Twenty-three trials (1,702 participants) were included in the review.
The included trials varied in quality, with 10 achieving a score of 5 (range: 2 to 5). Statistical significance was discussed in the results, but p-values were not available for all outcomes.
ADHD and related disorders (6 trials).
Two double-blind trials reported statistically significant findings resulting from mixed omega-3 and omega-6 (including EPA and DHA) supplementation. The first, involving 29 children with learning difficulties (but not clinically diagnosed ADHD), reported improvements in psychosomatic symptoms and inattention. However, only a modest difference was noted between placebo and treatment groups (-4.6 change from baseline score). The second trial, involving 117 children with Developmental Coordination Disorder, reported statistically significant improvements in reading and spelling age, and on opposition, cognitive problems, hyperactivity, anxiety, perfectionism, social problems, impulsivity and inattention. The magnitude of difference between placebo and treatment groups was small (range: -3.8 to + 6.2 change from baseline scores). Substantial clinical heterogeneity was noted amongst all trials in this group.
Anxiety disorders (2 trials).
One double-blind trial compared omega-3 (EPA and DHA) supplementation with a vegetable oil placebo in 24 men affected by substance abuse (but without clinically diagnosed anxiety disorder). The findings were statistically significant for improvements in tension and anxiety in favour of the treatment group (-2.9 change from baseline score).
Schizophrenia (5 trials).
Two double-blind trials (involving 71 medicated and unmedicated schizophrenic participants) of omega-3 (EPA and DHA) compared with a corn oil placebo reported statistically significant improvements in the Positive and Negative Syndrome Scale (range: -4.1 to -3.6 change from baseline score). Two further trials (155 medicated participants) compared various doses of ethyl ester EPA with a liquid paraffin placebo and found statistically significant improvements in symptom scores, although one trial reported improvements in both study groups.
Mood disorders and depression (10 trials).
The meta-analysis of 9 trials (involving 575 participants) investigating major depressive disorder and bipolar disorder showed that omega-3 fatty acid supplementation is significantly more effective than placebo in treating depression (SMD 0.91, 95% CI: 0.41, 1.42, p<0.001). A high level of heterogeneity was reported (I2=75%). Individual trial results for major depressive disorder, bipolar disorder and post-partum depression were reported in the paper.
A preliminary interpretation of the meta-regression analysis suggested that supplementation with EPA may be more beneficial than DHA in treating mood disorders (p=0.009).