NEW YORK (Reuters Health) - Despite some evidence linking depression with low levels of omega-3 fatty acids, there is currently no convincing evidence that omega-3 fatty acids alone can relieve depression, according to a report published this week.

And there is limited evidence that omega-3 fatty acids given in combination with antidepressant drugs can relieve depression -- but this needs to be confirmed before the routine use of omega-3 fatty acid supplements can be recommended for depression, the report states.

Circumstantial evidence has suggested a link between omega-3 levels and behaviour and mood disorders, such as depression, findings that have attracted widespread attention.

Omega-3 fatty acids, abundant in oily fish, nuts, seeds, and leafy green vegetables, "are involved in chemical messaging in the brain, and help regulate blood vessel activity and aspects of the immune system that affect the central nervous system," according to the report in Drug and Therapeutics Bulletin, a publication of the British Medical Journal Group. Many people take omega-3 supplements (fish oil), which are available over-the-counter.

Investigators pooled the results of published studies that evaluated the effect on depression of omega-3 fatty acid supplements alone or in combination with established antidepressant drugs.

As mentioned, there was little convincing evidence that omega-3 fatty acids alone can alleviate symptoms of depression and only limited evidence to back their use as a supplement to established antidepressant drugs.

In about nine studies that were reviewed, fish oil supplements were generally well tolerated; the most common side effects, "particularly with higher doses, are nausea, fishy belching and loose stools," the report states.

There have been concerns that fish oil supplements may contain environmental toxins, such as dioxins, PCBs, or mercury. Therefore, investigators believe it is important not to exceed the maximum recommended doses.

"Surveys of fish supplements by the Food Standards Agency have found that the levels of mercury they contain do not pose a risk to consumer safety," the report notes.

Moreover, because fish oil supplements contain vitamin A, pregnant women should take only low-dose omega-3 supplements, because of the potentially harmful effects of high levels of vitamin A on the fetus.