The health benefits of eating fish are pretty well-known. A lean source of protein, fish can be a rich source of healthful omega-3 fatty acids and has been shown to benefit heart, eye and brain health.
But for years, pregnant women have been advised to go easy on the fish. The U.S. government advises expecting mothers to eat no more than 12 ounces of seafood like salmon and shrimp per week, and to steer absolutely clear of bigger catch like swordfish and shark. The reason for this caution: concerns that mercury, found in nearly all seafood, could harm their babies' developing brains.
Now, fresh research suggests that advice might have been too restrictive.
Researchers at the University of Bristol in England have found that eating fish accounts for only 7 percent of the mercury in a person's body. "That was much lower than people have assumed," lead researcher Jean Golding tells The Salt. "It really implies that if women are worried about mercury affecting their children, stopping eating fish will not be beneficial."
The study, published in Environmental Health Perspectives, analyzed 103 types of food and drink items consumed by more than 4,000 pregnant women. The team at Bristol sent blood samples from the expecting mothers to the U.S. Centers for Disease Control and Prevention for analysis.
The women had a broad range of mercury levels in their blood, from as little as 0.17 micrograms per liter to as much as 12.8 micrograms per liter.
The researchers concluded that the 103 foods analyzed contributed about 17 percent to these differences seen across the women. The seafood items examined — white fish, oily fish and shellfish — were the biggest dietary contributors to the variation. But herbal teas, alcohol, white rice and fresh fruit were also associated with higher levels of blood mercury.
"Women should realize that any adverse effects there might be of a small amount of mercury is totally counterbalanced by the beneficial effects of eating fish," says Golding, a pediatric and perinatal epidemiologist at the University of Bristol.
The study looked at data from the long-term Children of the 90s study, which has been following mothers who were pregnant in 1991 and '92, and their children. A previous study based on the same group of women and their children showed that eating fish during pregnancy can benefit kids' IQ and eyesight.
Since diet is only one source of mercury — it's also found in some beauty products and medications, as well as in water and air — pregnant women can only do so much to limit their exposure. The bottom line, according to Golding, is that expecting moms should maintain a balanced diet.
And, Golding says, "definitely to include fish in that balance as well as fruits and vegetables." If women want to cut something out, she says, maybe it should be herbal tea rather than fish — "herbal tea is not something that is contributing anything particularly nutritious."
Dr. Margaret Dow, an obstetrician and gynecologist with the Mayo Clinic, says the study's findings didn't surprise her. A 1997 study, she notes, linked moms' fish consumption with slight deficits in their kids' language and memory. But other subsequent studies haven't been able to replicate the results. "The quality of those older data were not very good," Dow says.
Even so, Dow says she'll be sticking with the same nutritional advice she's always given her patients, based on the U.S. government recommendations.
"This is just one study," she says. "I imagine we will probably relax the fish recommendations in the very near future ... but it's difficult to make that jump off of one study."