Citrulline

Citrulline is a non-proteinogenic amino acid which in the blood comes almost exclusively from enterocytes (intestinal mucosal cells); it is also formed in the urea cycle of the liver, but is processed further there immediately. Low citrulline is looked at as an indication of a reduced number of functioning intestinal mucosal cells, for example with villous atrophy or inflammation of the small intestine. As a precursor of arginine it also contributes to the formation of nitric oxide in the vessel wall. Measurable in the Metabolome and in Prevent 360.

What citrulline does in the body

Citrulline is not a building block of proteins but an intermediate product. The citrulline in the blood comes almost entirely from the cells of the small intestinal lining: they make it mainly from glutamine. The liver hardly takes it up; instead the kidneys convert it into arginine and release this into the blood. In this way the gut supplies the whole body with arginine by way of the kidneys. Citrulline is also formed in the liver in the urea cycle, but it is processed further immediately there and not released. Citrulline is also produced during the formation of nitric oxide and can be turned back into arginine in the cells. Because almost only functioning small intestinal cells deliver citrulline into the blood, the value reflects how much active small intestinal lining is present.

Low citrulline – what happens in the body?

When fewer functioning small intestinal cells are present, less citrulline reaches the blood and the kidneys can make less arginine. Less arginine is then available for producing nitric oxide, for building protein and for immune cells. Low values occur when the small intestinal villi are wasted, for example with coeliac disease, with inflammation of the small intestine such as Crohn's disease, after gut infections and after surgery in which parts of the small intestine were removed. Little glutamine or protein and severe general illness are also associated with lower values.

High citrulline – what happens in the body?

Because the kidneys convert citrulline into arginine, the value rises when kidney function is impaired: less is processed and it accumulates in the blood. Very high values arise in rare inherited disorders of the urea cycle in which the enzyme that processes citrulline further is missing (citrullinaemia); ammonia then also builds up before the bottleneck and puts strain on the nervous system. Slightly raised values can also occur after meals containing a lot of citrulline.

The interpretation is given in your report.