Mike: Hello, I'm Mike Clarke, podcast editor for the Cochrane Library. Among the many Cochrane reviews of possible treatments for depression, the one looking at the effects of omega-3 fatty acids was updated in November 2021. Here's lead author, Katherine Appleton from Bournemouth University in the UK to tell us about the update and the latest evidence.

Katherine: Our review investigates the effects of omega-3 oils for major depressive disorder in adults. Major depressive disorder is characterized by depressed mood or markedly diminished pleasure or interest in all activities, which has lasted at least two consecutive weeks, results in significant distress or impairment in functioning and cannot be attributed to any physical cause. The condition is highly debilitating, for the individual, for those around them and for society, is difficult to treat and has a high rate of recurrence.
Omega-3 oils, or n-3 poly-unsaturated fatty acids (n-3PUFAs) to give them their technical name, have been suggested as a possible treatment for major depressive disorder for some time. This has been based in part on studies that found lower levels of n-3PUFAs in individuals with more depressive symptoms compared to those with less, and some trials using supplementation have demonstrated some positive effects. However, not all studies reveal these benefits, and investigations of the heterogeneity between studies has suggested limited effects in individuals with only mild depressive symptomology, but possible benefit in those with more severe symptoms.
Our review is an opportunity to look into this more thoroughly and this, the first update of our review published in 2015, contains an additional 9 independent studies, from 8 randomised trials. This brings the total to 35 independent studies, from 28 randomized trials.
Our primary outcomes continue to be depressive symptomology and adverse events, with secondary outcomes of depression remission and response, quality of life, and non-completion of studies.
Of the 35 included studies, 34, involving a total of nearly 2000 participants, compared n-3PUFA with placebo, while one study, with 40 participants, investigated the impact of n-3PUFA supplementation compared to antidepressant treatment.
For the placebo comparison, n-3PUFA supplementation resulted in a small-to-modest benefit for depressive symptomology, but the effect is unlikely to be meaningful to individuals with the condition and the certainty of the evidence was very low. We also found no good evidence of a difference between the n-3PUFA and placebo groups in depression remission rates, response rates, quality of life, trial non-completion or adverse effects.
The one study comparing n-3PUFAs with anti-depressants, involving only 40 participants, found no differences between the treatments in depressive symptomology, rates of response, or trial non-completion, but again this result is very uncertain.
In summary, therefore, there is insufficient high-certainty evidence to determine the effects of n-3PUFAs as a treatment for major depressive disorder. Although our primary analyses may suggest a small-to-modest, non-clinically beneficial effect of n-3PUFAs on depressive symptomology, the estimate is imprecise and the certainty of the evidence is very low. Our data may also suggest similar rates of adverse events and trial non-completion for n-3PUFA and placebo, but again the effect estimates are very imprecise. Although six years have passed since our earlier review, our conclusions remain unchanged and more complete evidence is still required to assess the potential positive and negative effects of n-3PUFAs for adults with major depressive disorder.

Mike: If you would like to read this latest version of Katherine's review and watch for its next update, simply go online to Cochrane Library dot com and search 'omega-3 for depression in adults'.