Researchers say omega-3 supplements were effective in treating major depression in patients without anxiety disorders. (CBC)

Omega-3 supplements help patients with major depression who do not have anxiety disorders, according to the largest study so far on the issue.

The effect on patients who took omega-3 capsules was comparable to that generally observed with conventional antidepressant treatment, researchers told a news conference Monday in Montreal.

A total of 432 patients were followed over a four-year period during the double-blind study conducted at the Centre hospitalier de l'Université de Montréal , McGill University in Montreal, Laval University in Quebec City and Queen's University in Kingston.

"We used a sample of people who are very much like the type of people who come to be treated in outpatient clinics for depression," said Nancy Frasure-Smith of McGill, who has a PhD in nursing and is one of the lead investigators. "So they had many, many other problems as well as depression."

For eight weeks, half of the participants took three capsules a day of an omega-3 supplement containing fish oil with 1,050 milligrams of eicosapentaenoic acid. The other half received a placebo.

The study found that the supplement helped about half of the sample of patients — those who had depression without anxiety disorders, said Frasure-Smith.

"The hardest-to-treat people are really those with anxiety as well, but there have also been major difficulties in treating people that you'd think would be simpler to treat — the people who simply have major depression," she said. "This is an alternative for them, which is worth exploring with their physicians."

Nancy Frasure-Smith of McGill, one of the lead investigators on the study, says major depression can be difficult to treat. (CBC)

The researchers could not say if food with omega-3, such as fish, would have the same effect, because their test involved only the supplement form.

The researchers recommend that people with major depression discuss the use of omega-3 with their physicians before supplementing their diets with the unsaturated fatty acid.

Some 11 per cent of men and 16 per cent of women suffer from major depression at some point in their lives, the researchers said.

The study was published in the Journal of Clinical Psychiatry.