The evidence has been building for decades.

People who live with depression have low blood levels of a specific molecule, new medical research has revealed. It's called acetyl-L-carnitine, and those with particularly severe, treatment-resistant or childhood onset depression were found to have the lowest levels.

Naturally produced by the body, acetyl-L-carnitine plays a crucial role in metabolising fat and the production of energy. It's also widely available as a dietary supplement - not some strange and esoteric thing.

Now researchers from multiple institutions have found a link to depression, noticing a clear correlation between the condition and noticeably low levels of acetyl-L-carnitine.

In recent years, more and more evidence has been building to suggest this link. Since at least 1991, medical researchers have been aware of acetyl-L-carnitine's potential to treat depression, particularly in geriatric and comorbid patients, with the substance showing greater efficacy than a placebo.

More recently, Carla Nasca of the Rockefeller University led a study on rodents, which found that acetyl-L-carnitine had a fast-acting antidepressant effect on rats, kicking into effect in just a few days, rather than the weeks it takes for drugs like SSRIs.

Now Nasca and colleagues have conducted a study on human patients to see if there's a basis for a similar trial in people.

"As a clinical psychiatrist, I have treated many people with this disorder in my practice," said Stanford University School of Medicine psychiatrist Natalie Rasgon.

"It's the number one reason for absenteeism at work, and one of the leading causes of suicide. Worse, current pharmacological treatments are effective for only about 50 percent of the people for whom they're prescribed. And they have numerous side effects, often decreasing long term compliance."

The research team recruited 71 patients with a diagnosis of depression. These were men and women, aged between 20 and 70. They also recruited 45 demographically matched healthy controls.

The patients had to fill out a detailed questionnaire, undergo a clinical assessment and medical history, and give a blood sample. Of the patients with depression, 28 had moderate depression and 43 had severe depression at the time of the study.

When compared to the age- and sex-matched healthy controls, the patients with depression had substantially lower levels of acetyl-L-carnitine.

Those with the most severe depression had the lowest levels. This included patients whose depression had resisted antidepressant drugs, those with early onset, and those who had experienced childhood abuse, neglect, poverty or violence.

These patients constitute around 25-30 percent of all people suffering depression, and are the most in need of help, the researchers said.

But there are a few steps to be done before acetyl-L-carnitine supplements can be approved as a treatment. In particular, clinical trials on human patients with depression, since, as we know, results from rodent models can't always be replicated in humans.

The researchers also don't know the reason for the correlation, or the effect it has. The rat research suggests that acetyl-L-carnitine plays a role in the brain, preventing the excessive firing of excitatory neurons, but this will need to be explored further as well.

"We've identified an important new biomarker of major depression disorder," Rasgon said.

"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."

And we'll be eagerly awaiting the results of those trials.

Meanwhile, the team's research can be found in the journal PNAS.