Citrulline
Citrulline is an amino acid the body makes in the gut and turns into arginine in the kidneys. It is not used to build protein. Watermelon is the richest food, and supplements sell it as L-citrulline or citrulline malate.
8 gof citrulline malate, the single dose most tested before exercise
~5.3 g of that is citrulline, in a 2:1 product
Daily reference · none: the body makes its own
- Main source in food Watermelon then other melons and cucumber
- Made in The small intestine mostly from glutamine
- Turned into Arginine in the kidneys
- Evidence Mixed small exercise studies
Studied doses from the NIH Office of Dietary Supplements review of exercise supplements. Not a recommended amount.
Foods With Citrulline
Citrulline is not part of protein, so high-protein foods do not supply it. It is found free in a few plants, almost all in the gourd family.
- Watermelonthe richest known food; the amount varies a lot by variety
- Other melons, cucumber and gourdssmaller amounts
- SupplementsL-citrulline or citrulline malate powders and capsules give gram doses that a normal serving of fruit does not
What Is Citrulline?
Citrulline was first isolated from watermelon juice and takes its name from the plant's Latin name, Citrullus. It is an amino acid, but not one the body builds into protein. Instead it is a go-between that the body passes from one organ to another.
How Citrulline Moves Between the Gut, Kidneys, and Blood Vessels
Cells lining the small intestine make citrulline, mostly from glutamine, and release it into the blood. The kidneys take it up and turn it into arginine. Cells lining blood vessels then use arginine to make nitric oxide, which relaxes the vessel wall, and citrulline is left over again.
The liver uses citrulline for a different job. It is one step in the urea cycle, the chain of reactions that turns toxic ammonia from protein breakdown into urea for the kidneys to remove.
Why Citrulline Raises Arginine Better Than Arginine
Swallowing arginine sounds like the direct route to more nitric oxide, but much of it is broken down by enzymes in the gut wall and liver before it reaches the rest of the body. Citrulline mostly escapes that first pass. In a study that compared the two, citrulline raised blood arginine more effectively than arginine itself.
What Exercise Studies Show About Citrulline
Most trials are small, short and use young men who train. Some found more repetitions in weight training or less muscle soreness after 8 g of citrulline malate taken an hour before exercise; others found no difference from a placebo. The NIH Office of Dietary Supplements considers the research too limited to say whether it helps.
A slice of watermelon is not a substitute for those doses. A usual serving gives less citrulline than the grams used in most studies, and the amount varies widely between varieties.
Common Citrulline Mistakes
Treating the label weight as citrulline. In citrulline malate, part of every gram is malic acid. See the table below.
Confusing it with citrus. Despite the name, oranges and lemons are not a source. The name comes from the watermelon genus.
Citrulline Supplement Forms
Supplements are the main source of citrulline in the amounts studied. The form decides how much citrulline a scoop holds.
| Form | Citrulline in 1 g | Citrulline in 8 g | Note |
|---|---|---|---|
| L-citrulline | 1,000mg | 8,000mg | Citrulline alone; the form used in the studies on blood arginine |
| Citrulline malate 2:1 | ~670mg | ~5,300mg | The most common ratio in sports products and in the exercise studies |
| Citrulline malate 1:1 | ~500mg | ~4,000mg | Half the weight is malic acid; check which ratio the label states |
Citrulline Side Effects
Studies lasting a few weeks report few problems, mostly stomach discomfort. Longer use has not been studied.
Citrulline and Medicines
Citrulline feeds the body's nitric oxide, which widens blood vessels. If you take medicine for blood pressure, chest pain or erections, ask a doctor or pharmacist first.
Sources: NIH Office of Dietary Supplements, standard biochemistry texts. Not medical advice.
Tracking Citrulline in Caltide
- Not totaled yetCaltide does not add up citrulline yet. Watermelon and every other food you log still count toward calories and the nutrients listed in the food library.
Citrulline Sources and References
The figures and claims on this page are checked against these sources.
Questions About Citrulline
The kidneys turn it into arginine, which the body uses to make nitric oxide, a signal that relaxes blood vessels. In the liver it is also a step in the urea cycle, which clears ammonia.
Watermelon, the fruit it is named after. Other melons and cucumbers, from the same plant family, have smaller amounts. Most other foods have little, because protein does not contain citrulline.
Swallowed arginine is largely broken down in the gut and liver before it reaches the blood. Citrulline passes through and is turned into arginine in the kidneys, so in a study that compared the two, citrulline raised blood arginine more effectively than arginine did.
Citrulline malate is citrulline bound to malic acid, often in a 2:1 ratio by weight, so 8 g of it holds about 5.3 g of citrulline. L-citrulline is citrulline alone. Read the label for the citrulline content.
The evidence is mixed. Some small studies of citrulline malate found more repetitions or less soreness, while others found no effect, and the NIH Office of Dietary Supplements rates the evidence as limited.
No. The small intestine makes citrulline from glutamine and other amino acids, so there is no daily requirement for it.
Short studies of a few weeks report few side effects, mostly an upset stomach. Long-term safety has not been studied, and anyone taking medicine for blood pressure or erections should ask a doctor first.
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