products |
The body also makes its own
Citrulline is not strictly essential; the body makes some of its own through the urea cycle. But the amount produced during normal metabolism generally is not enough to meaningfully boost nitric oxide output. If the goal is exercise performance or vascular health, researchers in this field largely agree that additional intake is necessary.
Dosage and Timing Protocol
Evidence-Based Dosage and Timing
Dosing by goal
The doses used in research vary by goal. For exercise performance, a common protocol is 6-8g of plain L-citrulline or 8g of citrulline malate taken 40-60 minutes before training. Studies targeting vascular health and blood pressure tend to use 3-6g daily, sustained for at least four to eight weeks before meaningful changes show up. Citrulline malate pairs citrulline with malic acid, which also feeds into the TCA (Krebs) cycle and supports energy metabolism, one reason it is the more common form in exercise products.
| Goal | Single Dose | Timing | Duration |
|---|---|---|---|
| Pre-workout performance | 6-8g (as citrulline) | 40-60 min before exercise | Single dose or ongoing |
| Vascular elasticity / blood pressure | 3-6g | Same time daily | 4+ weeks |
| Erectile function support | 1.5g, 3x daily | With meals | 1+ month |
| Recovery / muscle soreness | 8g | Immediately post-workout | Single dose to several days |
Practical tips for taking it
A few things make this easier to stick with in practice. Take it with water rather than on an empty stomach to cut down on GI discomfort. Citrulline malate has a noticeably sour, malic-acid tang, so diluting it in a small amount of juice or water improves compliance for most people. B-vitamins and antioxidant nutrients like vitamin C and polyphenols play a supporting role in nitric oxide precursor metabolism, so pairing supplementation with vegetables and fruit may add to the effect. Evidence on combining citrulline with caffeine or nitrate-rich beet juice is still limited, so it is worth watching your own response rather than assuming the effects stack. And because the powder form is moisture-sensitive, keep it sealed and try to use it up within three months of opening to preserve potency.
Why food alone rarely gets you there
Getting to the 3-8g doses used in research through food alone would mean eating several whole watermelons in a sitting, which is not realistic for most people. That is why the sports nutrition market leans on purified L-citrulline or citrulline malate in powder or capsule form. When choosing a product, it is worth checking for third-party certification, such as banned-substance testing, and confirming that the labeled dose matches what is actually in the container.
Pairing it with a broader blood-flow routine
Nutrition alone rarely maximizes blood flow improvements. Regular aerobic exercise, adequate hydration, and not smoking are the baseline factors that support endothelial function. Cutting sodium intake and eating more potassium-rich vegetables and fruit is another supporting strategy worth considering for vasodilation. For more on that angle, see our piece on dietary fiber and vascular health.
What Changes When Blood Flow Improves
What Changes When Nitric Oxide Goes Up
Vascular function markers
Nitric oxide relaxes vascular smooth muscle, triggering vasodilation. That shows up in measurements like blood pressure, pulse wave velocity, and flow-mediated dilation (FMD). Beyond the Figueroa (2017) study mentioned earlier, several meta-analyses have found that L-citrulline supplementation produces a modest but statistically significant drop in both systolic and diastolic blood pressure. Bailey et al. (2010, Journal of Applied Physiology) found that citrulline lowered resting diastolic blood pressure and, via Doppler ultrasound, showed increased blood flow to working muscle during high-intensity exercise.
What it feels like during training
- Muscular endurance: Several studies report a delayed onset of fatigue during high-intensity repeated efforts, and a number of resistance-training studies found more reps completed per set.
- Muscle soreness (DOMS): Some studies found reduced post-exercise soreness, but effect sizes and reproducibility vary considerably across trials.
- Oxygen efficiency during exercise: At a given intensity, oxygen uptake sometimes improves, which can translate into lower perceived exertion.
- Post-exercise lactate: A few studies found a blunted rise in blood lactate in citrulline groups, though this is not a consistent finding across the literature.
Setting realistic timelines
After a single dose, plasma arginine peaks within one to two hours, and any performance benefit in that window is a one-off effect tied to that session. Chronic markers like blood pressure and arterial elasticity are a different story; those changes typically start showing up only after four or more weeks of consistent intake, and how much they move depends heavily on baseline vascular health, sodium intake, and activity level. Not everyone sees the same magnitude of improvement.
Who responds more, and who responds less
People who start out with reduced vascular function or somewhat elevated blood pressure tend to see larger improvements, while young, healthy athletes whose vascular function is already good tend to see smaller gains. The likely explanation is a ceiling effect: if the nitric oxide pathway is already running near its optimum, there is less room for supplementation to add anything.
Growing interest among older adults
Because observational studies consistently link aging with a gradual decline in endothelial NO production, interest in citrulline supplementation has been growing among middle-aged and older adults. Large, long-term trials in this age group are still relatively scarce, though, so anyone managing a chronic condition or taking multiple medications should work out an appropriate dose with a physician rather than self-selecting a high dose.
Early research on cognitive function
There is also early research looking at whether NO-driven improvements in cerebral blood flow affect cognitive function. A handful of small studies have found improved markers of brain blood flow after citrulline intake, but sample sizes are small and there has not been enough replication to draw firm conclusions. This is an area that needs larger randomized controlled trials before it can be treated as an established clinical recommendation.
Side Effects and Precautions
What to Watch For
- GI discomfort: At high doses (10g or more), some people experience bloating or loose stools. Starting with a smaller amount and checking your tolerance first is the safer approach.
- Blood pressure sensitivity: Because of its vasodilating action, anyone with a tendency toward low blood pressure, or taking blood-pressure-lowering medication such as nitroglycerin-type drugs or PDE5 inhibitors, should talk to their doctor before starting. Combining citrulline with a PDE5 inhibitor prescribed for erectile dysfunction carries a theoretical risk of an excessive blood pressure drop.
- Pregnancy and breastfeeding: Safety data is limited, so professional guidance is recommended before use.
- Kidney function: Since the citrulline-to-arginine conversion happens mainly in the kidneys, anyone with kidney disease should discuss appropriate dosing with their doctor.
- History of herpes: Arginine has been reported in some studies to promote herpes virus replication, so people with a history of outbreaks should be aware that the arginine increase from citrulline metabolism could be a factor.
- Around surgery: Because it can affect blood pressure and blood flow, it is safer to stop taking citrulline one to two weeks before any scheduled surgery and let your care team know you have been using it.
- When combined with NIR sessions: Anyone on photosensitizing medications, such as tetracyclines or amiodarone, pregnant abdomens, sites of active malignancy, and the thyroid area should avoid direct irradiation, and protective eyewear should be worn. NIR is a wellness aid, not a substitute for professional medical care; if symptoms persist or worsen, see a physician first.
Overall, citrulline has a fairly favorable safety profile, but anyone on specific medications or managing an underlying condition should check with a professional before adding it. Choose supplements with verified quality control, and where relevant, look for health-functional-food certification from the appropriate regulatory body. Products bought through overseas direct-purchase channels carry a somewhat higher risk of the label not matching what is actually in the bottle, so it is worth developing the habit of checking ingredient sourcing and third-party verification data.
