Also known as: citrulline, citrulline malate, watermelon extract
The amino acid that replaced arginine in pre-workouts, for the good reason that arginine never survived digestion.
Reasonable evidence that citrulline raises plasma arginine and nitric oxide production; performance trials are smaller, mixed, and complicated by the malate component in most products.
Citrulline exists in your pre-workout because arginine failed. The logic behind arginine was sound — it is the direct substrate for nitric oxide synthesis, and nitric oxide dilates blood vessels — but swallowed arginine gets dismantled by enzymes in the gut wall before it reaches the bloodstream. Citrulline takes a detour through the kidneys and, absurdly, raises blood arginine better than arginine does.
That much is well established. What is less settled is whether the extra nitric oxide translates into performance you can measure. The trials are mostly small, and about half report modest improvements in resistance-training volume — an extra rep or two across multiple sets — while the rest find nothing. Endurance results are similarly split.
A complication muddies almost every study: most research and nearly all products use citrulline malate rather than pure citrulline, and malate is a Krebs-cycle intermediate with its own plausible ergogenic effects. Nobody has cleanly separated the two, so "citrulline malate works" and "citrulline works" are not interchangeable claims.
The subjective "pump" is real and is the reason the ingredient sells. Increased blood flow to working muscle produces genuine transient swelling. Whether that pump does anything for long-term hypertrophy is unresolved — the cell-swelling hypothesis is biologically reasonable and remains unproven in humans.
Practically: 6-8 g of citrulline malate about an hour before training is the studied protocol, or 3-6 g of pure citrulline if you would rather isolate the variable. It is well tolerated, cheap in bulk, and stacks logically with beetroot since they arrive at nitric oxide by different routes. Set expectations at the level the evidence supports, though. This is a compound with a well-mapped mechanism and an underwhelming outcome literature — worth trying if you like the pump, not worth defending as essential.
Oral arginine is rapidly metabolised by intestinal and hepatic arginase, so only part of an oral dose reaches systemic circulation. Citrulline follows a different absorption route, is converted to arginine largely in the kidneys, and generally raises plasma arginine more effectively than oral arginine — feeding the nitric oxide synthase pathway and downstream vasodilation.
Well tolerated at studied doses; unlike arginine it rarely causes GI distress, which is another reason it displaced it. No safety signals in the trial literature at 6-8 g.
This guide is educational, not medical advice. Talk to a physician or pharmacist before starting any supplement, especially alongside prescription medication or a medical condition.
Evidence review last updated