Though some research has suggested that fish oil may fight depression, the evidence from clinical trials is too mixed to draw any conclusions, according to a new research review.

In an analysis of 12 recent clinical trials, British researchers found little evidence that omega-3 polyunsaturated fatty acids (PUFAs) improved participants’ depression.

In general, they report in the American Journal of Clinical Nutrition, the studies were small, short-term and had substantial differences in their methodology that make it hard to draw firm conclusions.

One problem is that the trials included a wide range of patients, according to Dr. Katherine M. Appleton and her colleagues at the University of Bristol. Some studies examined adults with major depression, while others focused on bipolar disorder. Some assessed depression in people with other disorders, such as chronic fatigue syndrome or obsessive-compulsive disorder.

In addition, the treatment type and doses varied widely. In some studies, participants took omega-3 supplements along with their standard therapy, while other studies used only the supplements. One study looked at the effects of eating fish.

“Trial evidence that examines the effects of omega-3 PUFAs on depressed mood is limited and is difficult to summarize and evaluate because of considerable heterogeneity,” Appleton and her colleagues write.

What evidence there is, they conclude, offers “little support” for using fish oil to fight depression.

In theory, omega-3 fats could affect depression symptoms through their action in the brain. Several studies have suggested the fatty acids aid in the function of certain chemical messengers in the brain that are linked to depression.

In addition, some population studies have found that people who regularly eat fish have a relatively lower risk of depression.

However, the promise from studies like these is not always duplicated in clinical trials, where researchers rigorously test a treatment against a placebo, or inactive treatment.

According to Appleton’s team, larger, well-conducted clinical trials of fish oil for depression are still needed.

SOURCE: American Journal of Clinical Nutrition, December 2006.