There is no convincing evidence that taking omega-3 fatty acids on their own can alleviate depression, says Drug and Therapeutics Bulletin (DTB).
And there is only limited evidence that they can relieve depression when used in combination with antidepressant drugs, it adds.
Evidence from circumstantial research has suggested links between omega-3 levels and behaviour and mood disorders, such as depression. And the findings have attracted widespread attention.
Omega-3 fatty acids are polyunsaturated fatty acids, or PUFAs, which include ALA, EPA, and DHA. They 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.
The main dietary sources of EPA and DHA come from oily fish, while ALA is found mainly in nuts, seeds, and leafy green vegetables.
Omega-3 supplements are available over the counter in the form of fish oil.
The DTB review looked at published research on the clinical effectiveness of omega-3 fatty acids, on their own and in combination, as well as overall analyses of these studies (meta-analyses).
It concluded there was little convincing evidence for using the fatty acids alone as a treatment for depression. And it found only limited evidence to back their use as a supplement to antidepressants.
The review found that fish oil supplements are generally well tolerated by people who take them.
But there is evidence that they contain environmental toxins, which may be particularly concentrated in supplements made from fish livers. So it is important not to exceed the maximum recommended doses, says the DTB.
Furthermore, as fish oil supplements contain vitamin A, pregnant women should take only low doses of them, because of the potentially harmful effects of high levels of vitamin A on the developing fetus.