Only the EPA kind of fish oil moved depression scores
Across 26 trials in 2,160 people taking antidepressants too, omega-3 modestly beat placebo, and nearly all of it came from capsules built on EPA rather than DHA.
What the study found
Depression scores moved with only one of fish oil's two main fats
Which fat sat in the capsule decided the result. Across 26 double-blind trials in 2,160 people, omega-3 modestly beat placebo on depression, and nearly all of it came from capsules built on EPA rather than DHA, the two main omega-3 fats in fish oil. The DHA-heavy ones missed significance.
The caveat · Add-on to antidepressants only
Every trial gave the capsules alongside antidepressants rather than instead of them, so what was measured is an add-on effect. The EPA-heavy figure rests on a smaller set of trials than the overall one, which makes it the top of a range rather than a settled number. A correction to this analysis was published two years afterwards, so its exact figures are worth re-checking. Depression trials also carry a strong placebo response, which pushes small differences around.
What we don't know
Every trial gave the capsules alongside antidepressants rather than instead of them, so what was measured is an add-on effect. The EPA-heavy figure rests on a smaller set of trials than the overall one, which makes it the top of a range rather than a settled number. A correction to this analysis was published two years afterwards, so its exact figures are worth re-checking. Depression trials also carry a strong placebo response, which pushes small differences around.