A new study, the largest on the subject, shows that pregnant women who eat a lot of seafood have smarter babies than moms who don’t. This hasn’t gotten huge play. But it’s a big deal because it undercuts the government’s rationale for limiting seafood intake—the idea that a can of tuna fish in the wrong trimester could wreck your child’s attention span.
The study, published last week in the medical journal the Lancet, included 14,541 babies born in and around Bristol, England, in 1991 and 1992. The authors found that women who ate more than 340 grams of seafood a week (about two-thirds of a pound) had kids with significantly higher IQs than those who ate no fish. This was true even when the women’s socioeconomic status and the overall health of their diet were taken into account—in other words, more fish alone seemed to make the IQ difference.
The 340-gram cutoff was chosen because it’s the limit that the EPA and FDA have set for fish consumption. If they eat more fish than that, the agencies say, women may endanger their fetuses because of methyl mercury contained in the fish. But the authors of the British study concluded that they found “no evidence to lend support to the warnings of the U.S. advisory that pregnant women should limit their seafood consumption.” The government’s warning has had the effect of leading many women not to eat seafood at all. And that, it’s now clear, looks like a big mistake. In the British study, mothers who ate no seafood were 48 percent more likely than the seafood chowhounds to have children with a low verbal IQ at age 8. Their children were also worse behaved and less sociable than the kids of fish-snarfing moms.
The federal warnings about mercury consumption from fish are in large part based on the work of Philippe Grandjean, a Danish toxicologist who has studied the effects of maternal fish consumption on about 1,000 children born in the Faeroe Islands in the late 1980s. The Faeroese mothers ate varying amounts of pilot whale, which contains high levels of methyl mercury. When tested at ages 7 and 14, their children as a group had slower responses on finger-tapping tests and showed higher attention deficits. To be on the safe side, or so they thought, the FDA and EPA went with the Grandjean study rather than a large and multiyear study from the Seychelles islands that earlier contradicted Grandjean. Some of the participants of the Seychelles study had about three times as much mercury in their cord blood as the Faeroese women. Yet their children still ended up smarter and more alert than the ones whose moms did not eat much fish.
A variety of theories have arisen to explain this disparity. While the Seychelles women ate more fish, the Faroese women got much of their mercury bingeing on whale meat, and thus may have inflicted a single high dose of mercury on their fetuses. The Faeroese children-to-be also could have been exposed to more PCBs and other dangerous chemicals, in addition to mercury.
Now, the British study gives us more reason to think that the Seychelles findings, not the Faroese ones, are the ones to go with. When I e-mailed him in at a conference in Europe to ask Grandjean what he made of the new study, he spun out an idea he has developed in some recent papers (such as “Separation of Risks and Benefits of Seafood Intake,” available here) that the studies of maternal fish consumption tend to underestimate both the benefits of omega-3 fatty fish oils and the risks of methyl mercury. Because British women tend to eat cod, flounder, and other relatively low-mercury fish, Grandjean theorized, they may enjoy all the benefits and few of the risks of fish eating. But the same may not apply to American fish-eating moms, who are more likely to get their fish from a can of high-on-the-food-chain (and thus, high-mercury) tuna.
This explanation seems conjectural. Previous studies have shown that pregnant women in Britain have roughly two and a half times more organic mercury in their bloodstreams than pregnant American women do—presumably because of higher fish consumption. The new British study didn’t measure mercury blood levels in the moms or their babies. But in all likelihood, the more fish these British moms ate, the more mercury they got, whether the fish they were eating were particularly high in mercury or not. This tends to erode the Grandjean thesis.
The Environmental Working Group, an organization for Chicken Littles who think even the EPA limits on mercury ingestion are set too high, tried to spin the Lancet study. The British researchers found a boost in IQ of children from seafood nutrients but didn’t “look at how much higher children’s IQs would be if seafood were not polluted,” argued Jane Houlihan, EWG’s vice president for research. This kind of hypothetical is typical of those who have no real argument. It may still be the case that a diet heavy in metal-laden fish, like tuna and shark, is damaging to some fetuses. But such data aren’t part of this study, or the big Seychelles project. The current evidence argues, to the contrary, in favor of fret-free fish throughout pregnancy.