People with severe depression may have particularly low blood levels of an amino acid involved in brain function, a new study suggests.
The substance, called acetyl-L-carnitine (LAC), is naturally produced in the body. It aids in metabolism, and animal research suggests it prevents "excessive firing" of cells in certain parts of the brain.
LAC is also sold as a dietary supplement. It's marketed for conditions ranging from age-related memory loss to diabetic nerve damage.
A number of trials have tested the supplement against depression, too, but with mixed results, said Dr Natalie Rasgon, co-senior researcher on the new study.
The study was published online July 30 in the Proceedings of the National Academy of Sciences.
She and the research team came at it from a different angle. They looked at whether people with depression were, in fact, relatively deficient in LAC.
So they measured blood levels of the amino acid in 28 patients with moderate depression and in 43 with severe cases. Then they compared them with 45 adults who were demographically similar but depression-free.
Not a panacea
Overall, the researchers found, people with depression did have lower LAC levels. And levels were especially low in people with severe depression. The same was true of people who'd been resistant to treatment, and those whose depression arose early in life.
"We are not saying this is the cause of their depression," stressed Rasgon, a professor of psychiatry at Stanford University School of Medicine in California. "This is a correlation.
"We don't want people running out to buy this supplement, thinking this is a panacea and the solution to their problem," Rasgon said.
Instead, she explained, low LAC levels in the blood could serve as a "marker" of more severe, difficult-to-treat depression. If that turns out to be the case, doctors could potentially use LAC levels in diagnosing depression.
Dr Brian Brennan is a psychiatrist at McLean Hospital in Belmont, Massachusetts, and an assistant professor at Harvard Medical School. Previously, he led a small trial that tested LAC for easing bipolar depression symptoms. It found the supplement was no better than placebo (inactive) capsules.
What's "exciting" about the new findings, Brennan said, is they suggest LAC could help identify people with a more severe subtype of depression.
"In psychiatry, we lack substances that can serve as diagnostic markers," he explained.
Brennan, who was not involved in the study, also stressed it's not proof LAC supplements will help the severely depressed.
"The current evidence is mixed," he noted. "It's possible this could be a target for treatment, but that's a long way off."
The most commonly used antidepressants are selective serotonin reuptake inhibitors (SSRIs). These act on serotonin, a "chemical messenger" in the brain. But the drugs are not effective for everyone with depression.
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