L-Citrulline
Non-protein-forming amino acid from the urea cycle that raises the arginine level more reliably than arginine itself. The markers increase, the performance effect remains at the detection limit of 0.15 to 0.20, and the added malate contributes nothing.
Auch bekannt als: Citrullin, Citrulline, L-Citrulline, Citrullin-Malat
What it is
L-Citrulline is an amino acid that is not found in any protein. It is an intermediate product of the urea cycle, the cycle in the liver that converts the nitrogen produced during protein breakdown into excretable urea. It is named after the watermelon from which it was first extracted and in which it is found in significant quantities.
In the body, citrulline is the precursor to arginine, and arginine is the starting material from which NO synthases form nitric oxide. This explains why a substance that does not contain protein is found in every second pre-workout powder: it is the detour to a vasodilator signaling substance.
How it works
Why the detour is shorter than the direct route
Swallowing arginine directly works poorly. A large proportion is broken down by arginase in the intestinal wall and liver before it reaches the circulation. Citrulline bypasses this breakdown: it is reabsorbed, taken up in the kidneys and converted there into arginine, which then enters the circulation. Oral citrulline therefore raises the arginine level in the blood more reliably than oral arginine, and with it nitrate and nitrite in the blood also increase. For this common goal in a completely different way, see Nitrate and L-arginine.
Where the chain breaks
Everything is measured up to this point: citrulline increases, arginine increases, the end products of nitric oxide metabolism increase. The next step is the one that matters, and it is the weakest. Evidence for acute vasodilation and improved blood flow to skeletal muscle after citrulline administration is sparse and inconsistent.
This is where the secured ends. The mechanism is plausible and proven in its individual parts, the causal chain up to the dumbbell is not. What the user feels as a pump is a perception of tension and fullness in the muscle, and this arises even without any addition through repetitions with a short break. A noticeable pump is therefore not proof that more blood is flowing.
Ammonia
The second common reason is that citrulline accelerates the disposal of ammonia through the urea cycle, which is produced in the muscles during hard work and contributes to fatigue. Citrulline's place in this cycle is textbook knowledge. The fact that an additional dose reduces the ammonia peak after a training session and that this changes performance are two further steps, and the review article on the topic lists them under the proposed, not the proven, mechanisms.
What the studies show
The work on which the reputation rests. 41 men completed the same chest program twice over eight sets of bench presses to exhaustion at 80 percent of the one-rep maximum, once with 8 g of citrulline malate and once with placebo. From the third set evaluated, the number of repetitions increased, in the last set by 52.9 percent, and muscle soreness after 24 and 48 hours was 40 percent lower. These numbers are still on the labels today. They are also the reason the field has been studied further.
What came out of it. A meta-analysis of twelve studies with 198 participants, limited to at least 3 g of citrulline at least 30 minutes before the test and without added caffeine, creatine or nitrate, found an effect on strength and speed of 0.20 (confidence interval 0.01 to 0.39). The variation between studies was virtually zero, and the confidence intervals of each included study included zero. A clean, very small effect from lots of individual studies, none of which show anything on their own.
Endurance. Ten studies on aerobic performance resulted in a total of 0.15 (confidence range -0.02 to 0.32) and therefore no detectable effect, nor on the perception of exertion.
The widest cut. A meta-analysis of 30 controlled studies with 138 effect estimates from 644 participants found citrulline malate to be 0.16 for overall performance and no detectable effect on the perception of exertion. The prediction intervals were wide, the significance was limited, and the subgroup analyzes on gender, training level, type of load and time of intake did not separate anything. The data situation was more stable for a single dose before the session than for daily administration over weeks.
Malate. The trade mainly sells citrulline malate in a ratio of 2:1 and justifies the surcharge with the malate, which is said to have its own effects as an intermediate product in the citrate cycle. A crossover study with 43 trained people, 21 of whom were women, pitted 5.3 g of pure citrulline against the same amount of citrulline plus 2.7 g of malate against placebo, each 45 minutes before. There was no difference between the three arms, neither in the maximum neuromuscular performance nor in jumping power nor in the number of repetitions over three sets of squats and bench presses nor in the perception of effort.
The fact that this study found nothing for pure citrulline is the norm in this field and does not contradict the meta-analyses: with an effect size of 0.2, a single study needs several hundred participants to see something. The question it answers is different and is not affected by sample size because it is a within comparison: malate contributes nothing.
Dosage
- 6 to 8 g of pure L-citrulline, 45 to 60 minutes before the session. Nothing less than 3 g was expected in the studies evaluated
- If you buy citrulline malate in a ratio of 2:1, you will need more powder for the same amount of citrulline, around 9 to 12 g. The malate itself carries nothing
- A single dose before the session is better proven than daily intake for weeks
- No good reason to take it on non-training days
Side effects
Stomach pressure. Six to eight grams of a free amino acid at once is an osmotic load: the powder draws water into the intestines as long as it is not absorbed. In the work with 8 g of citrulline malate, 14.6 percent of participants reported stomach discomfort, and it was the only side effect reported. The symptoms depend on the amount taken at once and on an empty stomach, not the daily dose. What you can do: Take with plenty of water, not completely on an empty stomach, and if necessary divide into two portions twenty minutes apart. The acidic malate is harder for some people to tolerate than pure citrulline, which is the second reason to take the pure form.
Blood pressure. The intended mechanism is vasodilatation, and the downside of this would be a drop in blood pressure with dizziness when standing up. However, it is precisely this step that is the most poorly documented in the entire chain: the review describes the evidence for acute vasodilatation after citrulline administration as sparse and inconsistent. A blood pressure problem in healthy people has not been reported. What you can do: nothing as long as there is no blood pressure medication involved. If you take one, measure yourself in the first week because two vasodilator effects can basically add up, even if it is not measured for citrulline.
Remodeling in the kidney. The step from citrulline to arginine occurs predominantly in the kidney. If kidney function is impaired, both conversion and excretion are postponed, and this has not been investigated for the usual doses here. The concern is therefore weakly documented and at the same time unanswered: it is based on the known location of the transformation, not on any observed damage. What you can do: Anyone who has a known kidney disease leaves it alone and, according to the data above, loses almost nothing. If you're healthy, you don't need to do anything.
Classification
L-Citrulline is the rare case in which the drug does what it is supposed to and is still of little use. Arginine levels rise, levels of nitric oxide metabolism rise, and at the end of the chain there is an effect size of 0.2, the confidence interval of which touches zero. Anyone who fights for tenths in the competition can take credit for that. If you train to build muscle, you won't buy anything that can be measured in a year.
It's still worth taking a look at the famous 52.9 percent because it shows how this field works. A paper with 41 participants and a very large effect triggers twenty more, and their summary ends up with a twentieth of that. This is not a criticism of the first work, but rather the usual course of events. Only the first number is still on the cans today.
What remains practical: if you do, then pure citrulline instead of the more expensive malate form, 6 to 8 g, before the unit, and without the expectation of noticing the difference. The noticeable pump comes from the grounds, not the powder.
Sources
- Gonzalez AM, Trexler ET. Effects of Citrulline Supplementation on Exercise Performance in Humans: A Review of the Current Literature. J Strength Cond Res. 2020. PubMed
- Trexler ET et al. Acute Effects of Citrulline Supplementation on High-Intensity Strength and Power Performance: A Systematic Review and Meta-Analysis. Sports Med. 2019. PubMed
- Viribay A et al. Effects of Citrulline Supplementation on Different Aerobic Exercise Performance Outcomes: A Systematic Review and Meta-Analysis. Nutrients. 2022. PubMed
- Wang X et al. Effects of Citrulline Malate Supplementation on Exercise Performance: A Systematic Review and Three-Level Meta-Analysis. Nutrients. 2026. PubMed
- Perez-Guisado J, Jakeman PM. Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. J Strength Cond Res. 2010. PubMed
- Martin-Olmedo JJ et al. Malate or Not? Acute Effects of L-Citrulline Versus Citrulline Malate on Neuromuscular Performance in Young, Trained Adults: A Randomized, Double-Blind, Placebo-Controlled Crossover Trial. Int J Sport Nutr Exerc Metab. 2025. PubMed
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