Seafood — especially tuna, herring, salmon and sardines — contains omega-3 fatty acids, which prior studies have found counter depression and protect against heart disease.
The federal government has warned pregnant women about eating fish because of mercury contamination, but its recommendations still allow eating up to 12 ounces a week of a variety of species.
The new study is an analysis of 11,721 British women. Researchers found that the more omega-3 fatty acids a woman consumed in seafood during the third trimester, the less likely she was to show signs of major depression at that time and for up to eight months after the birth.
In fact, the rate of depression in the women with the highest intakes was only about half that of women with the lowest intakes, says senior author and psychiatrist Dr. Joseph R. Hibbeln.
Eating fish two or three times a week was typical of the highest-intake group, Hibbeln said. The federal 12-ounce-a-week advisory would allow for two servings of mixed species, he said, and federal data shows very little mercury in salmon, catfish and scallops. He also said fish-oil supplements are supposed to be free of mercury.
The depression study's results persisted after researchers accounted for other factors that influence risk of the illness. Still, more definitive studies should be done before omega-3 fatty acids are recommended to pregnant women to avoid or ease depression, said Hibbeln, chief of the outpatient clinic at the National Institute on Alcohol Abuse and Alcoholism in Bethesda, Md.
The study, which Hibbeln said received no funding from the fishing or nutritional-supplement industries, was released Tuesday at the annual meeting of the American Psychiatric Association.
About 10 percent of pregnant women develop depression severe enough to interfere with their functioning, and the figure jumps to 13 to 15 percent in new mothers. Postpartum depression can interfere with the adjustment to parenthood and the mother's interaction with the baby, which in turn puts the baby at risk for problems with language and physical development.
Researchers have studied the safety of getting pregnant women and nursing mothers to take antidepressants because of potential harm to the fetus and newborn. Results show the risk appears low, although there are few studies about the long-term effects, said Dr. Katherine Wisner of the University of Pittsburgh, who studies depression during and after pregnancy.
The new study shows a strong statistical relationship between low levels of omega-3 fatty acids and depressive symptoms, Wisner said.
Now researchers have to show the fatty acid levels really are responsible for that relationship, and that boosting the levels will combat depression, she said. That would be "a major finding," she said.
The federal government has warned pregnant women to avoid eating shark, swordfish, king mackerel and tilefish because they can contain high levels of mercury, which might harm the developing nervous system of the fetus. Wisner said fish in some areas pose other risks of contamination, and that women should follow local recommendations about eating them.
So why would omega-3 fatty acids reduce the risk of depression? Because they are key building blocks of the brain, Hibbeln said, and studies show that low levels in the diet are associated with low levels of a brain chemical called serotonin. Low levels of serotonin, in turn, are tied to depression.
Pregnant women could be particularly vulnerable to low levels of omega-3 fatty acids because the developing fetus draws on supplies stored in the mother's body, Hibbeln said. "If mothers don't eat enough, they're likely to become depleted," he said.