Citrulline is a non-essential amino acid that is definitely not abundant in our daily diet, but plays a particularly important role in many biological processes. Although not a component of most proteins in the human body, citrulline can be found in hair, skin and nerve cells.
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What is citrulline?
L-Citrulline is an amino acid, but unlike other amino acids in the body, it is not used to build structural proteins, such as enzymes or receptors. It was originally isolated from Koga & Odake in 1914 and consolidated in 1930 by Wada. Citrulline gets its name from the fruit from which it was originally derived – watermelon. The genus of the fruit has the Latin name Citrullus, hence Citrulline.
Citrulline, along with arginine and ornithine, are the three amino acids involved in the urea cycle, and as we will note below, there is a direct and significant link between them. The amino acid is also a well-known dietary supplement, mainly in the form of citrulline malate, advertised as improving sports performance.
Food sources and biosynthesis
One of the sources of citrulline is food, but it is practically very difficult to get the doses used in research. The richest food of l-citrulline is watermelon, where the concentration is about 2 mg / g, which means that to get a good amount close to that used in research, we need to eat about 3 kg of fruit. Other foods containing the amino acid are bitter melon, pumpkin, gourds and cucumbers, but the concentration in them is much lower.
The main way we get citrulline is through biosynthesis – production in the body. There are two main places where this happens. Citrulline is obtained as a by-product of chemical reactions in the urea cycle and the nitric oxide cycle. In the nitric oxide cycle, citrulline is obtained directly from the amino acid arginine, by releasing a molecule of nitric oxide from it.
In the urea cycle, through the enzyme arginase, l-arginine is converted to l-ornithine, which subsequently interacts with the enzyme ornithine carbamoyltransferase to form citrulline. A number of other chemical processes followed, after which the already formed citrulline was converted back to arginine.
Assimilation, regulation and metabolism
The absorption of orally administered citrulline occurs in the small intestine through sodium transporters and is extremely effective. Much more effective than the absorption of arginine. About 80% of the digested citrulline is transported to the kidneys, where it is converted to l-arginine, and the rest circulates freely in the blood. The above interactions between the amino acids arginine, ornithine and citrulline create a direct link between their levels in the blood.
Scientific studies have shown a very large increase in the levels of arginine (123%), ornithine (152%) and citrulline (173%) when taking 6 grams of citrulline malate from athletes before training. The regulation of citrulline in the blood depends mainly on it. Citrulline levels in the blood are in the range of 22.4-27μmol / L. As the concentration rises above these levels, a linear conversion of excess citrulline to arginine begins.
Proven and potential health benefits in humans
Citrulline is still poorly understood and has yet to be learned about the various positive or negative health properties. At this stage, the health benefits of citrulline are related to its ability to significantly increase arginine levels, which increases nitric oxide levels. Nitric oxide, in turn, is associated with the relaxation of blood vessels and improved blood circulation, leading to a reduction in blood pressure.
When watermelon extract containing 1350 mg citrulline and 650 mg arginine was taken, a reduction in systolic blood pressure of 7 mmHG was observed, but only in people with prehypertension. No such effect is observed in people with normal blood pressure.
Citrulline is also recommended as a pro-erectile agent, again due to the increase in nitric oxide levels, which also leads to an increase in the cGMP nucleotide. Increased cGMP is one of the main effects of medications such as Viagra.
A study in 24 men with moderate erectile dysfunction, with existing but weak erections, was able to significantly improve erections in 50% of study participants, compared with 8% in the placebo group. The daily dose is 1.5 g of citrulline, divided into 2 doses of 750 mg.
Citrulline and its role in sports
At this stage, citrulline finds a greater role in sports as a dietary supplement in the form of citrulline malate. As the name suggests, citrulline malate is a compound of citruline and malic acid. The various salts and esters of malic acid are called malate.
We presented citruline a little above, and for the little one we will only mention that it is part of the Krebs cycle and plays a role in energy production (ATP), as well as in the metabolism of carbohydrates, fats and proteins.
The compound is advertised as improving the controversial results – increasing energy, strength and endurance. Unfortunately, there are very few scientific studies behind these claims, each with its own shortcomings. It is important to note that studies have been performed on both pure citrulline and citruline malate. Studies using pure citruline have shown conflicting results, with citruline proving ineffective in most cases.
However, studies using citruline malate show very good, positive results, and it is the added malate and its participation in the Krebs cycle that is considered to be the key difference leading to improvement.
Two studies have shown a significant increase in the number of repetitions when performing training protocols for both the upper and lower body, when taking 8 grams of citruline malate from men with training experience of at least one year. Another study in highly trained teenage athletes observed significantly lower lactic acid levels and less fatigue when taking 3 and 6 grams of citruline malate.
Similar results were observed in various studies in rats and mice. As we have noted, the small number of studies still do not allow any firm conclusions to be drawn, but it is clear that citruine malate definitely has potential.
If you follow a training protocol that includes mostly moderate-intensity anaerobic exercise and short breaks between sets (a typical bodybuilding workout), citrulne malate is a supplement worth experimenting with.
Method of administration and possible side effects
To improve blood circulation or relieve erectile dysfunction, you can take a dose of 3 grams of citrulline or 6 grams of citrulne malate daily, divided into 2-3 doses. To improve athletic performance, take 6-8 grams of citrulne malate at a time, about 1 hour before your workout. The intake does not have to be on an empty stomach.
In a study of 8 people to determine the safety of different doses of citrulne, no side effects were observed at doses up to 15 grams. Stomach discomfort was reported by 15% of participants taking 8 grams of citrullie malate.
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