L-Citrulline
L-Citrulline · The Watermelon Amino Acid
Evidence grade 3 of 5, Mixed. Read how the Atlas grades evidence.
The evidence supports grade 3 (mixed/moderate RCT) for the two best-studied consumer uses, and the number should be read as use-specific rather than a blanket endorsement. For blood pressure, the largest current signal comes with a caution: the widely cited Mahboobi et al. meta-analysis in the Journal of Human Hypertension (2019), which reported systolic and diastolic reductions of roughly -7.5 and -3.8 mmHg across 15 trials and 424 participants, was retracted in 2021 after the authors identified fundamental data errors (standard error entered as standard deviation for two included trials). Independent later analyses find a smaller but consistent effect, on the order of -4 mmHg systolic and around -2.5 to -3.6 mmHg diastolic in middle-aged and older or pre-hypertensive adults, which keeps the direction of benefit while shrinking its size. For exercise, effects are small and inconsistent: Trexler and colleagues reported a pooled standardized mean difference near 0.2 for strength and power outcomes, Rhim et al. (Journal of Sport and Health Science, 2020) found citrulline reduced post-exercise perceived exertion and 24-hour muscle soreness across 13 RCTs and 206 participants but did not change blood lactate, and Harnden et al. (2023) found no significant endurance benefit across 9 trials and 158 participants. The pharmacokinetic premise is the most solid piece: Schwedhelm et al. (British Journal of Clinical Pharmacology, 2008) showed oral L-citrulline raises plasma L-arginine more effectively than L-arginine itself. Erectile-function evidence remains at pilot scale (Cormio 2011, 24 men). A 3 fairly represents real but modest RCT support that is separate from the larger marketed halo.
- FDA
- Legal supplement (DSHEA)
- WADA
- Not prohibited
Reference only. Not medical advice.
What can go wrong
Among the safest supplements at studied doses. Mild GI upset is the most common issue at high single doses and resolves with divided dosing; no serious adverse events documented in trials. Main caution is additive hypotension when combined with nitrates, PDE5 inhibitors or antihypertensives, so coordinate with a clinician.
L-citrulline is a non-proteinogenic amino acid first isolated from watermelon (Citrullus vulgaris), and it works mostly as a precursor: the kidneys convert it into L-arginine, the substrate for nitric oxide and vascular widening. Its selling point is a genuine pharmacokinetic quirk, since oral L-citrulline raises blood L-arginine more reliably than swallowing L-arginine directly, because a large share of oral arginine is broken down before it reaches the bloodstream. The honest headline is that this precursor logic is well established, but the downstream clinical payoffs are modest and use-specific. Short-term supplementation is associated with small blood pressure reductions in pre-hypertensive and older adults, small and inconsistent gains in strength and perceived exertion, and pilot-level improvement in mild erectile dysfunction. It is sold plain or as citrulline malate, which pairs it with malic acid.
The watermelon amino acid that delivers L-arginine's benefits without L-arginine's pharmacokinetic problem.
What people take it for
These are the reasons people give for taking it. They are not effects it has been shown to have.
How it is supposed to work
L-citrulline is a non-proteinogenic amino acid that occupies a unique pharmacokinetic position: it is the kidney-route precursor to L-arginine and substantially solves the first-pass metabolism problem that has limited oral L-arginine's utility as a nitric oxide supplement. When L-arginine is consumed orally, approximately 40-50% of the dose is destroyed by intestinal arginase enzymes before reaching systemic circulation, a substantial bioavailability loss that limits the plasma L-arginine elevation achievable with oral supplementation. L-citrulline bypasses this problem because intestinal arginase does not metabolize citrulline. Oral citrulline reaches systemic circulation largely intact, then is converted to L-arginine in the proximal renal tubules of the kidney through the sequential action of argininosuccinate synthase and argininosuccinate lyase. The result: oral L-citrulline produces substantially higher and more sustained plasma L-arginine elevation than equimolar oral L-arginine, first demonstrated definitively by Schwedhelm 2008. The downstream mechanism is nitric oxide synthesis. L-arginine is the direct substrate for nitric oxide synthase (NOS) enzymes, particularly endothelial NOS (eNOS) in vascular smooth muscle. Elevated arginine availability supports increased nitric oxide production, which produces vasodilation, improved blood flow, reduced platelet aggregation, and reduced vascular inflammation. The clinical implications span athletic performance (improved oxygen and nutrient delivery to working muscles, reduced perceived exertion, reduced post-exercise soreness), erectile function (vasodilation of corpus cavernosum), modest blood pressure reduction (peripheral vasodilation), and theoretical cardiovascular risk modification. L-citrulline also participates in the urea cycle, recycling ammonia from protein metabolism, a separate clinical role exploited in inborn errors of metabolism. Many commercial preparations combine L-citrulline with malic acid (forming citrulline malate, typically in 2:1 or 1:1 ratio), providing additional Krebs cycle intermediate support that may complement the nitric oxide mechanism, though most controlled trials supporting clinical effects use either pure L-citrulline or citrulline malate without rigorous head-to-head comparison.
How it is taken
L-citrulline dosing is indication-specific. For erectile dysfunction (the Cormio 2011 evidence base), 1.5-3g/day taken chronically over 4-8 weeks is the standard protocol. For cardiovascular and hypertension applications, 3-6g/day chronically. For athletic performance, the standard is 6-8g citrulline malate (delivering approximately 4-5g pure L-citrulline) taken 60 minutes before training. The pharmacokinetics support both acute pre-event dosing (peak plasma arginine elevation 60-90 minutes post-dose) and chronic daily dosing (sustained plasma arginine elevation supports the cardiovascular and ED applications). Take with or without food, absorption is acceptable either way, though some users find food helps with the mildly sour taste of pure powder. Citrulline malate has a more pleasant taste (malic acid provides apple-like notes) and is often preferred in powder form. Effects on cardiovascular and ED endpoints emerge over 4-8 weeks of consistent chronic use; acute ergogenic effects for athletic performance can be observed in single-session trials. Higher doses (>10g/day) have not been adequately studied; the standard ergogenic and clinical evidence clusters at 3-8g/day. Unlike beta-alanine, citrulline does not require loading, both acute and chronic protocols are supported by the evidence.
- Erectile dysfunction
- 1.5-3g/day chronic, 4-8 weeks
- Cardiovascular / hypertension
- 3-6g/day chronic
- Resistance training (pre-workout)
- 6-8g citrulline malate, 60 min pre-workout
- Endurance training
- 3-6g/day, chronic or pre-event
- Time to acute effect
- 60-90 min post-dose for peak plasma arginine
- Time to clinical effect (chronic)
- 4-8 weeks for ED, cardiovascular endpoints
- Citrulline malate ratio
- Typically 2:1 (citrulline:malate)
- Loading required
- No, both acute and chronic protocols supported
Which form to buy
Not all preparations of the same ingredient are the same thing, and the trials were run on particular ones.
Pharmaceutical-Grade L-Citrulline (Pure)
Crystalline white powder, water-soluble, used in clinical-grade trials and pharmaceutical compounding. The form used in Schwedhelm 2008 pharmacokinetic studies and Cormio 2011 ED trial. Available from major supplement manufacturers (NOW Foods, Bulk Supplements, Thorne, Pure Encapsulations) at reasonable cost per gram. Ideal for cardiovascular and ED applications where pure compound is preferred.
Citrulline Malate (1:1 or 2:1 Ratio)
The dominant commercial sport supplement form, citrulline combined with malic acid. The 2:1 ratio (2 parts citrulline to 1 part malate) is most common; 1:1 ratios are also available. Used in Pérez-Guisado 2010 resistance training trial and the majority of athletic performance research. Theoretical advantage of malate as Krebs cycle intermediate; pharmacological case for malate addition is suggestive rather than definitive.
L-Citrulline + L-Arginine Combinations
Combined supplementation theoretically provides immediate (arginine direct) and sustained (citrulline-to-arginine conversion) nitric oxide pathway support. Suzuki 2017 and subsequent work suggests the combination may produce more sustained plasma arginine elevation than either compound alone. Reasonable combination for users wanting comprehensive NO pathway support.
Pre-Workout Combinations (Beta-Alanine + Citrulline + Caffeine)
The dominant commercial pre-workout category includes citrulline at variable doses (often 4-8g per serving as citrulline malate). Mechanistically complementary with beta-alanine (different ergogenic mechanisms) and caffeine. Quality and dose disclosure varies enormously; verify citrulline content rather than relying on "pre-workout" labeling.
Watermelon (Citrullus vulgaris) Whole Fruit
The original natural source, watermelon contains approximately 250mg L-citrulline per cup, with higher concentrations in the white rind. A reasonable culinary contribution but does not deliver therapeutic supplement doses (would require multiple servings daily). Whole watermelon also provides hydration, lycopene, and other phytonutrients, reasonable dietary component but not substitute for therapeutic citrulline supplementation.
Watermelon Extract (Concentrated)
Concentrated watermelon extracts marketed as "natural citrulline" sources. Quality and citrulline content varies enormously across brands; many products contain less citrulline than advertised. Generally less cost-effective than pharmaceutical-grade L-citrulline for users seeking therapeutic doses.
Capsule Form L-Citrulline or Citrulline Malate
Capsule delivery typically 750-1500mg per capsule. Acceptable for users preferring capsule format; powder is more cost-effective for chronic 6-8g daily protocols. Citrulline is mildly acidic in powder form, capsule users may find taste preference advantage.
Branded Pre-Workout Brands (C4, Ghost, Cellucor, Optimum Nutrition Gold Standard Pre)
Mass-market sport supplement brands include citrulline at variable doses, often in proprietary blends that obscure exact citrulline content. Marketing emphasis on "pump" and vasodilation suggests citrulline content but does not guarantee therapeutic doses. Verify dose disclosure.
Who it works for, and who it does not
Athletes (resistance training)
The strongest commercial use case. Pérez-Guisado 2010, Glenn 2017, and subsequent work support modest improvements in repetitions to failure and post-exercise soreness at 6-8g citrulline malate pre-workout. Effect sizes meaningful in competitive contexts; modest in recreational training. Reasonable component of comprehensive sport supplementation.
Athletes (endurance)
Suzuki 2016 and subsequent endurance trials support modest improvements in time trial performance, oxygen utilization, and perceived exertion at 3-6g/day. Effect sizes smaller than for resistance training; reasonable component of comprehensive endurance protocols.
Erectile dysfunction (mild to moderate)
Cormio 2011 and subsequent work support modest erectile function improvements at 1.5-3g/day. Effect sizes substantially smaller than PDE5 inhibitors (sildenafil, tadalafil); reasonable for users preferring non-pharmaceutical approach or as adjunct to PDE5 inhibitors. Not appropriate as substitute for established ED treatment in severe cases.
Hypertension (mild to moderate)
Mechanistically reasonable based on NO-mediated vasodilation; clinical evidence is preliminary but consistent across small trials. Effect sizes modest (typically 3-7 mmHg systolic reduction). Reasonable adjunct; not substitute for first-line antihypertensives in established hypertension.
Post-exercise recovery (muscle soreness)
Pérez-Guisado 2010 and subsequent work support reduced post-exercise muscle soreness at standard ergogenic doses. Mechanistically reasonable based on improved blood flow and reduced inflammatory markers. Reasonable component of recovery protocols.
Wound healing adjunct
L-arginine has established clinical wound healing applications (Arginaid). L-citrulline as alternative route to arginine elevation is mechanistically reasonable but specific wound healing clinical evidence is limited. Coordinate with treating clinician for wound healing applications.
Heart failure (emerging)
Mechanistically reasonable based on NO pathway support and improved vascular function. Clinical evidence in heart failure is preliminary; specific recommendations beyond standard heart failure guideline-directed therapy not yet established. Coordinate with cardiology.
Pregnancy and lactation
Limited systematic safety data at supplemental doses. L-citrulline is endogenously present but specific gestational and lactational supplementation has not been adequately studied. Generally avoid supplemental L-citrulline during pregnancy and lactation without specialist guidance.
Taken with other things
Combinations people actually use. Evidence for one ingredient is not evidence for a pair.
L-Arginine
Mechanistically complementary, arginine provides immediate substrate availability; citrulline provides sustained plasma arginine elevation through kidney conversion. Combined dosing may produce more sustained NO pathway support than either alone. Common in comprehensive cardiovascular and ED protocols.
PDE5 Inhibitors (Sildenafil, Tadalafil)
Both mechanisms support nitric oxide pathway and vasodilation. Combined use is generally well-tolerated but theoretical additive hypotension warrants moderate caution, particularly in users on antihypertensives. Coordinate with prescribing physician for combined ED management.
Beta-Alanine
Common pre-workout combination. Different ergogenic mechanisms (citrulline: NO pathway vasodilation; beta-alanine: intramuscular pH buffering). Mechanistically complementary; supports comprehensive high-intensity performance.
Caffeine (Pre-Workout)
Common pre-workout combination. Caffeine provides CNS stimulation and acute ergogenic effects; citrulline provides vascular and NO pathway support. Different mechanisms, additive effects. Foundational pre-workout supplement combination.
Creatine Monohydrate
Mechanistically complementary for athletic performance. Creatine supports peak power and ATP regeneration; citrulline supports blood flow and metabolic clearance. Common in comprehensive resistance training nutrition.
Nitrates (Beetroot Juice, Dietary Nitrate)
Both pathways support nitric oxide-mediated vasodilation through different routes (citrulline: L-arginine to NO; nitrate: nitrate-to-nitrite-to-NO). Common combination in endurance sport protocols; cardiovascular effects are additive.
Antihypertensives (ACE inhibitors, ARBs, calcium channel blockers)
Additive blood pressure-lowering effects. Monitor BP when initiating citrulline in users on established antihypertensive therapy; medication dose adjustment may be needed. Generally well-tolerated combination at modest doses.
Pre-Surgery Period
Mild antiplatelet activity and vasodilation effects warrant 1-2 week discontinuation before elective surgery as conservative practice. Coordinate with anesthesiology for emergency or urgent procedures.
What to look for on the label
Look for: pure L-citrulline or citrulline malate specified clearly on label; explicit ratio disclosure for citrulline malate (1:1 or 2:1); Informed Sport or NSF Certified for Sport third-party verification (essential for athletes in tested competition); reasonable price per gram (citrulline is genuinely affordable at commodity pricing). Avoid: proprietary "pre-workout" blends that obscure citrulline dose; products at the low end of therapeutic range (<3g per daily serving); products from manufacturers without third-party testing disclosure; "watermelon extract" products that may not deliver standardized citrulline content; products marketed primarily for "pump" sensation rather than evidence-based ergogenic or clinical applications.
Safety in detail
- Among the safest amino acid supplements with extensive clinical trial safety data. No serious adverse events reported at standard doses across thousands of subject-exposures.
- Mild GI upset at high single doses (>6g); managed by dividing dose or using citrulline malate form (more pleasant taste, often better-tolerated).
- Blood pressure lowering effect can be clinically meaningful at higher doses (>3g/day); monitor BP in users on antihypertensive medications.
- Mild antiplatelet activity at high doses; pre-surgical discontinuation 1-2 weeks before elective procedures is conservative practice.
- Pregnancy and lactation: limited systematic safety data at supplemental doses; avoid outside specialist guidance.
- Pediatric use: limited systematic safety data; avoid supplementation outside specialist guidance.
- No documented hepatotoxicity, nephrotoxicity, or cardiac concerns at studied doses.
- Renal function: well-tolerated in users with normal renal function. The kidney-mediated arginine conversion mechanism warrants caution in severe renal impairment; coordinate with nephrology.
- Long-term safety profile is excellent across 15+ years of widespread supplement use and clinical trial follow-up.
What people get wrong
Often saidL-arginine and L-citrulline are interchangeable.
ActuallyThey are not, due to pharmacokinetics. Oral L-arginine loses approximately 40-50% of dose to intestinal arginase first-pass metabolism; oral L-citrulline largely bypasses this and produces superior plasma arginine elevation. For nitric oxide pathway support, L-citrulline is the more bioavailable option.
Often saidCitrulline malate is just citrulline.
ActuallyCitrulline malate combines L-citrulline with malic acid (typically 2:1 ratio). The malate addition provides Krebs cycle intermediate support that may complement the NO pathway mechanism. Most athletic performance research uses citrulline malate; pure L-citrulline is preferred for cardiovascular and ED applications.
Often saidWatermelon delivers therapeutic citrulline doses.
ActuallyWatermelon provides approximately 250mg L-citrulline per cup, a dietary contribution but well below therapeutic supplement doses (3-8g daily). Eating watermelon is healthy but not substitute for citrulline supplementation in users targeting therapeutic effects.
Often saidCitrulline produces immediate erection enhancement.
ActuallyThe Cormio 2011 evidence base is for chronic supplementation over 4-8 weeks. Acute pre-event dosing for immediate erectile effect is not well-supported; effects develop with sustained daily dosing.
Often saidHigher citrulline doses always produce better results.
ActuallyThe therapeutic dose range is 3-8g/day depending on indication. Doses above 10g/day have not been adequately studied and produce diminishing returns in clinical endpoints; primarily increased GI sensitivity.
Often saidCitrulline is banned in athletics.
ActuallyIt is not. Not on WADA prohibited list; freely used by Olympic and elite athletes. Informed Sport and NSF Certified for Sport third-party verification supports tested-competition use.
Often saidCitrulline causes dependence on supplementation.
ActuallyIt does not. Citrulline does not produce dependence, downregulation of endogenous NO pathways, or rebound effects on discontinuation. Standard amino acid supplementation pharmacology applies.
Who profits, and who pushes back
Mainstream cardiology
The blood pressure-lowering effects of L-citrulline at 3-6g/day are modest (typically 3-7 mmHg systolic), substantially smaller than established antihypertensive medications. Reasonable as adjunct or for users preferring non-pharmaceutical approach in mild hypertension; not substitute for first-line antihypertensives in established hypertension. Mainstream cardiology has not incorporated citrulline into routine hypertension management.
Sexual medicine and urology
The erectile function effects of L-citrulline at 1.5-3g/day are modest, substantially smaller than PDE5 inhibitors (sildenafil, tadalafil, vardenafil) which produce reliable clinical effects in most patients. Reasonable for users preferring non-pharmaceutical approach in mild ED or as adjunct to PDE5 inhibitors; not substitute for established ED treatment in severe cases.
Rainer Böger / Edzard Schwedhelm
Lead researchers on the 2008 pharmacokinetic study establishing L-citrulline's superiority over L-arginine for plasma arginine elevation. Foundational scientific contribution. Independent academic.
Joaquín Pérez-Guisado
Led the 2010 citrulline malate resistance training trial that established the foundational athletic ergogenic evidence. Independent academic with industry collaborations disclosed.
Luigi Cormio
Led the 2011 erectile dysfunction trial that established the modest ED indication at relatively low doses. Independent academic.
Eric Trexler
Lead author of the 2019 ISSN review consolidating citrulline ergogenic evidence. Among the most influential evidence-based fitness communicators on sport supplementation.
International Society of Sports Nutrition (ISSN)
Issued the 2019 position consolidating citrulline ergogenic evidence. Independent academic consensus body.
Pre-workout supplement industry
Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.
Substantial commercial interest in citrulline malate inclusion in pre-workout formulations. The mass-market global citrulline delivery vector. Marketing emphasis on "pump" and vasodilation aligns reasonably with NO pathway mechanism.
How we got here
L-citrulline was isolated and named in 1914 by Yotaro Wada at the Imperial University of Tokyo, working on the amino acid composition of watermelon (Citrullus vulgaris). The compound was named after its botanical source, establishing a unique naming tradition for an amino acid identified through dietary chemistry rather than animal biochemistry. The structural characterization by Wassersug in 1924 established the chemistry as a non-proteinogenic alpha-amino acid with a ureido side chain group, distinct from the standard twenty proteinogenic amino acids. The metabolic role characterization developed through the foundational urea cycle elucidation by Hans Krebs and Kurt Henseleit in 1932. Citrulline was identified as a key intermediate in ammonia detoxification, formed from ornithine and carbamoyl phosphate, then combined with aspartate to form argininosuccinate. The urea cycle role established citrulline's clinical importance in inborn errors of metabolism (particularly argininosuccinate synthase deficiency and OTC deficiency) and parenteral nutrition. The modern supplement era began in the 2000s as researchers began comparing oral L-citrulline versus oral L-arginine for supporting nitric oxide pathway endpoints. The 2008 Schwedhelm paper in the British Journal of Clinical Pharmacology was the foundational pharmacokinetic study, definitively demonstrating that oral L-citrulline produces superior plasma L-arginine elevation versus equimolar L-arginine due to bypass of intestinal arginase first-pass metabolism. This single finding catalyzed the modern supplement market shift from L-arginine to L-citrulline as the preferred NO pathway supplement. The athletic performance era developed through the 2010s, with Pérez-Guisado 2010 establishing the resistance training case, Cormio 2011 establishing the modest erectile dysfunction case at low doses, and Suzuki 2016 establishing the endurance case. The 2019 Trexler ISSN review consolidated the professional society position. The 2020s have seen continued expansion in pre-workout supplement combinations (citrulline malate is now near-universal in commercial pre-workouts), modest expansion in cardiovascular research, and the ongoing pharmaceutical use in clinical contexts. The consumer market story is essentially the displacement of L-arginine by L-citrulline based on pharmacokinetic superiority, a clear example of evidence-driven supplement market evolution. L-arginine retains specific clinical applications (wound healing through Arginaid, growth hormone testing) but has been largely supplanted in consumer NO pathway supplementation by L-citrulline.
- 1914Wada isolates citrulline from watermelon at Imperial University of Tokyo
- 1924Wassersug characterizes the chemical structure
- 1932Krebs and Henseleit elucidate the urea cycle; citrulline as key intermediate
- 2008Schwedhelm pharmacokinetic comparison establishes citrulline-over-arginine superiority
- 2010Pérez-Guisado resistance training trial, foundational ergogenic evidence
- 2011Cormio erectile dysfunction trial, establishes modest ED indication
- 2015Bailey pharmacokinetic refinement
- 2016Suzuki cycling endurance trial, endurance evidence consolidates
- 2017Glenn trained women trial, sex-specific evidence base expands
- 2019Trexler ISSN comprehensive review consolidates professional position
- 2020sCitrulline malate near-universal in commercial pre-workout supplements
What we do not know
- Whether citrulline malate produces clinically distinguishable outcomes versus pure L-citrulline, most evidence supports the malate addition as ergogenically helpful but rigorous head-to-head trials are limited.
- Optimal dose-response curve for cardiovascular and hypertension endpoints, most evidence clusters at 3-6g/day without comprehensive dose-finding studies.
- Whether the L-citrulline + L-arginine combination produces clinically meaningful advantage over L-citrulline alone in chronic dosing, combination evidence is preliminary.
- Whether female athletic ergogenic effects equal male effects, Glenn 2017 confirms female responsiveness but most early evidence was predominantly male.
- Whether older adult cardiovascular and physical function applications will develop into specific clinical protocols.
- Whether long-term high-dose use (>6g/day for years) has any cumulative effects not seen in current trial timeframes (typically <12 months).
- Whether watermelon-derived "natural" citrulline produces clinically distinguishable outcomes versus pharmaceutical synthesis, generally not, but consumer demand for "natural" sources persists.
- How citrulline-mediated blood pressure effects integrate with established hypertension management algorithms.
Legal status
Universally available globally as a dietary supplement. Legal under DSHEA in the United States and equivalent supplement regulations in most jurisdictions. Not on WADA prohibited list, used freely by Olympic and elite athletes. Pharmaceutical-grade L-citrulline is also used in clinical contexts (urea cycle disorders, parenteral nutrition specifications) at higher cost. Major retail brands include NOW Foods, Thorne, Pure Encapsulations, Bulk Supplements, Optimum Nutrition, BSN, and most pre-workout brands. Production is dominated by chemical synthesis or fermentation at commodity pricing; the watermelon-extracted "natural" form represents a small premium-priced niche. The molecule itself is well-characterized and pharmaceutical-grade material is widely accessible.
Status current as of July 2026. Regulatory positions change, sometimes quickly.
The market for this
What follows describes how this is bought and sold. It is not a buying guide and it is not an endorsement of any seller.
L-citrulline production is dominated by microbial fermentation and chemical synthesis at commodity pricing. Modern manufacturing produces pharmaceutical-grade material widely available from established supplement manufacturers. The molecule is well-characterized and quality variation across brands is modest; third-party testing (Informed Sport, NSF Certified for Sport) provides standard verification for athletic competition contexts. Citrulline malate manufacturing combines L-citrulline with malic acid in specific ratios, quality verification should confirm both component identity and ratio disclosure. Generic L-citrulline and citrulline malate are functionally equivalent across reputable manufacturers; premium pricing without specific justification (third-party testing, organic certification) is suspect.
At a glance
- Structure
- Non-proteinogenic alpha-amino acid with ureido side chain
- Botanical name origin
- Citrullus vulgaris (watermelon)
- Endogenous role
- Urea cycle intermediate; arginine precursor
- Bioavailability advantage
- Bypasses intestinal arginase first-pass metabolism
- Site of conversion to arginine
- Proximal renal tubules (kidney)
- Downstream mechanism
- Nitric oxide synthesis (eNOS substrate)
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about L-Citrulline. A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
- Where we land. We have not taken a position on this yet.
Sources
Oral L-citrulline raises plasma L-arginine dose-dependently more effectively than oral L-arginine because ~50% of oral arginine is lost to first-pass arginase metabolism; well tolerated with no related side effects in healthy volunteers.
Schwedhelm et al., British Journal of Clinical Pharmacology (PMC)
The Mahboobi et al. blood pressure meta-analysis (15 trials, 424 participants, ~-7.5/-3.8 mmHg) was retracted in 2021 after fundamental data errors, including standard error used instead of standard deviation, were identified.
Original (now retracted) meta-analysis reported systolic -7.54 mmHg and diastolic -3.77 mmHg across 15 trials and 424 participants, with greater effect at higher baseline pressure and durations of six weeks or longer.
Later meta-analysis of 15 RCTs (415 participants) found L-citrulline and watermelon intake reduced systolic BP by ~4.0 mmHg and diastolic by ~2.5 mmHg in middle-aged and older adults, a smaller but consistent effect.
Does l-citrulline supplementation and watermelon intake reduce blood pressure, ScienceDirect
Review reports minimum effective dose ~3 g/day and maximal ~10 g/day, named BP trials (Figueroa 2012, Wong 2016), mixed endothelial-function results, and tolerance up to 15 g/day in healthy volunteers.
Meta-analysis of 9 RCTs (158 participants) found no significant endurance benefit from acute citrulline; doses 1.47-9 g as bolus 60-120 min pre-exercise or 7-16 day loading.
Harnden et al., Journal of the International Society of Sports Nutrition (PMC)
Meta-analysis of 13 RCTs (206 participants) found citrulline significantly reduced post-exercise perceived exertion and 24-hour muscle soreness but did not lower blood lactate; most common dose 8 g citrulline malate.
Rhim et al., Journal of Sport and Health Science, PubMed record
L-citrulline plus transresveratrol (800 mg/300 mg) added to PDE5 inhibitors improved mean SHIM score versus placebo in a crossover pilot (13 completers); earlier Cormio 2011 pilot (24 men) reported erection-hardness improvement in 50% of citrulline users vs 8.3% on placebo.
Citrulline was first isolated from watermelon by Koga and Ohtake in 1914; Wada elucidated the structure and named it in 1930 after the genus Citrullus.
L-citrulline is not on the WADA prohibited list and is permitted for tested athletes, though WADA does not certify supplement products, leaving finished-product contamination risk.
Randomized HFpEF trial compared L-arginine (8 g/day) and citrulline malate (3 g/day) over two months, with both groups showing significant decreases in pulmonary artery pressure; later single-arm pilot suggests improved vascular function and capacity.
Effect of L-arginine or L-citrulline on BP and RV function in HFpEF, Cardiology Journal
Meta-analysis of citrulline for aerobic exercise outcomes reports modest effects on lactate, oxygen-uptake kinetics and perceived exertion, with authors noting the endurance evidence remains controversial.
Effects of Citrulline Supplementation on Aerobic Exercise Performance (PMC)
The studies that matter
Wada watermelon isolation1914
Yotaro Wada at the Imperial University of Tokyo isolated a previously-uncharacterized amino acid from watermelon (Citrullus vulgaris). The compound was named citrulline after its botanical source. Foundational discovery that named the molecule and established the watermelon natural source.
Wassersug structural characterization1924
Detailed chemical characterization of the citrulline structure as a non-proteinogenic alpha-amino acid with a ureido group on the side chain. Foundational structural biochemistry.
Krebs urea cycle elucidation1932
Hans Krebs and Kurt Henseleit established the urea cycle, identifying citrulline as a key intermediate in ammonia detoxification, the foundational metabolic role characterization.
Schwedhelm pharmacokinetic comparisonBr J Clin Pharmacol 2008
Definitive trial comparing oral L-citrulline versus oral L-arginine for plasma L-arginine elevation in healthy adults. Demonstrated substantially superior bioavailability of the citrulline route, establishing the modern rationale for L-citrulline over L-arginine supplementation.
Pérez-Guisado citrulline malate resistance trainingJ Strength Cond Res 2010
Spanish resistance training trial: 8g citrulline malate before resistance training increased number of repetitions performed and reduced post-exercise soreness versus placebo. Foundational resistance training ergogenic evidence.
Cormio erectile dysfunction trialUrology 2011
24-patient crossover trial: 1.5g L-citrulline daily for one month improved erectile dysfunction symptoms versus placebo. Established the modest erectile function evidence at relatively low doses.
Suzuki cycling endurance trialJ Int Soc Sports Nutr 2016
Japanese endurance trial: L-citrulline supplementation improved time trial performance and reduced perceived exertion in cycling. Established the endurance performance evidence.
Bailey citrulline-arginine plasma kineticsNitric Oxide 2015
Detailed pharmacokinetic study confirming that L-citrulline produces more sustained plasma arginine elevation than equimolar L-arginine, with implications for sustained nitric oxide pathway support.
Glenn trained women resistance trainingJ Strength Cond Res 2017
Sex-specific trial: citrulline malate improved repetitions to failure in resistance-trained women. Expanded evidence beyond predominantly-male trial populations.
Trexler ISSN citrulline reviewJ Int Soc Sports Nutr 2019
Comprehensive professional society review consolidating the citrulline ergogenic evidence base across resistance training, endurance, and recovery applications.