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L-Citrulline
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In one pass L-citrulline is the ingredient in pre-workout supplements sold for the pump and more blood flow.
Educational content, not medical advice — consult a clinician.
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Chapter 1
Why not just take arginine
L-citrulline is the ingredient in pre-workout supplements sold for the pump and more blood flow. In the body it first becomes arginine, which the lining of your blood vessels uses to make nitric oxide (), a gas signal that relaxes and widens vessels. The name comes from *citrullus*, Latin for watermelon, and watermelon (especially the pale rind next to the skin) is still its best-known food source.
It is an amino acid that is not built into proteins: it does not become part of muscle, and unlike leucine it does not switch on muscle building directly. Its real role is upstream on the nitric oxide route: citrulline becomes arginine, and arginine becomes nitric oxide.
The counterintuitive part: to raise the arginine in your blood, taking citrulline works better than taking arginine. A large share of swallowed arginine is broken down by the enzyme arginase in the gut and liver. Citrulline slips past that step, enters the blood and is turned back into arginine, mainly by the kidneys.
One safety point up front: if you use a nitrate heart medicine such as nitroglycerin, do not take it. If you have used it together with an erectile-dysfunction drug and feel dizzy, see your vision go dark, feel close to fainting or have chest pain, get medical care immediately.
It is an amino acid that is not built into proteins: it does not become part of muscle, and unlike leucine it does not switch on muscle building directly. Its real role is upstream on the nitric oxide route: citrulline becomes arginine, and arginine becomes nitric oxide.
The counterintuitive part: to raise the arginine in your blood, taking citrulline works better than taking arginine. A large share of swallowed arginine is broken down by the enzyme arginase in the gut and liver. Citrulline slips past that step, enters the blood and is turned back into arginine, mainly by the kidneys.
One safety point up front: if you use a nitrate heart medicine such as nitroglycerin, do not take it. If you have used it together with an erectile-dysfunction drug and feel dizzy, see your vision go dark, feel close to fainting or have chest pain, get medical care immediately.
Mechanism · Why arginine loses to citrulline
"I want more nitric oxide, so why not just take arginine (L-arginine)?" It is one of the most common misunderstandings in the supplement aisle.The route of swallowed arginine: it is absorbed in the small intestine and enters the portal vein. But arginase, the enzyme that splits arginine into urea and ornithine, works very efficiently in gut cells and in the liver, and a large share is broken down before it reaches the general circulation. Schwedhelm 2008 states in its abstract that the problem with oral arginine is extensive first-pass removal by intestinal arginase. What is left adds only a limited amount to the circulation, and high doses easily cause stomach upset and diarrhea.
The route of swallowed citrulline: it is absorbed in the small intestine. Citrulline is not a substrate for arginase, and the liver takes up little of it, so it passes straight into the general circulation. Two enzymes in the kidney's proximal tubules (ASS and ASL) then turn it back into arginine (textbook knowledge). Plasma arginine rises for longer, and the gut tolerates it much better.
So the most effective way to supplement arginine is not to take arginine but to take its upstream precursor. It is a classic case of pharmacokinetics (how a substance is absorbed, converted and cleared after it enters the body) beating intuition.
Read the numbers carefully: Schwedhelm 2008 (British Journal of Clinical Pharmacology, BJCP) gave 20 healthy volunteers six dosing regimens in turn, in a double-blind crossover design, each for 1 week. With citrulline 3 g twice a day (6 g/day), the area under the curve for plasma arginine (AUC, a measure of the total amount in the blood over time) was about 898 µmol·h/L, higher than the roughly 283 with about 3 g/day of oral arginine. The direction favors citrulline, but the doses were not equal, so do not read it as +227% in an equal-dose comparison.
Two more limits:
In the same study, citrulline raised the nitric oxide breakdown product nitrate and cGMP in urine, but no regimen improved flow-mediated dilation (FMD, an ultrasound test of how well an artery widens) above baseline.Bailey 2015 ran an equal-dose comparison in 10 healthy men (6 g/day of each, for 7 days). Arginine and citrulline raised plasma arginine by a similar amount, yet only citrulline lowered mean arterial pressure and improved high-intensity cycling performance.
So "citrulline works better" is clearest at the level of raising blood arginine. At the level of vessel function and exercise performance, there are still only a few small trials.
Chapter 2
How nitric oxide widens vessels
Citrulline does not widen vessels by itself. It first turns back into arginine, and then the cells lining the vessel (the endothelium) use that arginine to make nitric oxide ().
Nitric oxide is a gas. It diffuses freely from the endothelial cells into the smooth muscle wrapped around the vessel, the tense muscle relaxes, the vessel widens, and more blood flows to the muscles that are working. The engorged pump you feel during training is this.
The chemical relay in between goes like this. Nitric oxide switches on a signaling enzyme in the smooth muscle, which makes the second messenger cGMP (a small molecule that passes the signal along inside the cell). cGMP pushes calcium in the cell down, the muscle relaxes, and the vessel opens up.
Sildenafil (Viagra), nitroglycerin and a range of supplements sold for vessel health all work on this same route. Citrulline raises the arginine at the entrance, sildenafil blocks the breakdown of cGMP, and nitroglycerin supplies nitric oxide directly. The three act at different points on one route, which is why their effects add up when they are combined.
Nitric oxide is a gas. It diffuses freely from the endothelial cells into the smooth muscle wrapped around the vessel, the tense muscle relaxes, the vessel widens, and more blood flows to the muscles that are working. The engorged pump you feel during training is this.
The chemical relay in between goes like this. Nitric oxide switches on a signaling enzyme in the smooth muscle, which makes the second messenger cGMP (a small molecule that passes the signal along inside the cell). cGMP pushes calcium in the cell down, the muscle relaxes, and the vessel opens up.
Sildenafil (Viagra), nitroglycerin and a range of supplements sold for vessel health all work on this same route. Citrulline raises the arginine at the entrance, sildenafil blocks the breakdown of cGMP, and nitroglycerin supplies nitric oxide directly. The three act at different points on one route, which is why their effects add up when they are combined.
Safety · Which drugs it must not be stacked with
The mechanism is real, and so are the interaction risks.Do not combine it with nitrate heart medicines
Nitroglycerin and isosorbide mononitrate (common angina drugs) supply nitric oxide directly.Sildenafil and tadalafil (PDE5 inhibitors, the common erectile-dysfunction drugs) stop cGMP from being broken down, so it stays active in the vessels longer. Taking a nitrate together with one of these is already a contraindicated combination that can cause severe low blood pressure.Adding citrulline on top pushes the nitric oxide supply up from upstream as well. Reasoned from the mechanism, that amplifies the vessels throughout the body at once, and blood pressure could fall to a dangerous level within minutes. No one has tested this three-way combination in a trial, so the only guide is the mechanism, and its answer is do not do it.
If you feel dizzy, see your vision go dark, feel close to fainting or have chest pain after combining them, get medical care immediately (go to the emergency department).
Situations to watch:
Already on blood-pressure drugs (especially ACE inhibitors, or calcium channel blockers, CCBs): in Bailey 2015, healthy men already had lower mean arterial pressure after 7 days of citrulline, so check your blood pressure when combining them; it may run lower than usual.Migraine with aura: the nitric oxide route is thought to take part in attacks, so by the mechanism a few people might have attacks triggered.Severe kidney failure: turning citrulline back into arginine happens mainly in the kidney, and that process changes when the kidneys fail (textbook knowledge).
None of this means never take it. It means do not stack it casually. A supplement is completely safe only when it does nothing; the clearer a supplement's mechanism, the more its interactions are worth reading carefully.
Chapter 3
It also helps clear ammonia
Citrulline also appears on another metabolic map: the urea cycle. This is the body's central workshop for turning excess ammonia (NH₃, a toxic waste left when protein is broken down) into urea that leaves in the urine, and it runs mainly in liver cells (textbook knowledge).
Simplified to five steps: ammonia and carbon dioxide first form carbamoyl phosphate, which joins ornithine to form citrulline. Citrulline becomes argininosuccinate, then arginine; arginine gives off urea and leaves ornithine, which returns to the start. Citrulline is an intermediate of this cycle, which is how biochemistry textbooks introduce it.
That gives citrulline two roles that look unrelated:
During exercise: it raises arginine, more nitric oxide is made, and you get the pump (what the chapter How nitric oxide widens vessels covers).In metabolism: it feeds raw material to the urea cycle and may reduce the build-up of blood ammonia after exercise. Some researchers think this is one way it eases the feeling of fatigue, acting on the ammonia load in the brain rather than on acid in the muscle.
One molecule doing two jobs at once is why studies on whether citrulline reduces fatigue often read as a muddle: a performance gain may come from blood flow, from clearing ammonia, or from both. Reading it as a single-route supplement misreads it.
Simplified to five steps: ammonia and carbon dioxide first form carbamoyl phosphate, which joins ornithine to form citrulline. Citrulline becomes argininosuccinate, then arginine; arginine gives off urea and leaves ornithine, which returns to the start. Citrulline is an intermediate of this cycle, which is how biochemistry textbooks introduce it.
That gives citrulline two roles that look unrelated:
During exercise: it raises arginine, more nitric oxide is made, and you get the pump (what the chapter How nitric oxide widens vessels covers).In metabolism: it feeds raw material to the urea cycle and may reduce the build-up of blood ammonia after exercise. Some researchers think this is one way it eases the feeling of fatigue, acting on the ammonia load in the brain rather than on acid in the muscle.
One molecule doing two jobs at once is why studies on whether citrulline reduces fatigue often read as a muddle: a performance gain may come from blood flow, from clearing ammonia, or from both. Reading it as a single-route supplement misreads it.
Clinical · Where citrulline is used in medicine
Citrulline's most established medical uses are, ironically, not in the gym (what follows is clinical textbook knowledge):1. Urea cycle enzyme defects (inherited metabolic diseases)
Examples are ornithine transcarbamylase (OTC) deficiency and argininosuccinate synthetase (ASS) deficiency, also called citrullinemia.In these patients the urea cycle is broken at one step, and ammonia cannot be cleared.Treatment often adds citrulline or arginine, depending on where the break is, to reconnect the cycle downstream of it: citrulline for OTC deficiency, arginine for citrullinemia.This is amino acid therapy designed around a specific enzyme defect and prescribed by metabolic specialists, not supplement marketing.
2. Short bowel syndrome (SBS)
In patients who have lost a large part of the small intestine to surgery, the gut makes less citrulline of its own.So the plasma citrulline level is often used to estimate how much working small intestine is left.This is a diagnostic use, not a treatment.
3. Vessel-lining function in sickle cell disease
Some studies are exploring whether citrulline improves nitric oxide metabolism and endothelial function in children with sickle cell anemia.The evidence is still accumulating; the mechanism is plausible.
These medical uses and taking 8 g for a pump involve the same molecule with completely different dose logic. It is a reminder that the same molecule is often only one clinical setting and one prescription away from being a drug rather than a supplement.
Chapter 4
How much it helps performance
The evidence on exercise performance is real, just a notch smaller than the supplement ads. Several randomized crossover trials of citrulline before training point the same way: a few more reps before failure, lasting longer in high-intensity cycling, a slightly faster time trial, and a little less soreness afterward.
Taken together, most of these effects sit in the 1–15% range: real but limited, not a doubling. For example, 22 trained men who took it for 7 days rode a 4 km time trial 1.5% faster.
The number ads inflate most comes from one trial in which 8 g of citrulline malate gave 52.92% more bench-press reps to failure. That figure is the gap in the last set only (the 8th); in the earlier sets the gap was much smaller. It comes from a single-dose crossover trial in 41 men, with reps to failure as the endpoint. Treat it as the best case, not the average you should expect.
Taken together, most of these effects sit in the 1–15% range: real but limited, not a doubling. For example, 22 trained men who took it for 7 days rode a 4 km time trial 1.5% faster.
The number ads inflate most comes from one trial in which 8 g of citrulline malate gave 52.92% more bench-press reps to failure. That figure is the gap in the last set only (the 8th); in the earlier sets the gap was much smaller. It comes from a single-dose crossover trial in 41 men, with reps to failure as the endpoint. Treat it as the best case, not the average you should expect.
Mechanism · Why a pump is not muscle growth
The pump (engorgement) is real: vessels widen, blood flow to the working muscle rises, and the muscle looks and feels different for a short while.But be clear: the pump itself is not a signal for muscle growth.
One common account holds that muscle growth (hypertrophy) is driven by three things: mechanical tension, metabolic stress, and repair after muscle damage. The pump belongs roughly to part of the second. Its effect is not zero, but pre-workout blend marketing has inflated it into no pump, no growth.
Back to what citrulline actually does:
What it does: improves blood flow during and after training, gives a stronger subjective sense of fullness, and delays local fatigue a little. In Pérez-Guisado 2010, soreness 24 and 48 hours after training was 40% lower; in the same trial 14.63% of participants reported stomach discomfort, the only side effect recorded.What it does not do: directly stimulate muscle protein synthesis (that is the job of leucine, total protein and training load), raise the most you can lift once (your ), or make up for a flawed training plan or short sleep.
So in the order of supplement priorities, it belongs to the second tier, to consider only after training, sleep, protein, creatine and caffeine are all in place. The same tier includes Beta-alanine (which buffers acid in muscle) and beetroot nitrate (another route to nitric oxide). They work in different ways, and there is no need to stack them all.
Chapter 5
How much, and who needs it
For citrulline to work, the dose has to land in the range the trials used. A scoop of pre-workout blend is also packed with caffeine, Beta-alanine and a pile of other ingredients, and the label often gives only the total rather than the amount of each, so the citrulline share may be far below the trial dose. You are buying the advertised dose, not the research dose. Plain citrulline powder bought on its own gives you control over the dose.
It works through nitric oxide, so people taking nitrate angina drugs should not use it, and combining it with erectile-dysfunction drugs carries a risk of an acute drop in blood pressure. This chapter goes on to the dose range used in trials, the difference between daily and single use, and how to judge whether you need it at all.
It works through nitric oxide, so people taking nitrate angina drugs should not use it, and combining it with erectile-dysfunction drugs carries a risk of an acute drop in blood pressure. This chapter goes on to the dose range used in trials, the difference between daily and single use, and how to judge whether you need it at all.
In practice · Effective doses and the blend trap
Doses used in trials:Plain citrulline (L-citrulline): commonly 6–8 g, about 60–90 minutes before training. Bailey 2015 used 6 g a day for 7 days; Suzuki 2016 used only 2.4 g a day and still saw a faster time trial.Citrulline malate (CM): 8 g (of which roughly 4–5 g is citrulline and the rest is the malate part). Pérez-Guisado 2010 gave a single dose before training.Daily versus single use: the cycling trials mostly gave it for 7 days, while the bench-press trial used one dose; no trial has compared the two approaches head to head.
The key trap: blends may hold only a sliver of citrulline
Turn any tub of pre-workout blend around and look at the proprietary blend:
One scoop usually packs caffeine, Beta-alanine, citrulline, taurine, tyrosine and more.When the label lists only the blend total, you cannot know how much citrulline a scoop contains, and its share is often far below the dose used in trials.You are buying the advertised dose, not the research dose.
Buying the plain ingredient is cheaper and easier to control: you can weigh every gram and know what you took.
Do not stack with:
Nitrate angina drugs (do not combine)PDE5 inhibitors (sildenafil, tadalafil): risk of an acute drop in blood pressureCare is needed if you already take several blood-pressure drugs.If you have migraine with aura, watch how you respond.
In practice · Do you need citrulline
Ask four questions, in order:Q1: Is your training plan stable?
No: get your training stable first; supplements cannot make up for that.Yes: go to Q2.
Q2: Are protein, sleep, creatine and caffeine in place?
No: sort out those four first; they are far better value than citrulline.Yes: go to Q3.
Q3: What kind of training do you do?
High-volume bodybuilding, sets of 8–15 reps, chasing the pump: citrulline is a meaningful small boost.Pure strength work at 1–3RM: little value, so you can skip it.Long-distance aerobic work: almost no relevant evidence, so skip it.Middle distances (400–1500 m) or high-intensity intervals: it may help, but Beta-alanine and nitrate fit better.
Q4: Is there anything you need to avoid?
Taking a nitrate drug: do not take it.Taking a PDE5 inhibitor: do not add it on your own.Taking blood-pressure drugs: ask your doctor first and keep measuring your blood pressure.Pregnant or breastfeeding: there are no data, so skip it.
One last rule: buy single-ingredient plain citrulline powder (not a blend). Start at 3 g, take it for a week to check tolerance, then increase to 6–8 g. It is cheap, the dose is clear, and interactions are easier to manage.
It is not a must-take foundation supplement. But once you are already training seriously and want a little extra, it is one of the few options with a clear mechanism, clear pharmacokinetics and few side effects.
References · 5
- Schwedhelm, E., Maas, R., Freese, R., et al. (2008). Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. British Journal of Clinical Pharmacology, 65(1), 51-59. Citrulline 3 g bid (6 g/day) arginine AUC 898±67 vs arginine IR 1 g tid (~3 g/day) 283±51 µmol h/L. Not an equal-dose comparison; do not cite as '+227% at equal doses'. Abstract: double-blind, randomized, placebo-controlled crossover in 20 healthy volunteers, six dosing regimens of placebo, citrulline and arginine for 1 week each. Citrulline raised plasma arginine AUC and Cmax dose-dependently and more effectively than arginine (P < 0.01); 3 g twice daily raised the arginine/ADMA ratio (186 to 278) and urinary nitrate and cGMP; no regimen improved flow-mediated dilation over baseline (abstract, PMID 17662090). 10.1111/j.1365-2125.2007.02990.x
- Friebe, A., & Koesling, D. (2009). The function of NO-sensitive guanylyl cyclase: what we can learn from genetic mouse models. Nitric Oxide, 21(3-4), 149-156. 10.1016/j.niox.2009.07.004
- Pérez-Guisado, J., & Jakeman, P. M. (2010). Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. Journal of Strength and Conditioning Research, 24(5), 1215-1222. 41 men, randomized double-blind 2-period crossover: a single 8 g dose of citrulline malate or placebo before a 16-set pectoral training session. Repetitions to fatigue at 80% of 1RM rose from the third set on, reaching 52.92% more repetitions in the last set; muscle soreness was 40% lower at 24 and 48 h; 14.63% reported stomach discomfort (abstract, PMID 20386132). 10.1519/JSC.0b013e3181cb28e0
- Bailey, S. J., Blackwell, J. R., Lord, T., Vanhatalo, A., Winyard, P. G., & Jones, A. M. (2015). L-citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans. Journal of Applied Physiology, 119(4), 385-395. 10 healthy men, randomized placebo-controlled crossover, 7 days each of placebo, L-arginine 6 g/day and L-citrulline 6 g/day. Citrulline raised plasma arginine as much as arginine did, lowered mean arterial pressure, sped VO2 kinetics in severe-intensity cycling, extended severe-intensity exercise tolerance (589 vs 661 s) and slightly raised total work in the performance test (123 vs 125 kJ, P < 0.05); arginine changed none of these (abstract, PMID 26023227). 10.1152/japplphysiol.00192.2014
- Suzuki, T., Morita, M., Kobayashi, Y., & Kamimura, A. (2016). Oral L-citrulline supplementation enhances cycling time trial performance in healthy trained men: double-blind randomized placebo-controlled 2-way crossover study. Journal of the International Society of Sports Nutrition, 13, 6. 22 trained men: 2.4 g/day L-citrulline or placebo for 7 days, plus 2.4 g 1 h before a 4-km cycling time trial on day 8. Citrulline raised plasma arginine and cut completion time by 1.5% (p < 0.05), and improved subjective muscle fatigue and concentration right after exercise. Three of the four authors list Kyowa Hakko Bio Co., Ltd. (abstract and affiliations, PMID 26900386). 10.1186/s12970-016-0117-z