Amino Acids & Proteins

Citrulline (L-Citrulline)

Also known as: L-citrulline, Citrulline malate, Watermelon extract

The watermelon amino acid that raises nitric oxide — real endurance and pump benefits at 6–8 grams.

By IngredientWiki Editorial Team
Fact-checked by IngredientWiki Editorial Team
Updated 2026-09-26
2 min read

In short

L-citrulline is a non-protein amino acid — famously abundant in watermelon — that the kidneys convert into arginine, which in turn raises nitric oxide production. Nitric oxide dilates blood vessels, improving blood flow and oxygen delivery. This pathway makes citrulline one of the better-evidenced pre-workout ingredients: trials show it reduces fatigue during resistance training, improves aerobic performance, and lowers post-exercise soreness. Crucially, citrulline raises blood arginine more effectively than arginine supplements themselves.

Named after watermelon (Citrullus), its richest food source
Converts to arginine → boosts nitric oxide
Better at raising arginine than arginine supplements
Effective dose: 6–8 g citrulline, or 8–10 g citrulline malate

What is Citrulline (L-Citrulline)?

Citrulline solves a biochemistry problem: arginine supplements are poorly absorbed and rapidly broken down, so they barely raise blood arginine — while citrulline, its precursor, slips through intact and lets the kidneys manufacture arginine steadily. That clever metabolic detour made citrulline the pre-workout industry's nitric-oxide booster of choice, and the trial data largely backs the positioning: real improvements in training volume, endurance, and recovery at gram-level doses. Watermelon even supplies a natural version, though at doses too low to matter for performance.

Where it comes from

Natural sources

Watermelon (especially the rind) — the richest source
Cucumbers and other melons
Produced endogenously in the urea cycle

How it's made

Supplements use fermentation-derived L-citrulline, alone or bound to malic acid (citrulline malate).

What it does in products

Benefits

Resistance-training volume

strong evidence

Trials find citrulline lets lifters complete more reps with less fatigue — the dose-dependent benefit that underpins its pre-workout use.

Aerobic endurance

moderate evidence

Meta-analyses show improved time-trial performance and perceived exertion with citrulline before endurance exercise.

Reduced muscle soreness

moderate evidence

Several trials report less soreness 24–48 hours after training — a consistent secondary benefit.

Blood pressure

emerging evidence

Nitric-oxide boosting modestly lowers blood pressure in some trials of hypertensive adults — a possible off-label benefit, but not a substitute for treatment.

Side effects & safety

Stomach discomfort at high doses (uncommon)Dose-related; splitting doses or using malate form helps.

Generally safe within tested ranges (rare)Trials up to 15 g daily show good tolerability.

Safety considerations

Limited data; concentrated citrulline supplements are best avoided in pregnancy.
Additive with blood-pressure medications and PDE5 inhibitors (sildenafil) — avoid combining.
No other major interactions documented.
Not for: Low blood pressure or concurrent vasodilator therapy

How it's used

In supplements

Performance dose: 6–8 g L-citrulline (or 8–10 g citrulline malate, which is ~56% citrulline) 30–60 minutes before training. Daily use in this range is well tolerated.

Regulatory status

Dietary supplement under DSHEA; L-citrulline is GRAS for food use.
GRAS (generally recognized as safe) for food use.

Citrulline vs arginine: the absorption trick

Supplemental arginine is largely destroyed in the gut and liver before reaching circulation, which is why arginine pills barely move blood arginine. Citrulline bypasses that first-pass destruction, and the kidneys convert it to arginine at a steady rate. The result is a paradox: citrulline supplements raise arginine levels more than arginine supplements do — one of the few cases where the precursor outperforms the product.

Frequently asked questions

Citrulline malate or L-citrulline — which is better?

They are functionally similar; malate adds malic acid (energy-cycle support) and is gentler on some stomachs. Compare gram-for-gram of citrulline: 10 g of 2:1 malate delivers about 5.6 g of citrulline.

When should I take citrulline?

30–60 minutes before training for performance. Effects on blood flow peak within the first hour and last several hours. Daily dosing is fine; consistency matters more than timing for recovery benefits.

Does watermelon juice work as a pre-workout?

A practical serving delivers well under a gram of citrulline — far below the 6–8 g performance dose. Watermelon is a great food, but not a substitute for the supplement at effective levels.

The bottom line

Citrulline is one of the few pre-workout ingredients that survives scrutiny: real, dose-dependent benefits for training volume, endurance, and recovery via the nitric-oxide pathway. At 6–8 grams before training, it earns its place in the stack.

References

Citrulline and exercise performance (meta-analysis). PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=citrulline+exercise+performance+meta-analysis
Citrulline and muscle soreness — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=citrulline+malate+muscle+soreness+randomized
L-citrulline compound summary. PubChem (National Library of Medicine). https://pubchem.ncbi.nlm.nih.gov/compound/9750
Citrulline and blood flow (review). PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=citrulline+nitric+oxide+review