The "pump," that muscle congestion that swells under effort, is the most sold promise of pre-workouts, and citrulline is the ingredient behind it, present in 71% of the top one hundred best-sellers. It also has a particularity that its category neighbors don't have: a positive meta-analysis.
This guide explains what this meta-analysis shows and doesn't show, what the Spanish trial worth 53% more repetitions—which started the trend—is really worth, the dose that was tested, and why citrulline replaced arginine. It extends our pre-workout guide, ingredient by ingredient.
A real but small effect. The meta-analysis from Sports Medicine, twelve studies and 198 participants, finds a standardized effect of 0.20 on strength and power with at least 3 g of citrulline, 30 minutes before, with no heterogeneity or publication bias. No single study demonstrates it; their combined effect makes it appear. The landmark trial, Cordoba 2010, 8 g of malate: 52.9% more repetitions on the last bench press set and 40% less muscle soreness. Never replicated to this extent: this is the high end of the range.
The dose : 6 to 8 g of citrulline malate, 30 to 60 minutes before; 4 g, the market average, is the low end. Why not arginine : when ingested, it breaks down before reaching the blood; citrulline passes through, and raises blood arginine levels more than arginine itself does. A modest supplement, behind caffeine, creatine, and beta-alanine.
What citrulline is, and where the pump comes from
Citrulline is an amino acid that doesn't enter proteins, discovered in watermelon, Citrullus, which is where its name comes from. In the body, the kidneys convert it to arginine, and arginine is the precursor tonitric oxide, the molecule that dilates blood vessels. More blood flow to the working muscle, that's the congestion, the pump, and the hypothesis is that this flow brings more oxygen and nutrients and better eliminates exercise waste. Citrulline malate combines citrulline with malic acid, an intermediate in the cell's energy cycle, most often in a two-to-one ratio.
The meta-analysis: small, but real
The included studies evaluated the effects of citrulline on high-intensity performance indices in healthy men and women: strength and power on tests involving repeated actions of large muscle groups, weightlifting sets or 30-second or shorter sprints. Studies were excluded if they lacked a placebo, involved clinical populations, administered less than 3 g of citrulline, administered it less than 30 minutes before testing, or combined it with creatine, caffeine, nitrates, or other ergogenic ingredients. Twelve studies, thirteen independent samples, and 198 participants were included. Between-study variance, heterogeneity, and inconsistency were low, and no asymmetry in the funnel plot was present. The meta-analysis identified a significant benefit of citrulline, with a small standardized mean difference of 0.20. The effect was small and confidence intervals from each individual study crossed the null effect line; the results can nonetheless matter for elite athletes, where outcomes are decided by small margins.
Trexler ET, Persky AM, Ryan ED, Schwartz TA, Stoner L, Smith-Ryan AE. Sports Med 2019;49(5):707-718. DOI: 10.1007/s40279-019-01091-z
What makes this meta-analysis valuable is its rigor : citrulline alone, minimum dose, minimal timing, no blending, no patients. On these grounds, the effect exists, and it is small, 0.20, where 0.50 is medium and 0.80 is large. Concretely, it's a few extra reps on a long set, or slightly more power on a short sprint. No single study shows this; their combined effect reveals it, with no heterogeneity or publication bias, which makes it credible. For an elite athlete, a few percent matters; for a practitioner, it's a minor benefit.
The 53% trial: what it's worth
Forty-one men completed two consecutive chest training sessions of sixteen sets, in crossover and double-blind fashion, with 8 g of citrulline malate in one session and placebo in the other. Strength was tested by the number of repetitions to fatigue at 80 percent of maximum over eight sets of bench press. The number of repetitions was significantly higher under citrulline malate from the third evaluated set onward, with the increase correlated to the number of sets and reaching 52.92 percent more repetitions at the last, with 100 percent responders. Muscle soreness was reduced by 40 percent at 24 and 48 hours. The only adverse effect was gastrointestinal discomfort in 14.63 percent of subjects.
Pérez-Guisado J, Jakeman PM. J Strength Cond Res 2010;24(5):1215-1222. DOI: 10.1519/JSC.0b013e3181cb28e0
This trial is the one that all packaging cites, and it is well-constructed: crossover, double-blind, 41 men. But 53% more repetitions and 100% responders, that's an exceptional result, and the meta-analysis that includes it nine years later finds on average an effect ten times more modest, with no study reproducing it at that magnitude. So it must be read as the upper end of the range, what citrulline can achieve under the best conditions, long training with short rest where fatigue accumulates, and not as what should be expected. The reduction in muscle soreness, 40%, is an interesting secondary result that has also not been confirmed at that level.
Why citrulline, not arginine
Arginine is the direct precursor of nitric oxide, and it was the historical ingredient of the pump. The problem is that when swallowed, it doesn't reach the bloodstream : the intestine and liver degrade it substantially through an enzyme, arginase, before it reaches circulation, and its effect on nitric oxide at supplement doses is not demonstrated, as our pre-workout guidereminds us. Citrulline, on the other hand, escapes this degradation, passes into the blood, is converted to arginine by the kidneys, and raises blood arginine levels more than arginine itself does. It is this detour that explains why citrulline has a positive meta-analysis and arginine doesn't, and why it has replaced arginine in serious formulations.
Dose, timing, and the market
| Parameter | What studies tested | The market |
|---|---|---|
| Dose | 3 g of citrulline minimum in the meta-analysis; 6 to 8 g of malate in conclusive trials, i.e., 4 to 5.5 g of citrulline | 4.0 g on average in the top 100 best-sellers, the lower end of the range |
| Timing | 30 to 60 minutes before exercise | Consistent with pre-workout intake |
| Type of effort | Repeated resistance training sets, sprints of less than 30 seconds | Promised for everything; documented for this |
| Expected effect | Small, 0.20; a few repetitions on a long set | Often presented with the 53% from the Cordova trial |
| Adverse effects | Gastrointestinal discomfort at 14.6% at 8 g | Rarely mentioned |
Pure creatine monohydrate, Creapure label, 3 g per day, every day. Strength, power and lean mass across hundreds of trials, whereas citrulline adds a few reps across twelve studies. Foundation first, supplement second.
See CreatineFor the immediate effect before training, our caffeine guide gives the dose by body weight.
Frequently Asked Questions
What is citrulline and why is "pump" mentioned?
Citrulline is a non-protein amino acid found in watermelon, which is where its name comes from, in fact, Citrullus. In the body, it is converted to arginine by the kidneys, and arginine is the precursor to nitric oxide, the molecule that dilates blood vessels. More blood flow to the muscle during exercise creates the congestion, the pump, and this is the hypothesis behind its use before training. Citrulline malate combines citrulline with malic acid, an intermediate in the cell's energy cycle, generally in a two-to-one ratio.
Does citrulline really improve strength?
Slightly, and it's measured. The meta-analysis published in Sports Medicine by the University of North Carolina brought together twelve studies and 198 participants on citrulline taken alone, at least 3 g, at least 30 minutes before strength or power exercise. The effect is significant but small, an effect size of 0.20, and the confidence interval of each individual study crosses zero. Heterogeneity between studies was null and no publication bias was detected. The authors estimate this could matter for elite athletes where everything comes down to marginal gains.
What about the trial showing 53% more reps?
This is the trial that launched citrulline malate in gyms, published in 2010 by the University of Córdoba. Forty-one men completed two chest sessions of sixteen sets, in a crossover double-blind design, with 8 g of citrulline malate or placebo. The number of reps on the bench press was higher with citrulline starting from the third set, with 52.9% more reps on the final set, and muscle soreness was reduced by 40% at 24 and 48 hours. This is a spectacular result, well above what the meta-analysis finds on average afterward, and which has not been reproduced at this scale: it should be read as the upper end of the range, not the expected outcome.
What dose of citrulline?
The meta-analysis only retained studies using at least 3 g of citrulline, taken at least 30 minutes before exercise; the most conclusive trials used 6 to 8 g of citrulline malate, or approximately 4 to 5.5 g of pure citrulline, 30 to 60 minutes before. The average dose of the hundred best-selling pre-workouts is 4.0 g, at the lower end of the range. Gastrointestinal discomfort reported in the Córdoba trial at 8 g is the only known side effect.
Why citrulline rather than arginine?
Because ingested arginine doesn't reach the bloodstream. It is largely broken down in the intestines and liver before entering circulation, by an enzyme called arginase, and its effect on nitric oxide at supplement doses is not demonstrated. Citrulline escapes this breakdown, enters the bloodstream, is converted to arginine by the kidneys, and raises blood arginine more than arginine itself does. This is why arginine has been replaced by citrulline in serious formulations.
Is citrulline useful for endurance?
The cited meta-analysis covers strength and power, weightlifting sets and sprints under 30 seconds, and excludes endurance. Data on long-distance endurance are more heterogeneous and are not the subject of a comparable positive meta-analysis. For endurance, caffeine has by far the strongest evidence, and our dedicated article details the dosing.
What to remember about citrulline?
That it's the only pump ingredient with a positive meta-analysis, twelve studies and 198 participants, for a small effect, 0.20, a few extra reps on longer sets. That the 53% trial is the upper end of the range, not the average. That the dose is 6 to 8 g of malate, 30 to 60 minutes before, and that 4 g is the low end. That arginine doesn't work the same way because it doesn't reach the bloodstream. And that it's an honest supplementary ingredient, behind caffeine, creatine, and beta-alanine.
Sources
The studies cited below were identified via PubMed.
- Trexler ET, Persky AM, Ryan ED, Schwartz TA, Stoner L, Smith-Ryan AE. Acute effects of citrulline supplementation on strength and high-intensity power: systematic review and meta-analysis. Sports Medicine, 2019;49(5):707-718. DOI
- Pérez-Guisado J, Jakeman PM. Citrulline malate improves anaerobic performance and relieves muscle soreness. Journal of Strength and Conditioning Research, 2010;24(5):1215-1222. DOI
- Jagim AR, Harty PS, Camic CL. Ingredient profiles of multi-ingredient pre-workout supplements. Nutrients, 2019;11(2):254. DOI
