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“A number of other reviews investigating the impact of omega-3 fatty acids on depression and depressive disorders have also been conducted, and all of these also find discrepancies between studies, and inconsistencies in findings, and essentially find it difficult to draw convincing conclusions,” said lead author Katherine Appleton of the University of Bournemouth in the U.K.

“All reviews also conclude with the need for further evidence,” she said.

The new review included 26 randomized controlled trials involving almost 1,500 adults with major depressive disorder in total. The trials compared people who were given omega-3 fatty acid capsules and people given placebo pills.

Only one study compared omega-3 capsules to an antidepressant medication.

While people who took the omega-3 capsules did report lower levels of depression symptoms compared to the placebo group, there was only a small difference that would likely not be meaningful for most people, the authors write in the Cochrane Library.

Side effects and adverse events may have increased for some people taking omega-3 capsules, but the results varied widely from study to study. The quality of evidence from each study was low to very low, in the authors’ rating.

In the one study including antidepressants, there was no difference between omega-3 fatty acid supplements and antidepressants for depressive symptoms.

The results are “promising, but not conclusive,” said Giuseppe Grosso, a nutritional epidemiologist at the University of Catania in Italy, who was not part of the review.

It’s not clear why, or how, the omega-3 fatty acids naturally found in fish oils would improve depressive symptoms, although inflammation and cell communication changes, among other pathways, have been suggested, Appleton told Reuters Health by email.

“One of the problems with understanding results from clinical trials in general is that they are looking for an average effect, while many experts agree that the ideal approach would be to figure out which subsets of depressed patients could benefit from particular treatments, an approach the (National Institutes of Health) has called 'personalized medicine',” said Dr. Elizabeth Sublette of Columbia University in New York, who was not part of the new review.

“At this stage we don't yet know how to predict which depressed patients will respond to omega-3 supplements,” Sublette told Reuters Health by email. “They remain one potential treatment that may be worth trying, and I would encourage both doctors and patients to keep an open mind about this topic until more evidence of higher quality can be obtained.”

Most omega-3 supplements are composed of natural oils, so there is no evidence to suggest the results of the trials would have been different if the omega-3 acids came from eating fish rather than taking capsules, she said.

A bottle of 50 to 100 capsules of omega-3 fish oil supplement costs between $10 and $20 at most drugstores.

More research is needed to assess the potential positive and negative effects of using these supplements to treat major depressive disorder, Appleton and her colleagues write.

“Many possible treatments for depression are currently being investigated, but the evidence for many of these treatments is still incomplete,” she said.

SOURCE: bit.ly/1HFLp0f The Cochrane Library, online November 5, 2015.