L-Citrulline for Men: Blood Flow, Performance & Evidence (UK Guide)

By Jordan Harvey · 17 July 2026 · Updated 21 August 2026 · 8 min read

L-citrulline is an amino acid that has quietly built a reputation in the men’s health and sports-nutrition world, largely on the back of its link to blood flow. Here is the honest summary up front: citrulline is one of the better-studied amino acids for raising the body’s arginine levels, and a modest body of human trials has looked at it for exercise and circulation. The headline that surprises most people is that citrulline, taken by mouth, raises blood arginine more reliably than arginine itself. This UK guide explains what citrulline is, why it differs from L-arginine, what the trials actually show, how it is dosed, whether it is safe, and what to look for when buying.

What Is L-Citrulline?

L-citrulline is a non-essential amino acid, meaning your body can make it on its own and you do not strictly need it from food. It was first isolated from watermelon — Citrullus is the Latin name for the watermelon genus, which is where the name comes from. Beyond watermelon, small amounts appear in other foods, but the quantities used in research are far higher than diet alone would supply.

In the body, citrulline plays a role in the urea cycle (how the body clears ammonia) and, more relevant here, it is a stepping-stone to L-arginine. As a supplement it is sold in two main forms: pure L-citrulline and citrulline malate, the latter being citrulline bound to malic acid (a compound found in apples). Both deliver citrulline; the malate version simply carries extra weight from the malic acid.

L-Citrulline vs L-Arginine: The Key Difference

This is the single most useful thing to understand, because it is counter-intuitive. L-arginine is the amino acid that directly feeds nitric oxide production — so you might assume taking arginine is the most direct route. In practice, a large share of swallowed arginine is broken down in the gut wall and liver by an enzyme called arginase before it ever reaches the bloodstream. This is why oral arginine often produces a smaller, shorter rise in blood arginine than the dose would suggest.

Citrulline sidesteps this. Because it is not a target for arginase in the gut, swallowed citrulline passes through to the kidneys, where it is converted into arginine and released into the blood. The practical result, repeatedly observed in pharmacokinetic studies, is that oral citrulline raises plasma arginine more, and for longer, than the same dose of oral arginine.

This is a research observation about the citrulline ingredient at the doses studied. It is not a health claim, and it does not transfer to any finished product, including Blue Power, which does not contain L-citrulline.

The Nitric Oxide Pathway, Briefly

Why does the arginine link matter? Because arginine is the raw material the body uses to make nitric oxide — a short-lived signalling molecule (written NO) that tells the smooth muscle in blood-vessel walls to relax. When those vessels relax, they widen, a process called vasodilation. Wider vessels can carry more blood, which is the mechanism behind much of the interest in citrulline for both exercise and general circulation.

It is worth being precise here: raising arginine availability is a plausible mechanism, but a mechanism is not the same as a guaranteed outcome in a finished product. The trials below are where the story gets tested, and the picture is encouraging in places and modest in others.

What the Evidence Actually Shows

L-citrulline has no authorised European (EFSA) health claim, so nothing below should be read as a benefit you can expect from any product. Each finding is reported as a passive trial observation at the dose studied, with the standard caveat that ingredient findings do not transfer to any finished supplement.

Endurance exercise Moderate evidence, mixed

The most-cited exercise finding comes from cycling. In Suzuki et al. (2016, n=22 trained male cyclists, 2.4 g/day L-citrulline for seven days plus a dose before the test), citrulline was associated with a faster 4 km cycling time-trial and lower self-reported fatigue than placebo at the dose studied (Suzuki et al., J Int Soc Sports Nutr, 2016). It is a single, modest-sized trial, and not every endurance study has been positive, so the signal is real but should not be oversold.

Resistance exercise and recovery Limited evidence

In resistance training, the form usually studied is citrulline malate at around 8 g. In a placebo-controlled crossover (Wax et al., 2015, n=12 resistance-trained men), citrulline malate was associated with a greater number of repetitions to failure across lower-body sets at the dose studied (Wax et al., J Strength Cond Res, 2015). Other crossover studies have reported reduced post-exercise muscle soreness, though sample sizes are small and results across the literature are not uniform.

These are findings about the citrulline ingredient at the doses studied in the cited trials. They are not health claims and do not transfer to any finished product, including Blue Power, which does not contain L-citrulline.

Circulation and erectile studies Cautious / early

A small amount of clinical work has looked at citrulline and circulation in men with a specific complaint. In Cormio et al. (2011, n=24 men with mild erectile difficulty), L-citrulline was associated with an improvement in self-reported sexual function scores compared with placebo at the dose studied; the authors described the effect as modest and called for larger trials (Cormio et al., Urology, 2011). This was a small, short pilot study in men with an existing condition.

The result above is a research observation about the citrulline ingredient at the dose studied in a small clinical trial. It is not a product claim, it does not establish any benefit, and it does not transfer to Blue Power or any other finished supplement. Erectile difficulty can have medical causes — see your GP rather than self-treating with a supplement.

Typical Dosage & Forms

There is no officially established dose, because there is no authorised health claim to anchor one. Across the trial literature, two patterns emerge: a daily 1.5–3 g of pure L-citrulline for general circulation interest, and a pre-exercise 6–8 g of citrulline malate for training studies. Because citrulline malate is part malic acid by weight, an 8 g dose of the malate delivers roughly the same citrulline as a smaller dose of the pure form.

Is It Safe? Side Effects

In trials, L-citrulline is generally well tolerated, with side effects uncommon and usually mild — mostly minor digestive upset at higher doses. It does not tend to cause the stomach discomfort that high-dose arginine sometimes does. As with any amino acid taken in supplement amounts, there is limited long-term safety data in healthy men.

L-Citrulline vs Other Blood-Flow Ingredients

If your interest in citrulline is about circulation, it helps to see where it sits among the related options. For broader context, see our guide to L-arginine, our overview of the nitric oxide pathway, and our pillar on herbal supplements for men.

The honest takeaway: citrulline and arginine are linked, but citrulline is the more reliable way to raise blood arginine because it avoids the gut breakdown that limits oral arginine. None of these should be treated as a guaranteed circulation fix, and the exercise evidence for citrulline, while encouraging, rests on small trials.

Frequently Asked Questions About L-Citrulline

The Bottom Line

L-citrulline is one of the more interesting amino acids for men focused on circulation and training, mainly because of one well-documented quirk: it raises blood arginine more reliably than arginine taken directly. That arginine feeds the nitric oxide pathway that relaxes and widens blood vessels. The human exercise evidence — cycling and resistance training — is encouraging but rests on small trials, and citrulline carries no authorised health claim, so every finding here is an ingredient observation rather than a guaranteed outcome.

If you are weighing citrulline against L-arginine, the absorption advantage is the main reason people choose it. Whatever you decide, buy from suppliers that disclose the form and provide a Certificate of Analysis, and check with your GP first if you take nitrate, PDE5, blood-pressure or blood-thinning medication, or have a medical condition.