L-Citrulline: Evidence-Based Guide to Benefits, Forms, and Dosing

L-Citrulline: Evidence-Based Guide to Benefits, Forms, and Dosing

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Table of Contents

Overview

L-citrulline is a non-essential, non-proteinogenic amino acid that functions as an intermediate in the urea cycle and serves as a precursor to L-arginine, the direct substrate for nitric oxide (NO) synthesis [1][2]. The "L" prefix indicates the naturally occurring stereoisomer. The compound was first isolated from watermelon (Citrullus vulgaris) in 1914 by Japanese researchers Yotaro Koga and Ryo Odake, and its name derives from the Latin word for watermelon [3][4].

The primary interest in L-citrulline supplementation stems from its ability to raise plasma L-arginine levels more effectively than oral L-arginine itself. When L-arginine is taken orally, a substantial portion is degraded by arginase enzymes in the gut and liver during first-pass metabolism, limiting the amount that reaches systemic circulation [5][6]. L-citrulline bypasses this first-pass metabolism because it is not a substrate for arginase. Instead, it is absorbed intact in the small intestine, enters systemic circulation, and is converted to L-arginine primarily in the kidneys via the sequential action of argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) [1][7][8]. This conversion pathway — known as the citrulline-NO cycle — regenerates L-arginine for sustained nitric oxide production by endothelial nitric oxide synthase (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS) [9][10].

Nitric oxide is a key signaling molecule that promotes vasodilation by activating soluble guanylyl cyclase and increasing cyclic GMP (cGMP), which relaxes vascular smooth muscle and allows more blood to flow through arteries [1][11]. This mechanism underpins the theoretical rationale for L-citrulline's cardiovascular, exercise performance, and erectile dysfunction applications.

L-citrulline also plays important roles beyond nitric oxide production. As an intermediate in the urea cycle, it participates in ammonia detoxification — the conversion of toxic ammonia (generated from amino acid catabolism) into non-toxic urea for renal excretion [12][13]. Additionally, research suggests L-citrulline can directly stimulate muscle protein synthesis via the PI3K/mTOR signaling pathway, independent of its conversion to arginine [14][15].

The body produces L-citrulline endogenously through multiple pathways. In the liver, it is synthesized as part of the urea cycle from ornithine and carbamoyl phosphate via ornithine transcarbamylase (OTC) [12]. In the intestinal enterocytes, it is generated from glutamine and glutamate through a series of mitochondrial enzymes [16]. The kidneys produce minor amounts via the NOS pathway. Age-related declines in enzymatic activity reduce endogenous production, particularly in older adults [17].

L-citrulline exhibits high oral bioavailability (approximately 97%) and is efficiently absorbed in the gastrointestinal tract without significant hepatic first-pass metabolism [18]. Peak plasma concentrations occur approximately one hour after ingestion [19]. It appears as a white crystalline powder with high water solubility (approximately 200 g/L at 20°C), making it suitable for aqueous formulations [20][21].

Despite the strong mechanistic rationale, clinical evidence for L-citrulline supplementation has been mixed. Studies have generally shown little to no benefit for exercise performance when taken as a single dose before exercise, modest potential for blood pressure reduction, and some evidence for mild erectile dysfunction [1][22]. The evidence is summarized in detail in the sections that follow.

Forms and Bioavailability

L-Citrulline vs. L-Citrulline Malate

L-citrulline is commercially available in two primary forms: pure L-citrulline and L-citrulline malate (a combination with malic acid, technically DL-malic acid). Understanding the differences between these forms is critical for interpreting research and selecting appropriate dosing.

Pure L-citrulline is essentially 100% citrulline by weight. It has a very faint, savory flavor and is neither acidic nor basic. It does not absorb water from the air (non-hygroscopic), so it is unlikely to become clumpy as a powder or crumbly as a tablet [1]. Most clinical studies evaluating cardiovascular and erectile dysfunction endpoints have used this form.

L-citrulline malate is only 56.6% citrulline by weight, with the remainder being malic acid [1][23]. This gives it a tangy, acidic flavor. L-citrulline malate absorbs water from the air (hygroscopic), which means powders will clump and tablets may crumble if exposed to humid conditions [1]. It is generally not suitable for tablet formulations for this reason. The malate form is more commonly marketed for exercise performance, though this specific advantage has not been demonstrated in controlled studies.

A direct comparison study among 43 healthy, resistance-trained men and women (average age 24) showed that taking citrulline malate (5.3 g citrulline plus 2.7 g malate) 45 minutes before low-to-moderate-volume resistance training did not improve muscle strength or endurance compared to supplementation with the same dose (5.3 g) of pure L-citrulline without malate, and neither form showed improvement compared to placebo (Martin-Olmedo et al., Int J Sport Nutr Exerc Metab, 2024) [24]. A systematic review also concluded that the addition of malate to citrulline has not been demonstrated to enhance exercise performance (Gough et al., Eur J Appl Physiol, 2021) [23].

"2:1" Labeling Confusion

Products labeled as "2:1 L-citrulline malate" or "L-citrulline malate 2:1" present a labeling ambiguity. It is unclear whether "2:1" refers to a molecular ratio (where 72.3% of the ingredient should be L-citrulline) or a weight ratio (where 67% should be L-citrulline). Independent testing of five different "2:1" products in England revealed citrulline amounts that were consistently less than expected, yielding actual ratios of 1.11 to 1.92 parts citrulline to each part malate — meaning only 52% to 66% was actual citrulline (Chappell et al., J Int Soc Sports Nutr, 2018) [25].

Bioavailability and Pharmacokinetics

L-citrulline exhibits high oral bioavailability, approximately 97%, with efficient absorption occurring in the small intestine via amino acid transporters [18]. It reaches systemic circulation largely intact without significant hepatic first-pass metabolism — a key advantage over oral L-arginine, which is substantially degraded by arginase in the gut and liver [5][6].

Peak plasma citrulline and arginine concentrations occur approximately one hour after oral ingestion [19]. Studies have confirmed that both L-arginine and L-citrulline supplementation cause similar increases in blood levels of arginine, but citrulline supplementation achieves this more efficiently because it is not degraded during first-pass metabolism (Bailey et al., J Appl Physiol, 2015) [26].

L-citrulline supplementation can bypass arginase-mediated limitations on L-arginine availability, leading to more sustained NO production compared to direct L-arginine supplementation [5][27][28]. Animal models have demonstrated that L-citrulline restores intracellular NO levels by preventing arginase activity increases and enhancing arginine recycling [29].

Summary of Form Selection

Form Citrulline Content Flavor Hygroscopic Best Use
Pure L-Citrulline ~100% Faint, savory No General health, blood pressure, erectile dysfunction. Better studied form.
L-Citrulline Malate ~56.6% Tangy, acidic Yes Often marketed for exercise but no proven advantage over pure form.
"2:1" L-Citrulline Malate 52-72% (variable) Tangy, acidic Yes Labeling ambiguous; actual citrulline content often less than expected.

If you are trying to get a specific dose of L-citrulline from a product, you must account for the citrulline content of the form being used. An 8 g dose of citrulline malate provides only approximately 4.5 g of actual citrulline [1].

Evidence for Benefits

Exercise Endurance and Performance

L-citrulline has been evaluated for exercise performance enhancement based on the rationale that increased nitric oxide production improves blood flow, oxygen delivery, and ammonia clearance during exercise. Results have been mixed, with multi-day supplementation showing more promise than single acute doses.

Multi-day supplementation (positive results): A study of 10 healthy, recreationally active young men who took either 6 g of L-arginine, 6 g of L-citrulline, or placebo for seven days found that both L-arginine and citrulline caused similar increases in blood levels of arginine, but only citrulline supplementation improved exercise tolerance (by 12%) and increased the total amount of work completed (by 7%) during a severe-intensity cycling exercise test compared to placebo (Bailey et al., J Appl Physiol, 2015) [26].

Single-dose supplementation (negative results): Most studies using a single pre-exercise dose of citrulline have failed to show benefit:

  • A study of 12 recreationally trained young men and women (average age 24) found that a single dose of 8 g of citrulline malate (providing approximately 5.33 g of citrulline) taken one hour before high-intensity exercise did not improve performance or decrease fatigue compared to placebo (Farney et al., J Strength Cond Res, 2017) [30].
  • A study among young, healthy men and women in the UK found that an 8 g dose of citrulline malate taken one hour before resistance exercise did not increase muscle strength or reduce muscle fatigue, and actually worsened delayed-onset muscle soreness (DOMS) in the three days following exercise compared to placebo (Chappell et al., J Int Soc Sports Nutr, 2018) [25].
  • Drinking a combination of L-arginine (2.5 g) and L-citrulline (5.6 g) before resistance exercise was no more effective than placebo in increasing muscle perfusion (blood flow) and muscle size after exercise in young men who regularly practiced resistance exercise (Burkle et al., JISSN, 2022) [31].
  • A study of 43 healthy, resistance-trained men and women (average age 24) showed that taking citrulline malate (5.3 g citrulline plus 2.7 g malate) 45 minutes before low-to-moderate-volume resistance training did not improve muscle strength or endurance compared to the same dose (5.3 g) of pure L-citrulline without malate, and neither form was better than placebo (Martin-Olmedo et al., Int J Sport Nutr Exerc Metab, 2024) [24].

Older adults: A study of 44 older men and women (average age 65) found that 3 g of citrulline malate taken daily in addition to regular exercise (a 45-minute combination of walking, slow running, balance exercises, and strength training) for six weeks slightly improved walking speed but did not significantly improve muscle strength or endurance compared to exercise plus placebo (Caballero-Garcia et al., Nutrients, 2021) [32].

Meta-analyses and systematic reviews: A systematic review concluded that citrulline malate supplementation has not demonstrated reliable improvements in exercise performance, and that the addition of malate provides no proven ergogenic advantage (Gough et al., Eur J Appl Physiol, 2021) [23]. Some meta-analyses report modest increases in repetition performance (approximately 3 ± 5 weighted repetitions) in resistance training with citrulline malate, but results vary substantially by protocol [33]. A 2025 systematic review indicates an overall positive effect on vascular health in middle-aged and older adults, which may indirectly support exercise capacity [34].

Muscle soreness and perceived exertion: Research suggests L-citrulline may reduce post-exercise ratings of perceived exertion (RPE) and delayed muscle soreness without altering blood lactate levels directly. Acute supplementation with 8 g citrulline malate taken 1-2 hours before exercise has been reported to significantly reduce delayed muscle soreness at 24 hours post-exercise (effect size 0.99), with some effects at 48 hours [33]. Citrulline malate has also been shown to enhance creatine phosphate resynthesis, which supports faster ATP regeneration and may delay fatigue in repeated bouts of anaerobic exercise [33].

Power output and resistance training: An acute dose of 8 g citrulline malate has been reported to increase the number of repetitions to failure during multiple sets at 80% of one-repetition maximum (1RM), leading to greater overall work capacity in some studies [33]. Similarly, studies on resistance training protocols report improved performance in upper-body exercises, with faster recovery enabling higher power maintenance across sets. However, the direct comparison trial by Martin-Olmedo et al. (2024) found no benefit over placebo [24].

CrossFit and functional training: A randomized crossover study on CrossFit athletes found that acute citrulline malate supplementation improved overall workout performance metrics, such as total repetitions and time efficiency in mixed aerobic-anaerobic circuits [35]. However, investigations into sex-specific responses remain limited, with no dedicated female-only endurance trials identified.

Muscle protein synthesis: Beyond performance, research suggests L-citrulline can directly stimulate muscle protein synthesis. Studies in rats showed that citrulline supplementation combined with endurance training enhanced muscle protein synthesis by 48% compared to controls [36]. In humans, citrulline administration in post-absorptive states after a low-protein diet boosted muscle protein synthesis by approximately 20% via mTOR activation [37]. L-citrulline modulates the PI3K/mTOR pathway to stimulate muscle protein synthesis independently of its conversion to arginine [14][15]. Through its conversion to ornithine, L-citrulline also contributes to polyamine synthesis via the ornithine decarboxylase pathway, supporting cellular proliferation and anabolic signaling [67][68].

Synthesis: Multi-day supplementation (7+ days) at doses of 6 g/day of pure L-citrulline has shown some benefit for exercise tolerance in small studies. However, single pre-exercise doses — including 8 g of citrulline malate — have consistently failed to improve performance in well-controlled trials. The exercise literature is dominated by small studies with heterogeneous protocols. The addition of malate to citrulline provides no demonstrated advantage over citrulline alone. Some evidence supports reduced muscle soreness at 24 hours post-exercise, but this has not translated to consistent performance gains.

Blood Pressure and Cardiovascular Health

Because citrulline is converted to arginine, which in turn may increase nitric oxide production and promote vasodilation, it has been evaluated for cardiovascular benefits. Evidence has been mixed.

Meta-analyses: Several meta-analyses of randomized controlled trials have evaluated citrulline for blood pressure reduction. Results have been inconsistent, with most positive findings limited to specific populations:

  • A systematic review and meta-analysis reported a modest but significant reduction in systolic blood pressure of approximately 4 mmHg compared to placebo, with effects primarily observed in individuals with elevated blood pressure rather than normotensive subjects [38].
  • An updated meta-analysis from 2024 confirmed these findings, reporting significant reductions in both systolic and diastolic blood pressure across multiple trials, attributing the benefits to improved endothelial function via nitric oxide pathways [38].
  • A 2025 systematic review and meta-analysis also indicates overall positive effects on vascular health in middle-aged and older adults [34].
  • However, an earlier review found that most studies evaluating citrulline for lowering high blood pressure were conducted in healthy individuals rather than hypertensive populations, and results were mixed (Barkhidarian et al., Avicenna J Phytomed, 2019) [39].

Postmenopausal hypertension (negative): A study of 25 postmenopausal women with high blood pressure (systolic blood pressure 130 mmHg or higher) showed that supplementing with 5 g of L-citrulline in the morning and at night for 4 weeks did not significantly improve systolic or diastolic blood pressure, mean arterial pressure, or heart rate, despite increasing flow-mediated dilation (a measure of blood vessel health) from 4.8% to 6.2% (Maharaj et al., Nutrients, 2022) [40]. This suggests that improved endothelial function from citrulline does not always translate into measurable blood pressure reduction.

Arterial stiffness: L-citrulline supplementation has shown some ability to improve arterial stiffness, as measured by reductions in pulse wave velocity, especially in middle-aged and older populations at risk for cardiovascular disease. Studies demonstrate that short-term supplementation can functionally enhance arterial compliance independent of blood pressure changes, with notable decreases in systemic arterial stiffness observed in overweight men under stress conditions [41]. A 2022 trial in hypertensive postmenopausal women showed improvements in femoral-ankle pulse wave velocity after eight weeks of combined L-citrulline supplementation and resistance training [42].

Diastolic heart failure: A small, uncontrolled study among people with diastolic heart failure showed that taking 3 g of citrulline daily for two months reduced systolic blood pressure by 10.8%, diastolic blood pressure by 8.3%, and decreased pulmonary artery pressure by 9 mmHg compared to baseline. However, there was no placebo control group, which is essential to confirm the benefit. There was also no improvement in other measures of heart failure such as left or right ventricular ejection fraction or ability to walk on a treadmill (Orozco-Gutierrez et al., Cardiol J, 2010) [43].

Systolic heart failure: A small study in Mexico among people with systolic heart failure showed that taking 3 g of citrulline daily for 17 weeks did not significantly reduce systolic or diastolic blood pressure, although left ventricular ejection fraction at rest and with stress increased by about 20% and 13% compared to baseline, and these increases were significant compared to the control group. However, there was no significant between-group difference in right ventricular ejection fraction (Balderas-Munoz et al., Cardiol J, 2012) [44].

Heart failure and atrial fibrillation risk: There does not appear to be evidence that citrulline prevents heart failure or atrial fibrillation. In fact, an observational study found that higher blood levels of citrulline were associated with a 19% greater risk of heart failure and no significant effect on the risk of atrial fibrillation (Goni et al., Am J Clin Nutr, 2022) [45]. This finding is observational and may reflect reverse causation (sick individuals may have altered amino acid metabolism), but it raises a cautionary note.

Combination therapies: Post-2020 trials have explored L-citrulline in combination with other supplements. A 9-week study combining L-citrulline with nitrate-rich beetroot extract showed potentiated reductions in blood pressure and improved vascular health metrics [46]. Research on combinations with polyphenols from cranberry and grape seed has shown promising effects on ambulatory blood pressure in women with prehypertension [47].

Use in older adults: L-citrulline supplementation may be particularly relevant for middle-aged and older adults due to age-related declines in nitric oxide production and endothelial function. Meta-analyses show significant improvements in flow-mediated dilation (FMD) in this population after chronic use. Studies in adults around 70 years have demonstrated that 6 g/day increases lower limb blood flow during exercise by approximately 11% [48]. Doses of 3-6 g/day are commonly effective and well-tolerated in this population, with lower risk of gastrointestinal side effects compared to L-arginine.

Synthesis: The blood pressure evidence for L-citrulline is inconsistent. Meta-analyses suggest a modest reduction of approximately 4 mmHg in systolic blood pressure, primarily in people with elevated blood pressure. However, a well-controlled trial in hypertensive postmenopausal women showed no blood pressure reduction despite improved endothelial function. Small heart failure studies showed some hemodynamic improvements but lacked placebo controls or were underpowered. The observational association between higher citrulline levels and increased heart failure risk warrants further investigation.

Erectile Dysfunction

The most consistent clinical evidence for L-citrulline comes from studies on mild erectile dysfunction.

Key clinical trial: A study in Italy among 24 men with mild erectile dysfunction found that taking 750 mg of L-citrulline twice daily (1.5 g/day total) for one month resulted in improvements in erection hardness in 12 of the 24 men (50%), compared to improvements in only 2 of the men when they were given placebo. The men who improved shifted from an Erection Hardness Score (EHS) of 3 (partially hard) to 4 (fully hard). No adverse events were reported. The researchers noted that L-citrulline is less effective than prescription medication for erectile dysfunction (Cormio et al., Urology, 2011) [49].

Mechanism: L-citrulline supports erectile function by serving as a precursor to L-arginine, which enhances nitric oxide production, promoting vasodilation and improving penile blood flow essential for achieving and maintaining erections [49][50]. This mechanism addresses endothelial dysfunction, a common underlying factor in erectile dysfunction, by bypassing the presystemic metabolism of L-arginine and efficiently increasing its bioavailability for NO synthesis [50].

Combination with transresveratrol: A randomized, double-blind, placebo-controlled trial demonstrated that combining L-citrulline with transresveratrol enhanced erectile function in men already using PDE5 inhibitors (e.g., sildenafil), particularly in non-responders to L-arginine alone. Improvements were noted in intercourse frequency and overall sexual satisfaction (Sansone et al.) [51].

Comparison with PDE5 inhibitors: Evidence-based reviews suggest that higher daily doses of L-citrulline (5-8 g, often split into multiple doses) may provide mild benefits similar to low-dose tadalafil (e.g., 5 mg daily), such as improved spontaneous erections and blood flow. However, direct head-to-head trials establishing exact equivalence are lacking [52][53]. L-citrulline is generally considered less potent than prescription PDE5 inhibitors.

Comparison with L-arginine: L-citrulline is generally superior to direct L-arginine supplementation (typically 2-5 g daily) for erectile function due to its better bioavailability, as it bypasses first-pass metabolism in the liver and intestines, leading to more efficient and sustained increases in plasma L-arginine levels [50][54]. Evidence as of 2026 continues to favor L-citrulline over oral L-arginine for improving mild ED. The combination of L-arginine and L-ornithine is marketed as a nitric oxide precursor but lacks strong clinical evidence specifically for erectile dysfunction [55].

Libido: Evidence specifically for improving libido (sexual desire) is limited, with most research focused on erectile performance rather than desire [50].

Patient demographics: Benefits appear most pronounced in men with mild ED and without severe underlying vascular disease, such as those aged around 56 years in the key trial, where baseline endothelial function is preserved enough for NO-mediated interventions to be effective [49].

Limitations: Available reviews have identified gaps in long-term safety data for prolonged use in ED management, as existing studies are short-term and lack large-scale assessments [56]. Additional research, including direct head-to-head comparisons with sildenafil, is needed. No adverse events were reported in existing trials, supporting short-term safety.

Synthesis: L-citrulline at 1.5 g/day improved erection hardness in 50% of men with mild ED in the only dedicated randomized controlled trial. It is less effective than prescription PDE5 inhibitors but has an excellent safety profile. It may serve as an adjunct therapy, particularly when combined with transresveratrol. Larger trials are needed to confirm efficacy and establish optimal dosing.

Brain Function and Neuroprotection

Animal evidence: A study in mice found that feeding them L-citrulline daily after temporarily blocking blood flow to a portion of the brain (simulating an ischemic stroke) reduced death of neurons and loss of capillaries and improved memory deficits. The authors included employees of Kyowa Hakko, which provided the product and is a major manufacturer of L-citrulline (Yabuki et al., Brain Res, 2013) [57]. This study has conflict-of-interest concerns and has not been replicated in humans.

Cerebrovascular function: Preliminary evidence suggests L-citrulline may improve cerebrovascular function, potentially aiding recovery and providing indirect benefits for mental clarity via enhanced nutrient and oxygen delivery to the brain [58]. However, direct clinical data for cognitive benefits in humans remain very limited, particularly in sedentary healthy adults [59].

Depression and anxiety: There is no clinical evidence from randomized controlled trials supporting L-citrulline supplementation as an effective treatment for depression, anxiety, or as an antidepressant. Observational studies have reported lower serum L-citrulline levels and altered arginine-citrulline ratios in patients with major depression compared to healthy controls, suggesting a possible association with the nitric oxide pathway, but no intervention studies demonstrate benefits from supplementation for these conditions [60].

Synthesis: The brain and neuroprotection evidence for L-citrulline is preliminary and almost entirely from animal models or observational data. There is no convincing evidence of cognitive benefits in humans. Clinical trials are needed before any recommendations can be made.

Fatty Liver Disease

Animal evidence: Studies in animals suggested a potential benefit of L-citrulline supplementation for fatty liver disease (Jegatheesan et al., Clin Nutr, 2016; Levy, Br J Clin Pharmacol, 2017) [61][62].

Human trial (negative): A study in Mexico among 42 adolescents with abdominal obesity and metabolic-associated fatty liver disease (MAFLD) found that 3 g of L-citrulline taken twice daily (6 g/day total, before breakfast and after dinner) did not decrease liver fat accumulation, total and LDL cholesterol, or alkaline phosphatase (ALP) levels, or show any other benefit compared to placebo (Tovar-Villegas et al., Gastroenterol Insights, 2024) [63].

Synthesis: Despite promising animal data, the only human trial of L-citrulline for fatty liver disease showed no benefit. This application lacks clinical support.

Ammonia Clearance and Urea Cycle Support

L-citrulline plays a critical role in ammonia detoxification as an intermediate in the urea cycle. It facilitates the conversion of toxic ammonia into non-toxic urea for renal excretion [12][13]. This function is particularly relevant in:

  • Hepatic tissues: Disruptions in citrulline metabolism can lead to hyperammonemia [64].
  • Urea cycle disorders: Citrullinemia type I (caused by argininosuccinate synthetase deficiency) and citrullinemia type II (caused by mutations in the SLC25A13 gene encoding citrin) both result in elevated plasma citrulline and impaired urea cycle function [65][66].
  • Exercise: Citrulline's role in ammonia clearance may contribute to reduced fatigue during sustained exercise efforts by facilitating ammonia detoxification [33].

Citrulline supplementation is sometimes used clinically in urea cycle disorder management, though this is a specialized application under medical supervision [65].

Muscle Protein Synthesis

Beyond its role in nitric oxide production, L-citrulline has been investigated for its ability to directly stimulate muscle protein synthesis:

  • L-citrulline modulates the PI3K/mTOR signaling pathway to stimulate muscle protein synthesis independently of its conversion to arginine [14][15].
  • In rats, citrulline supplementation combined with endurance training enhanced muscle protein synthesis by 48% compared to controls [36].
  • In humans, citrulline administration in post-absorptive states after a low-protein diet boosted muscle protein synthesis by approximately 20% via mTOR activation [37].
  • Through its conversion to ornithine, L-citrulline contributes to polyamine synthesis via the ornithine decarboxylase pathway, supporting cellular proliferation and anabolic signaling [67][68].

These findings are preliminary and require larger human trials to confirm clinical relevance, particularly for populations at risk of sarcopenia. A completed clinical trial (NCT02417428) evaluated L-citrulline combined with exercise for managing sarcopenia in elderly people, examining changes in body composition and muscle function [69].

COVID-19 and Vascular Health

Research has shown that patients with severe COVID-19 exhibit decreased plasma citrulline levels associated with increased oxidative stress and poor oxygenation [70]. Low citrulline concentrations in COVID-19 patients correlate with systemic inflammation and gastrointestinal symptoms, suggesting a role in vascular dysfunction [71]. A clinical trial evaluating intravenous L-citrulline for reducing the need for mechanical ventilation in acute hypoxemic respiratory failure due to COVID-19 was completed in 2021 [72]. These findings remain preliminary and do not support supplementation as a COVID-19 treatment.

By Indication

Dosing recommendations vary depending on the intended use and the form of citrulline being used. Note that L-citrulline malate is only approximately 56.6% citrulline, so doses must be adjusted accordingly.

Purpose Form Dose Duration Notes
General cardiovascular support Pure L-citrulline 3-6 g/day, divided Ongoing Multi-day use shows more benefit than single doses [1][19]
Blood pressure Pure L-citrulline 5-6 g/day, divided (morning and evening) 4-8+ weeks Most positive meta-analysis data at ~6 g/day [38][40]
Exercise performance Pure L-citrulline 6 g/day for 7+ days 7+ days before event Single pre-exercise doses have not shown benefit [23][26]
Exercise performance L-citrulline malate 8 g (~4.5 g citrulline) Single dose, 30-60 min pre-exercise Commonly used but evidence weak; most single-dose trials negative [23][24][25]
Mild erectile dysfunction Pure L-citrulline 1.5 g/day (750 mg twice daily) 1+ month Based on Cormio et al. trial [49]; higher doses (up to 6 g/day) sometimes suggested
Older adults (vascular health) Pure L-citrulline 3-6 g/day Ongoing Well-tolerated; lower GI risk than L-arginine [48]

Timing and Administration

  • Peak plasma concentrations occur approximately 1 hour after oral ingestion [19].
  • Multi-day supplementation (7+ days) has generally shown more benefit than single acute doses, particularly for exercise-related outcomes [1][26].
  • Dividing the dose (e.g., morning and evening) may sustain arginine and nitric oxide levels throughout the day.
  • L-citrulline can be taken with or without food. Some sources suggest taking it on an empty stomach for optimal absorption, though there is no strong evidence that food significantly reduces effectiveness [19].

How to Read a Supplement Label

  • Products labeled "L-citrulline" are expected to be approximately 100% citrulline.
  • Products labeled "L-citrulline malate" are expected to be 56.6% citrulline. An 8 g dose provides approximately 4.5 g of actual citrulline.
  • Products labeled "2:1 L-citrulline malate" may be 52-72% citrulline depending on interpretation and manufacturing accuracy [1][25].
  • If a product lists "3 g L-citrulline malate," only approximately 1.7 g is actual citrulline.
  • Always check whether the label specifies the amount of L-citrulline itself or the total weight of the citrulline malate compound.

Storage

If buying citrulline as a powder, be aware that L-citrulline malate absorbs water from the air (hygroscopic) and will clump if left exposed to humid air. Keep the container closed when not in use. Regular L-citrulline does not absorb water from the air and does not have this issue [1].

Safety and Side Effects

General Safety Profile

L-citrulline has self-affirmed GRAS (Generally Recognized as Safe) status for oral use in food products [73]. Animal studies indicate low toxicity with a wide margin of safety for acute exposure [74]. In clinical trials lasting from one week to several months, daily doses of citrulline up to about 10 g/day have generally been safe and well-tolerated [1][75]. Higher acute doses up to 15 g have been reported as safe in short-term studies [75].

Common Side Effects

Side effects are mild and infrequent, occurring in less than 5% of users based on aggregated clinical data [75]:

  • Gastrointestinal upset — nausea, diarrhea, or stomach discomfort
  • Heartburn — particularly with the malate form, which is more acidic than pure L-citrulline
  • Tooth enamel effects — L-citrulline malate taken as a drink may affect tooth enamel due to its acidity

L-citrulline supplementation appears to be less likely to cause gastrointestinal upset than L-arginine supplementation [1]. The malate form is more acidic than regular L-citrulline, making it more likely to cause heartburn and exacerbate gastroesophageal reflux [1].

Specific Populations

People with pulmonary arterial hypertension: Increased urinary frequency, cough, and swelling of arms and legs have been reported with L-citrulline use (Allerton et al., Nutrients, 2018; Kashani et al., J Cardiol, 2014) [22][76].

People with low blood pressure: Because citrulline is converted to arginine and may increase nitric oxide production, it could potentially reduce blood pressure further. Rare instances of hypotension have been noted, particularly in individuals with pre-existing low blood pressure due to enhanced vasodilation. Caution is advised, especially if symptomatic or combined with other vasodilators [75].

Urea cycle disorders: Certain urea cycle disorders, such as argininosuccinate synthetase deficiency, are associated with elevated blood levels of citrulline. Citrulline supplementation could affect tests used to detect and/or monitor these disorders (Berry et al., J Pediatr, 2001) [77].

Pregnancy and pediatric use: Data on safety in pediatric populations and during pregnancy are sparse, with most evidence derived from small-scale or animal studies. Ongoing clinical investigations, such as those evaluating enteral supplementation in preterm infants, report no adverse events to date but emphasize the need for further research [78][79]. In pregnancy models, supplementation has shown potential benefits without observed toxicity, but human trials like the AGREE study are still assessing long-term outcomes [80][81].

Kidney disease: Caution is advised in individuals with severe kidney disease due to limited data on amino acid metabolism in this population, though some studies suggest potential renal protective effects [75].

Surgery: Some sources recommend stopping L-citrulline supplementation at least 2 weeks before scheduled surgery due to potential interference with blood pressure control during and after the procedure [75].

Bleeding Risk

L-citrulline supplements have no established bleeding risk. No side effects related to bleeding, hemorrhage, or blood thinning have been reported. No known interactions with anticoagulants or antiplatelet drugs (e.g., aspirin, warfarin) are documented in major databases [75].

Alcohol Interactions

No significant adverse interactions have been reported for combining L-citrulline with alcohol [82][83][84].

Drug Interactions

Because citrulline is converted to arginine in the body, it might — like arginine — potentially enhance nitric oxide production and lower blood pressure. This creates interactions with several medication classes:

Medication Class Examples Interaction Clinical Significance
Blood pressure-lowering medications ACE inhibitors, ARBs, calcium channel blockers, beta-blockers May enhance blood pressure reduction Monitor blood pressure; use with caution [1]
Nitrates Nitroglycerin, isosorbide dinitrate May amplify vasodilatory effects Potential for excessive hypotension [1][85]
PDE5 inhibitors Sildenafil (Viagra), tadalafil (Cialis) May amplify vasodilatory effects Potential for excessive blood pressure lowering; use with caution [1][85]

Key Points on Drug Interactions

  • The primary concern is additive blood pressure-lowering effects when L-citrulline is combined with medications that also lower blood pressure or promote vasodilation [1][85].
  • PDE5 inhibitors work on the same nitric oxide-cGMP pathway that L-citrulline supports. Combining them could amplify vasodilatory effects and potentially cause excessive blood pressure lowering [85].
  • Although one clinical trial specifically evaluated L-citrulline combined with PDE5 inhibitors and found no adverse events [51], this was in a controlled research setting. Caution is still advised in clinical practice.
  • Unlike magnesium supplements, L-citrulline does not have known chelation interactions that reduce the absorption of other medications.
  • Individuals taking any blood pressure-lowering medication, nitrates, or PDE5 inhibitors should consult their healthcare provider before starting L-citrulline supplementation.

Dietary Sources

L-citrulline is primarily obtained from plant-based foods, with the highest concentrations found in members of the Cucurbitaceae (gourd) family.

Top Food Sources

Food Citrulline Content Notes
Watermelon (flesh) 0.7-3.5 g/kg fresh weight Highest natural source. Varies by cultivar; 'Crimson Sweet' up to 2.85 g/kg, 'Dixielee' 2.36 g/kg [86]
Watermelon (rind) Higher than flesh Often discarded but particularly rich in citrulline [86]
Casaba-type melon 0.86 g/kg Moderate source [86]
Mouse melon (Melothria scabra) 0.64 g/kg Uncommon food [86]
Horned melon (Cucumis metuliferus), rind 0.45 g/kg Niche cucurbit [86]
Cucumber 0.23-0.28 g/kg Moderate source; varieties 'Expedition' and 'Dasher II' [86]
Pumpkin and squash 0.04-0.07 g/kg Low amounts; varieties like 'Enterprise' and 'Payload' [86]

Practical Notes

  • Watermelon is the dominant dietary source — it contains 7-41 times more citrulline than other cucurbits [86].
  • Achieving therapeutic doses from food is impractical. To obtain 6 g of L-citrulline from watermelon, one would need to consume approximately 2-8.5 kg (4-19 lbs) of fresh watermelon, depending on cultivar and citrulline concentration [18][86]. Supplements provide concentrated, standardized doses that are far more practical for therapeutic applications.
  • Processing reduces citrulline content. Juicing or drying watermelon can decrease citrulline levels; fresh consumption preserves the most citrulline [86].
  • Animal-derived foods contain only trace amounts of L-citrulline, primarily from endogenous production rather than substantial direct dietary content [18].
  • Oral bioavailability of dietary citrulline is high (approximately 97%), so what is consumed from food is efficiently absorbed [18].
  • Watermelon vs. supplements for therapeutic use: There is no reliable scientific evidence that consuming watermelon is superior to L-citrulline supplements. Achieving therapeutic doses (e.g., 1.5-6 g) from watermelon alone would require impractically large amounts, and no studies directly compare the two [17].

Endogenous Production

The body produces L-citrulline through several internal pathways:

  • Liver: Synthesized as part of the urea cycle from ornithine and carbamoyl phosphate. However, hepatic production does not contribute net citrulline to systemic circulation — it is used internally within hepatocytes [16].
  • Intestinal enterocytes: The primary source of circulating citrulline. Generated from glutamine and glutamate through mitochondrial enzymes including phosphate-dependent glutaminase, pyrroline-5-carboxylate synthase, and ornithine aminotransferase. This pathway is particularly active in the small intestine [16].
  • Kidneys: Minor amounts produced via the NOS pathway, but the kidneys primarily convert circulating citrulline to L-arginine rather than producing it for export [16].
  • Endogenous production declines with age due to reduced enzymatic activity, particularly OTC and ASS expression. Health status also plays a key role — liver disease impairs urea cycle function, and hormonal controls such as glucagon further regulate synthesis [17].

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