L-Citrulline Simply Explained

1. What It Is About

L-Citrulline is an amino acid. Amino acids are the small building blocks from which protein is made. Citrulline occurs in high concentration in watermelon [59]. The body can form arginine from it, another amino acid, and from arginine a messenger substance that dilates the blood vessels. In the specialist literature, citrulline is therefore regarded as a precursor of arginine.

Two forms are investigated: pure L-citrulline and citrulline malate, a salt of citrulline and malic acid. When reading the research, one feature stands out: a large proportion of the most frequently cited studies examine not citrulline but arginine, or both substances together. Anyone who wants to know what is known about citrulline therefore reads a considerable part of the work on another substance.

A second point: measurements are made predominantly of blood values and performance measures such as riding time, blood pressure or vascular stiffness, not disease courses. The evaluated collection comprises 83 controlled human studies, 47 observational studies, 25 review articles and 43 animal experiments.

2. What Studies Show

The best investigated effect is what citrulline does in the blood. In a 2008 study, 20 healthy volunteers each took citrulline, arginine or a placebo, that is a sham preparation without active ingredient, for one week 1. Citrulline raised the arginine level in the blood more than arginine itself, and the increase depended on the dose. Several papers explain this by the fact that citrulline is broken down less in the intestine and liver than arginine 9.

Findings also exist for the heart and circulation. In a 2015 study, 10 healthy men took citrulline for seven days. Their mean blood pressure was lower afterwards, and they lasted longer on the bicycle: on average from 589 to 661 seconds 2. In a 2012 study, 20 of 35 heart-failure patients received citrulline for four months. The pumping power of their hearts increased measurably at rest and under load; this did not occur in the control group without citrulline 5. These blood-pressure and endurance findings are indications but not yet confirmed.

In 15 men with an average age of 58 years, arterial stiffness was lower after seven days with citrulline than with placebo, and the breakdown products of the vasodilating messenger substance in the blood rose 3.

In sport the picture is mixed. In a 2016 study, 22 trained men were slightly faster in a 4-kilometre time trial on the bicycle than with placebo and felt less muscularly fatigued afterwards 4. In 41 overweight asthmatics, citrulline and arginine values in the blood rose under citrulline, and a measure of inflammation in the exhaled air increased; the study lasted two weeks and had no control group 8.

In 50 patients with non-alcoholic fatty liver, a fat accumulation in the liver, inflammatory markers in the blood had fallen more after three months with citrulline than under placebo 7.

3. What Remains Unclear

Short sport studies examined healthy, trained men. Blood values and performance values were measured, not the course of diseases. Whether altered blood values translate into longer-term benefit is so far only a hypothesis.

A large proportion of the most frequently cited studies examine arginine instead of citrulline. Studies that combine both substances or mix citrulline with other substances do not permit any statement about citrulline alone. Pure citrulline and citrulline malate are not consistently separated in this process.

In sport the findings contradict one another: in one investigation in men and women, citrulline malate brought no advantage in a strength protocol with many repetitions, and muscle soreness was even stronger than under placebo 6. A large summary of studies on amino-acid administration in pregnancy found no influence of these amino acids on birth weight 10.

4. What This Means in Everyday Life

Citrulline is not a substance about which one can say in a few sentences what it does. There are indications that it reliably raises the arginine level in the blood, and in short investigations blood pressure, vascular stiffness or endurance performance changed in some people. These investigations usually comprise only a few to several dozen participants.

Anyone who deals with the topic should separate two things: what was measured, and what of it is noticeable in everyday life? A measured blood value initially says something about that blood value; whether complaints improve as a result is an open question. It is also worth looking into the study itself, because part of the work examines arginine and only co-measures citrulline.

The next point to clarify is whether the changes in blood and vascular values are confirmed in longer studies with disease courses and whether citrulline malate and pure citrulline act differently.

All details with study numbers and sources: detailed page on L-citrulline. For children: L-citrulline for children.