There are many hypothesis that omega 3 fat acids are important for the development of depression. There are also studies showing that the levels of the two most important omega 3 fat acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) decreases in red blood cells with depression.
Now there are a number of randomized, double-blind studies that suggest that omega 3 could also have some effect in the treatment of depression, at least for some patients. The studies are consistently small, often with only 20 to 30 patients, and have varying patient populations and treatments which differ widely in terms of the balance of EPA and DHA.
In six of the eight studies in the treatment of depression with omega-3 has been shown more effective than by using placebo. There have also been occasional positive studies in bipolar disorder and perinatal depression. But there are also a lot of studies that did not show any effect at all, but no studies have been showing that the placebo is better than omega 3, which could possibly indicate a small and elusive treatment effect.
It is also proposed mechanisms for how omega 3 fat acids could be important for the development of depression. Omega 3 is a building block of all cell membranes and is important to the receptors and cell permeability of various molecules, and it also has anti-inflammatory properties.