How Does L Citrulline Work?
Citrulline shows up on almost every pre-workout label, usually described in one word: pump. That word skips the entire biology.
The mechanism is specific and well mapped: an amino acid gets absorbed, converted in the kidneys, and feeds a signaling molecule that relaxes blood vessels. Whether that translates into better sets in the gym is a separate, messier question, and the research pack answers both.
How does l citrulline work? It works by raising your body's supply of arginine, the amino acid your cells use to make nitric oxide. Swallowed citrulline passes through the gut and liver largely intact, and the kidneys convert much of it to arginine. In human trials, oral l-citrulline and citrulline malate increased plasma citrulline and arginine concentrations along with total nitrate and nitrite, the standard blood markers of nitric oxide production [1].
Nitric oxide signals the smooth muscle in artery walls to relax, which widens the vessel and allows more blood to reach working tissue [2]. That vasodilation is the mechanism pre-workout formulas are chasing when they talk about pumps and blood flow. Citrulline is also an intermediate in the urea cycle, the pathway that handles ammonia [2].
Mechanism is not the same as measurable performance. Reviews and meta-analyses of citrulline and citrulline malate in trained adults report mixed outcomes for strength and endurance, and several controlled trials found no significant advantage over placebo [3][4][5]. The pathway is well described; the training payoff is modest and inconsistent.
How does l citrulline work in the body, step by step?
From the gut to the kidneys
L-citrulline is a nonessential amino acid found in food, most famously watermelon. It is absorbed in the small intestine and, unlike several other amino acids, a large share of it survives the first pass through the gut wall and liver. The kidneys then convert citrulline into arginine, which raises circulating arginine beyond what the dose of citrulline alone would suggest [1].
Arginine to nitric oxide
Arginine is the substrate for nitric oxide synthase, the enzyme family that produces nitric oxide in the lining of your blood vessels. More available arginine means more raw material for that reaction, and supplementation trials have tracked the downstream result as higher total nitrate and nitrite in plasma [1]. Nitric oxide then relaxes vascular smooth muscle, which is the vasodilation step described in reviews of citrulline and cardiometabolic measures [2].
The urea cycle side door
Citrulline is not only a nitric oxide precursor. It is a working intermediate of the urea cycle, the route the body uses to process ammonia, and this is one of the theoretical reasons it has been studied for fatigue during repeated hard efforts [2][1].
That is the full chain: absorb, convert to arginine, feed nitric oxide, widen vessels, support blood flow. Every claim a label makes about pumps sits on this sequence.
Why does citrulline raise arginine better than arginine itself?
This is the part that confuses people. If arginine is the molecule that makes nitric oxide, why not just take arginine?
Because a meaningful portion of oral arginine is broken down by arginase in the intestine and liver before it ever reaches general circulation. Citrulline largely sidesteps that route and is converted to arginine in the kidneys instead, which is why reviews describe citrulline as a way to augment arginine bioavailability rather than as a weaker substitute [1].
That does not make arginine useless. It is still a direct substrate, and it is still used in formulas, often alongside citrulline so both entry points to the pathway are covered. If you want the arginine side of the story in detail, that is covered in our breakdown of the l arginine evidence.
What the blood markers do and do not tell you
Raising plasma arginine and nitrite is a biochemical outcome, not a performance outcome. Reviews on nitric oxide precursors for strength performance are explicit that marker changes have not translated cleanly into consistent lifting results across studies [6]. Acute improvements in blood flow during exercise have also been inconsistent in the literature [1].
So the honest framing is this: citrulline reliably moves the inputs to the nitric oxide pathway, and the pathway reliably relaxes blood vessels. How much that shifts your top set is where the evidence loosens up.
What does l citrulline do during a workout?
During training, the proposed effect is increased blood flow to active muscle, which could mean more oxygen and nutrient delivery and faster clearance of metabolic byproducts [1]. The felt version of that is the tight, full sensation lifters call a pump, and the mechanics of it are covered further in what l-citrulline does for workouts.
The trial data are mixed, and it is worth being specific.
- A meta-analysis of citrulline malate and muscle strength in resistance-trained adults found no overall effect versus placebo (SMD 0.13, 95% CI -0.21 to 0.46) [3].
- A randomized, double-blind crossover trial in 43 trained men and women using 5.3 g of citrulline 45 minutes before testing found no improvement in maximal neuromuscular performance, ballistic strength, or strength-endurance [5].
- A meta-analysis of foods rich in l-citrulline found no effect on endurance performance (SMD -0.03, p = 0.24), while nitrate-rich and polyphenol-rich foods showed trivial but significant benefits [4].
- A separate meta-analysis examined the acute effect of citrulline malate on repetitions completed during strength training [7].
There are more favorable signals in specific populations. In 34 professional female soccer players, four weeks of 500 mg nitrate plus 8 g citrulline malate was associated with higher maximal speed and a smaller post-match decline in Wingate power than placebo, though that was a combination protocol [8]. In 44 adults aged 60 to 73 following an exercise program, six weeks of citrulline supplementation improved walking speed significantly versus placebo, with strength and endurance showing only non-significant trends [9].
Does the form, amount, or timing change how it works?
L citrulline versus citrulline malate
Citrulline malate pairs citrulline with malic acid, a Krebs cycle intermediate that is often credited with extra ergogenic value. A randomized, double-blind, placebo-controlled crossover trial compared 5.3 g of citrulline against 5.3 g of citrulline plus 2.7 g of malate in trained adults and found neither form improved neuromuscular performance or strength-endurance over placebo [5]. On current evidence, the malate does not appear to add a measurable edge.
How much l citrulline before a workout?
Amounts belong to the studies that used them. The crossover trial above used 5.3 g taken 45 minutes before testing [5], and the female soccer protocol used 8 g of citrulline malate daily for four weeks [8]. Reviews note that oral citrulline and citrulline malate both raise plasma citrulline and arginine, so the conversion step is not form-dependent [1].
For a finished product, follow the label. Surge pre-workout capsules list 200 mg of l citrulline (HCl) and 200 mg of citrulline malate per serving alongside l arginine, l arginine AKG, beta alanine, and niacin, and the label serving is 2 capsules 20 to 30 minutes before training. The gram-level numbers above come from research protocols, not from that label.
Timing logic follows absorption and conversion, which is why pre-workout protocols are dosed in the 30 to 60 minute window before training [5]. For the broader question of onset across pre-workout ingredients, see how long pre-workout takes to work.
Who should be cautious with l citrulline?
Citrulline acts on the nitric oxide pathway, and nitric oxide is involved in regulating vascular tone throughout the body [2]. That makes a short caution list worth reading before you add it to a stack.
- Anyone taking medication that affects blood pressure, circulation, or blood vessel dilation. The overlap in mechanism is the reason to ask a professional first, not a reason to self-adjust anything [2].
- Pregnant or nursing adults. Controlled data for citrulline supplementation in these groups are limited, so this is not the place to experiment [10].
- Anyone under 18. Pre-workout formulas as a category are not designed for this group [10].
- Anyone managing a health condition. Dietary supplements are not reviewed or approved by the FDA before they are sold, so the burden of judgment sits with you and your clinician [10].
If you are combining citrulline with an energy product, remember that multi-ingredient formulas stack their own cautions, and stimulant-containing products in particular warrant attention to total daily intake [11]. Surge's listed ingredients are nitric oxide precursors, beta alanine, and niacin rather than a stimulant blend, but the same rule applies: read the full label on everything you take in a day.
Talk with a healthcare professional before starting any supplement, especially if you are pregnant, nursing, under 18, taking medication, or managing a health condition. Tolerance also varies by person, so start at the label serving and pay attention to how you respond.
Frequently asked questions
What does l citrulline do in a workout?
It supplies arginine for nitric oxide production, which relaxes blood vessels and supports blood flow to working muscle [1][2]. That is the basis of the pump sensation. Performance results are less consistent: a meta-analysis in resistance-trained adults found no overall strength effect versus placebo, and a food-based endurance meta-analysis found no effect for citrulline [3][4].
How much l citrulline before workout?
Amounts belong to the studies. A crossover trial in trained adults used 5.3 g taken 45 minutes before testing, and a four-week protocol in female soccer players used 8 g of citrulline malate daily [5][8]. For a finished product, use the label serving only. Surge lists 200 mg l citrulline (HCl) plus 200 mg citrulline malate, taken as 2 capsules 20 to 30 minutes before training.
Does l citrulline work immediately?
Absorption and conversion to arginine take time, which is why trials dose it roughly 45 minutes before testing [5]. Blood markers of nitric oxide production rise after oral dosing, but acute performance effects have been inconsistent across studies [1].
Is l citrulline safe?
It is a food-derived amino acid studied in healthy adults, but it acts on the nitric oxide pathway [2]. People taking medication affecting blood pressure or circulation, anyone pregnant or nursing, and anyone under 18 should speak with a clinician first [10].
Key takeaways
- L citrulline works by surviving first-pass metabolism, converting to arginine in the kidneys, and feeding nitric oxide production [1].
- Nitric oxide relaxes vascular smooth muscle, which is the actual mechanism behind pump and blood flow claims [2].
- Citrulline raises plasma arginine more effectively than oral arginine because it bypasses much of the gut and liver breakdown [1].
- Performance evidence is mixed: a strength meta-analysis and a crossover trial in trained adults found no significant advantage over placebo [3][5].
- Adding malate did not outperform plain citrulline in a double-blind crossover trial using 5.3 g [5].
If you want the nitric oxide precursors in one capsule serving rather than a scoop, Surge pre-workout capsules combine l citrulline, citrulline malate, l arginine, and l arginine AKG to support blood flow and pumps during training. Made in the USA in an FDA-registered, GMP-certified facility and third-party tested for purity and potency. Read the label, check the serving, and see whether it fits your routine.
References
- Effects of Citrulline Supplementation on Exercise Performance in Humans: A Review of the Current Literature. Journal of strength and conditioning research (2020). PubMed 31977835
- l-Citrulline Supplementation: Impact on Cardiometabolic Health. Nutrients (2018). PubMed 30029482
- Effects of Citrulline Malate Supplementation on Muscle Strength in Resistance-Trained Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of dietary supplements (2022). PubMed 34176406
- Effect of food sources of nitrate, polyphenols, L-arginine and L-citrulline on endurance exercise performance: a systematic review and meta-analysis of randomised controlled trials. Journal of the International Society of Sports Nutrition (2021). PubMed 34965876
- Malate or Not? Acute Effects of L-Citrulline Versus Citrulline Malate on Neuromuscular Performance in Young, Trained Adults: A Randomized, Double-Blind, Placebo-Controlled Crossover Trial. International journal of sport nutrition and exercise metabolism (2025). PubMed 39662304
- Supplementation with Nitric Oxide Precursors for Strength Performance: A Review of the Current Literature. Nutrients (2023). PubMed 36771366
- Acute Effect of Citrulline Malate on Repetition Performance During Strength Training: A Systematic Review and Meta-Analysis. International journal of sport nutrition and exercise metabolism (2021). PubMed 34010809
- Impact of Chronic Nitrate and Citrulline Malate Supplementation on Performance and Recovery in Spanish Professional Female Soccer Players: A Randomized Controlled Trial. Nutrients (2025). PubMed 40733006
- L-Citrulline Supplementation and Exercise in the Management of Sarcopenia. Nutrients (2021). PubMed 34579009
- MedlinePlus: Dietary Supplements. medlineplus.gov
- NIH National Center for Complementary and Integrative Health: Energy Drinks. www.nccih.nih.gov
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational and educational purposes only and is not medical advice. Dietary supplements are not a substitute for a balanced diet, training, or sleep. Talk with a qualified healthcare professional before starting any supplement, especially if you are pregnant, nursing, under 18, taking medication, or managing a health condition.