John: Major depressive disorder is becoming more common and is typically very difficult to treat, making the need to find effective treatments ever more important. In a new Cochrane Review from November 2015, Katherine Appleton, from Bournemouth University in England, and colleagues have brought together the evidence on the effects of omega-3 oils. She tells us more in this Evidence Pod.

Katherine: Major depressive disorder is characterised by high levels of depressed mood or a markedly diminished pleasure or interest in all activities. It causes significant distress or disruption to functioning for an individual, and has a negative impact on both them and on society. We investigated whether n-3 polyunsaturated fatty acids, also known as n-3PUFAs, or omega-3 oils, might be a useful treatment for adults; and have found that the evidence is currently too weak to support their use.
N-3PUFAs are a family of polyunsaturated fatty acids that are important for biological functioning. We get them by eating foods such as fatty fish, some other seafoods, certain nuts and seeds, and certain other animal products depending on the animal’s diet.
Various evidence has suggested that n-3PUFAs may play a role in depressive conditions. Animal studies suggest possible pathways, population-based studies show associations between fish intake and depressive symptomology, clinical studies have found lower levels of n-3PUFAs in people with depressive conditions compared to those without, and some randomized trials have demonstrated benefits compared to placebo. However, these associations or benefits have not been detected in all studies, results have varied across studies and analyses of the differences between studies have suggested more benefits for individuals with greater depressive symptomology at baseline. Therefore, we decided to focus our efforts on studies involving individuals with major depressive disorder to assess whether or not there really is a benefit.
We looked for any randomized controlled trial conducted in adults with major depressive disorder that had compared n-3PUFAs against an alternative such as a placebo, anti-depressant treatment or standard care. Our outcomes of interest were depressive symptomology (measured using a validated rating scale), adverse events, depression response and remission, quality of life and failure to complete the trial.
We found 20 trials in total, that encompassed 26 relevant studies. 25 of the studies compared n-3PUFAs to placebo and had recruited nearly 1500 participants. The other study involved 40 participants who were randomised to n-3PUFAs or antidepressant treatment.
In the studies against placebo, n-3PUFA supplementation resulted in a small to modest benefit for depressive symptomology; but the effect is unlikely to be clinically meaningful, is very imprecise and is based on evidence that we judged to be of very low quality. Even though we had focused the review on a specific patient group, those with major depressive disorder, we still found considerable differences between study findings, which could not be explained by hypotheses based on study population or ongoing treatment for depression. Similar numbers of individuals experienced adverse events, depression response and remission in the intervention and placebo groups; and the effects on quality of life and failure to complete were similar as well, but the evidence for these other outcomes was judged also to be of very low quality.
The one study that compared n-3PUFAs to antidepressant treatment did not detect any differences in depressive symptomology, depression response, or failure to complete, while other outcomes could not analysed. Again though, the evidence here was judged to be of very low quality.
In summary, there currently isn’t enough good quality evidence to determine the effects of n-3PUFAs as a treatment for major depressive disorder. It’s still an area of uncertainty that should be tackled in randomised trials, but these trials need to be large enough to detect clinically meaningful differences and conducted in ways that minimise bias.

John: To read the current version of the Cochrane Review, and to watch for updates if more evidence becomes available, simply go to Cochrane Library dot com and search for 'despression and omega-3'.