Scientists have identified a substance in blood as a biomarker for depression, a finding that could open a novel way to a new class of antidepressants that could have no side-effects and faster-acting functions than those in current use.

Depression is a common mental disorder, with more than 300 million people of all ages suffering from the disorder globally, according to the World Health Organization (WHO). The findings showed that people with a particular type of depression have decreased blood levels of the molecule acetyl-L-carnitine (LAC), also widely available as a nutritional supplement in drugstores.

Those with severe or treatment-resistant depression, or whose bouts of depression began earlier in life, have particularly low blood levels of the substance. LAC is a crucial mediator of fat metabolism and energy production throughout the body, plays a special role in the brain, where it works at least in part by preventing the excessive firing of excitatory nerve cells in brain regions called the hippocampus and frontal cortex.

The results are "an exciting addition to our understanding of the mechanisms of depressive illness", said Natalie Rasgon, Professor at the Stanford University. "In patients with depression, something is causing a problem in the mechanisms related to the biology of LAC," said Carla Nasca, from the Rockefeller University in New York City.

"And, surprisingly, the deficiency in LAC is even stronger in patients that don't respond to standard antidepressants," Nasca added. The study, published in the journal PNAS, involved 20 to 70-year-old men and women, diagnosed with depression and, amid episodes of acute depression, and had been admitted for treatment.

The patients' LAC blood levels were found to be substantially lower in both men and women, regardless of age. LAC levels were also lower among those patients reporting a childhood history of abuse, neglect, poverty or exposure to violence. However, Rasgon cautioned against rushing to the store to pick up a bottle of acetyl-L-carnitine and self-medicating for depression.