Depression can be a frustrating illness, as sufferers often have to try numerous types of medication before finding one that works – if any work for them at all, that is. There could be new hope, however, in the form of an existing off-the-shelf product.

Led by Stanford University's Dr. Natalie Rasgon and Rockefeller University's Dr. Bruce McEwen, scientists analyzed blood samples from 71 men and women aged 20 to 70, who had been diagnosed with moderate to severe depression. When compared to 45 demographically-matched healthy individuals, it was found that these people had significantly lower blood levels of a naturally-occurring substance known as acetyl-L-carnitine – the more severe their depression, the lower the levels were.

In previous studies conducted by Rockefeller's Dr. Carla Nasca, depression-like symptoms in rodents were also found to go hand-in-hand with lowered levels of the chemical. When acetyl-L-carnitine was orally or intravenously administered to the animals, however, those symptoms were eliminated within a matter of days. By contrast, traditional antidepressants typically take two to four weeks to have an effect in both people and rodents, and that effect is often negligible.

These studies suggested that in the hippocampus and frontal cortex of the animals' brains, the substance was reducing depressive symptoms by preventing excessive firing of excitatory nerve cells. The new study, which was also initiated by Nasca, indicates that the same may be true of human brains – and acetyl-L-carnitine is already commercially available as a nutritional supplement.

That said, Rasgon warns that people suffering from depression shouldn't start tossing the stuff back.

"We've identified an important new biomarker of major depression disorder," she says. "We didn't test whether supplementing with that substance could actually improve patients' symptoms. What's the appropriate dose, frequency, duration? We need to answer many questions before proceeding with recommendations, yet. This is the first step toward developing that knowledge, which will require large-scale, carefully controlled clinical trials."

A paper on the research was recently published in the journal Proceedings of the National Academy of Sciences.