Oils from fatty fish like salmon could relieve depression
They have found that feeding patients with manic depression oil supplements relieved some of their symptoms.
Experts say the study is limited, but called it "a landmark attempt" which demanded closer scrutiny.
The researchers, led by Andrew Stoll, director of the pharmacology research laboratory at Harvard University's McLean Hospital, studied 30 patients.
Roughly half were given up to seven fish oil supplements a day and half got a placebo.
The patients were also given psychological testing every two weeks over the four-month period of the study.
The supplements contained omega-3 fatty acids, which are naturally found in fatty fish like salmon and cod.
Mr Stoll said the effects were very significant.
Anti-depressants
Omega-3 fatty acids increase levels of the neurotransmitter serotonin in a similar way to anti-depressants like Prozac.
Mr Stoll said it was also thought that the acids replenished the outer part of brain cells which receive chemical signals.
Other research shows that omega-3 fatty acids can have a beneficial effect on a range of medical problems, including heart disease, arthritis, breast cancer and Crohn's disease.
Mr Stoll said that Western diets tended to be low in fish and food containing omega-3 fatty acids, but he said oil supplements could compensate for this.
He added that manic depressives could take the supplements in addition to anti-depressants.
The study is published in the American Medical Association's Archives of General Psychiatry.
In an accompanying editorial, experts from Case Western Reserve University in the USA call the research "a landmark attempt".
Dr Francisco Fernandez said patients were becoming increasingly interested in natural, non-toxic treatments.
He added that the research suggested fish oil supplements could be as effective as mental health drugs.
He called for government-backed research into the area since he believes drug companies are unlikely to fund it because it is unlikely to be a profit-maker.