01Research Directions

L-Citrulline research directions

01Research Directions

Why this compound has a literature

L-Citrulline is a non-proteinogenic amino acid and an intermediate of the urea cycle. Research interest in it stems from a specific metabolic route: unlike oral arginine, ingested citrulline largely escapes intestinal and hepatic arginase and is converted to arginine in the kidney, which made it a candidate for raising systemic arginine availability. It is studied in vascular pharmacology, sports science, clinical nutrition and obstetrics, and the quality of evidence differs considerably between these fields.

02Literature

What the literature investigates

Pharmacokinetics and the renal conversion ceiling

Two studies define the dosing landscape. Moinard et al. (2008), in the Citrudose study, gave eight fasting healthy men single oral loads of 2, 5, 10 or 15 g in random order and measured plasma amino acids over eight hours. Only citrulline, ornithine and arginine concentrations were affected, urinary citrulline excretion remained below 5% even at high doses, and plasma insulin and growth hormone were unaffected; critically, at the highest doses citrulline accumulated in plasma while arginine rose less than expected, which the authors attributed to saturation of renal conversion. Schwedhelm et al. (2008) reported in a crossover of twenty healthy volunteers that citrulline dose-dependently raised plasma arginine AUC and Cmax more effectively than arginine itself, and that 3 g twice daily raised the arginine/ADMA ratio and urinary nitrate and cGMP — though no treatment improved flow-mediated dilation over baseline. Together these suggest a plateau above which additional dose is not proportionally converted.

High-intensity strength and power outcomes

Trexler et al. (2019) published a systematic review and meta-analysis in Sports Medicine covering acute citrulline supplementation and high-intensity strength and power outcomes across the available randomised trials. Gonzalez and Trexler (2020) reviewed the same domain narratively in the Journal of Strength and Conditioning Research and reported that oral L-citrulline and citrulline malate raise plasma citrulline and arginine along with total nitrate and nitrite concentrations, while noting that the translation from those biochemical changes to performance measures is where the literature diverges. Study designs are typically small acute crossovers in trained young adults with repetitions-to-failure or peak-power endpoints, and effect estimates in this area have generally been small. Formulators should read strength claims about this ingredient against the size of the reported effects rather than against statistical significance alone.

Endurance and aerobic outcomes, where findings diverge

Harnden, Agu and Gascoyne (2023) conducted a systematic review and meta-analysis of randomised controlled trials examining whether acute citrulline ingestion affects time-to-completion and time-to-exhaustion in young healthy adults, and explicitly noted that findings in this literature have been inconsistent. Viribay et al. (2022) took a broader set of aerobic endpoints — including lactate, oxygen-uptake kinetics and rating of perceived exertion — in a systematic review and meta-analysis in Nutrients. The endurance literature is smaller and more heterogeneous than the resistance-exercise literature, with variation in dose, timing, training status and exercise modality that is difficult to separate statistically. Where a supplier or formulator is designing an endurance product, the honest reading is that this evidence base does not currently converge on a single protocol.

Vascular and clinical trials, including null and retracted results

Luo et al. (2025) pooled fifteen randomised controlled trials (24 datasets, 415 participants) on L-citrulline supplementation and watermelon intake against blood pressure in middle-aged and older adults, reporting reductions in systolic and diastolic pressure and a larger subgroup difference where citrulline was combined with arginine. This area requires caution: an earlier meta-analysis of citrulline and blood pressure published in the Journal of Human Hypertension was subsequently retracted, so secondary literature in this field should be checked against current PubMed records before being cited. In the clinical domain, Winer et al. (2025) reported a multicentre randomised double-blind trial in 115 women with preeclamptic pregnancies in which oral citrulline did not prolong the interval to delivery or alter neonatal outcomes, and in which liver enzymes and systolic blood pressure were higher at delivery in the treated group.

03Specification

What this means when you specify the material

Acute performance protocols in this literature clustered at 6-8 g taken roughly 40-60 minutes before exercise, and the pharmacokinetic work used single loads of 2-15 g, with evidence of saturating renal conversion at the upper end. That saturation is the practically useful finding for spec-setting: increasing the citrulline load in a formula is not linearly reflected in plasma arginine. L-Citrulline is hygroscopic and freely soluble, so moisture specification, particle size and packaging drive most of the handling risk in powder and stick-pack formats.

04References

Cited literature

Research context only. Each entry describes what a published study examined and reported. These are not claims about this raw material, and not claims about any finished product. Regulatory and labelling judgements belong to the brand placing the product on its market.

A dose-ranging pharmacokinetic study in eight fasting healthy men using single oral loads of 2, 5, 10 or 15 g reported that only citrulline, ornithine and arginine plasma concentrations were affected, urinary citrulline excretion stayed below 5%, plasma insulin and growth hormone were unaffected, and at the highest doses citrulline accumulated while arginine rose less than expected, which the authors attributed to saturation of renal conversion. Moinard C, Nicolis I, Neveux N, Darquy S, Bénazeth S, Cynober L. Br J Nutr. 2008;99(4):855-862. PMID 17953788 DOI 10.1017/S0007114507841110
A double-blind randomised placebo-controlled crossover study in twenty healthy volunteers reported that oral L-citrulline raised plasma L-arginine AUC and Cmax dose-dependently and more effectively than oral L-arginine, and that 3 g twice daily raised the arginine/ADMA ratio and urinary nitrate and cGMP excretion, while no treatment improved flow-mediated dilation over baseline. Schwedhelm E, Maas R, Freese R, Jung D, Lukacs Z, Jambrecina A, Spickler W, Schulze F, Böger RH. Br J Clin Pharmacol. 2008;65(1):51-59. PMID 17662090 DOI 10.1111/j.1365-2125.2007.02990.x
A systematic review and meta-analysis of acute citrulline supplementation reported on high-intensity strength and power outcomes across the available randomised trials. Trexler ET, Persky AM, Ryan ED, Schwartz TA, Stoner L, Smith-Ryan AE. Sports Med. 2019;49(5):707-718. PMID 30895562 DOI 10.1007/s40279-019-01091-z
A review of the citrulline literature reported that oral L-citrulline and citrulline malate supplementation increase plasma citrulline and arginine concentrations along with total nitrate and nitrite concentrations. Gonzalez AM, Trexler ET. J Strength Cond Res. 2020;34(5):1480-1495. PMID 31977835 DOI 10.1519/JSC.0000000000003426
A systematic review and meta-analysis of randomised controlled trials examined whether acute citrulline ingestion affects time-to-completion and time-to-exhaustion endurance outcomes in young healthy adults, noting that findings in this literature have been inconsistent. Harnden CS, Agu J, Gascoyne T. J Int Soc Sports Nutr. 2023;20(1):2209056. PMID 37155582 DOI 10.1080/15502783.2023.2209056
A systematic review and meta-analysis assessed citrulline supplementation across aerobic exercise performance outcomes including lactate, oxygen-uptake kinetics and rating of perceived exertion. Viribay A, Fernández-Landa J, Castañeda-Babarro A, Collado PS, Fernández-Lázaro D, Mielgo-Ayuso J. Nutrients. 2022;14(17):3479. PMID 36079738 DOI 10.3390/nu14173479
A systematic review and meta-analysis of fifteen randomised controlled trials (24 datasets, 415 participants) examined L-citrulline supplementation and watermelon intake against blood pressure in middle-aged and older adults, reporting reductions in systolic and diastolic pressure and a larger subgroup difference where citrulline was combined with L-arginine. Luo P, Chen J, Liu K, Zhang J. Clin Nutr ESPEN. 2025;69:653-664. PMID 40789388 DOI 10.1016/j.clnesp.2025.07.1130
A retraction note was published for an earlier systematic review and meta-analysis of L-citrulline supplementation and blood pressure, indicating that secondary literature in this specific area should be checked against current records before citation. Mahboobi S, Tsang C, Rezaei S, Jafarnejad S. J Hum Hypertens. 2021;35(4):381. PMID 31690817 DOI 10.1038/s41371-019-0280-1
A multicentre randomised double-blind trial in 115 women with preeclamptic monofetal pregnancies reported that oral L-citrulline supplementation did not prolong the interval between inclusion and delivery and did not alter neonatal weight or pregnancy outcomes, while liver enzymes and systolic blood pressure at delivery were higher in the treated group than in placebo. Winer N, Misbert E, Masson D, et al. Am J Clin Nutr. 2025;121(2):488-496. PMID 39638148 DOI 10.1016/j.ajcnut.2024.12.001

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