People with major depressive disorder (MDD) have lower levels of naturally occurring arginine in their body than their non-depressed controls, a study has found.

Arginine, an amino acid which the body uses to produce nitric oxide is a nervous system and immune defence mediator, which also plays a role in vascular regulation. Reduced arginine bioavailability is also known to be an independent risk factor of cardiovascular diseases, the researchers noted. "It is possible that depression-induced inflammatory responses lead to reduced arginine levels. This may result in insufficient production of nitric oxide for the needs of the nervous system and circulation. However, we don't know yet what exactly causes reduced arginine bioavailability in people with depression," said lead author Toni Ali-Sisto, researcher at the University of Eastern Finland.

"Although our study shows that people with depression have reduced arginine bioavailability, this doesn't mean that taking an arginine supplement would protect against depression. That's an area for further research," Ali-Sisto said. The study, published in journal Affective Disorders, involved 99 adults with diagnosed major depressive disorder and 253 non-depressed controls. The concentrations of three amino acids, namely arginine, citrulline and ornithine, were analysed from their fasting glucose samples, and this data was used to calculate their global arginine bioavailability ratio (GABR).

The GABR is an indicator of the body's arginine levels, and the ratio has previously been used to measure the body's capacity to produce nitric oxide. The results showed that people with depression had weaker arginine bioavailability than their non-depressed controls. However, in people who had recovered from depression the arginine bioavailability was found to be slightly higher than in people who remained depressed, the researchers said.