What is citrulline?

In one line

Citrulline is an amino acid that the body turns into arginine, which becomes the raw material for nitric oxide, a signal that widens blood vessels.

What is citrulline?
The name you keep seeing on pre-workout supplement labels
On the labels of supplements taken before exercise, the names 'citrulline' or 'citrulline malate' come up often. This post explains what citrulline is, where it travels in the body, and what it does there, step by step.
In one line
A raw material for the signal that widens blood vessels
Citrulline is a substance found in especially large amounts in watermelon. In the body it is turned into another substance called arginine, and arginine is the raw material for a signal that widens blood vessels. That is why it is often added to supplements taken before exercise.
First, what is an amino acid?
The small pieces that make up protein
Amino acids are the small pieces that make up protein. The body builds its proteins by linking 20 kinds of amino acids together in chains. Citrulline is also an amino acid, but it is not one of these 20, so it is not used as a piece for building protein.
Where is it found?
Rich in watermelon, rare in other foods
Citrulline is almost absent from ordinary foods, but watermelon has a lot of it. 1 kg of watermelon contains about 1.6-3.5 g. So even to get the smallest amount used in supplement studies from watermelon alone, you would need to eat about 1-1.5 kg of watermelon a day.
The body makes it too
Made in the small intestine, turned into arginine in the kidneys
The body makes citrulline on its own even if you do not eat it. It is made mainly in the small intestine and released into the blood, and the kidneys take it up and turn it into arginine. So citrulline in the blood is sometimes called 'packaged arginine' that carries arginine to the kidneys.
In the liver it does another job
One step in getting rid of waste through urine
When the body uses protein, a waste product called 'ammonia' is left over. The liver turns ammonia into a substance called 'urea' and sends it out in urine. Citrulline appears briefly in the middle of the liver's urea-making process and is then turned into the next substance.
Arginine and nitric oxide
When arginine becomes nitric oxide, citrulline is left over
In the body, arginine becomes a gas called 'nitric oxide'. Nitric oxide signals the muscle in blood vessel walls to relax, which widens the vessels. What is left after arginine gives off nitric oxide is citrulline, and this citrulline is turned back into arginine and used again.
Why take it instead of arginine?
Swallowed arginine is largely lost in the gut and liver
When you swallow arginine, a large part of it is broken down in the gut and liver. Breaking down means cutting it into smaller pieces and turning it into other substances, so less of it reaches the blood. Citrulline passes through the gut and liver almost untouched. So blood arginine rose more after taking citrulline than after taking arginine itself.
As a supplement
Plain citrulline, or citrulline malate
Supplements come as plain citrulline, or as 'citrulline malate', which is citrulline joined to 'malic acid'. Malic acid is an acid also used to adjust the sour taste of foods. In exercise studies, the most common approach was 8 g of citrulline malate taken about 1 hour before exercise.
Effects studies found
Short, hard efforts went slightly better than with a fake pill
When people took citrulline and then did strength-training sets or all-out efforts of 30 seconds or less, their results were slightly better than with a fake pill. The difference was small. A recent analysis that looked only at citrulline malate also found a small improvement in exercise results.
A common misconception
Is 8 g of citrulline malate the same as 8 g of citrulline?
No. Citrulline malate is citrulline joined to malic acid, so part of the 8 g is the weight of the malic acid. So even at the same 8 g, it holds less citrulline than a powder that is citrulline only.
Summary
An amino acid that becomes arginine and feeds nitric oxide
Citrulline is an amino acid that is rich in watermelon and also made by the body; in the body it turns into arginine and becomes raw material for nitric oxide, which widens blood vessels. Citrulline malate has malic acid attached, so the same weight holds less citrulline. Related terms: arginine, nitric oxide, pre-workout.