Everybody knows to avoid tuna when pregnant, right? Not so fast. Eating tuna might actually yield better results, says a large new study.

Eating ocean fish is good for you, but some fish have significant levels of methylmercury which is bad for you, so you should avoid those fish, right? Wrong, says a new study.

Mothers who ate seafood, even when it contained high levels of methyl mercury, had smarter kids than those who didnʻt eat seafood, says the comprehensive, peer-reviewed study.  

"Moderate and consistent evidence indicates that consumption of a wide range of amounts and types of commercially available seafood during pregnancy is associated with improved neurocognitive development of offspring as compared to eating no seafood," it said.

This flies in the face of conventional wisdom, and some medical wisdom. Both the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration recommend against pregnant women eating ahi, over concerns about methyl mercury exposure.

There is no question that thereʻs methylmercury in yellowfin, bigeye and bluefin tuna, and that the amount has been increasing in recent years. There are also significant amounts of mercury in blue marlin and other species. 

The Hawai`i Department of Health warns against pregnant women eating any blue marlin, swordfish and shark and recommends severe limits on consumption of tunas. 

Yet the new study suggests women who eat some ocean fish, even when mercury levels are high, actually have kids who have better mental outcomes. The authors wrote: " No net adverse neurocognitive outcomes were reported among offspring at the highest ranges of seafood intakes despite associated increases in mercury exposures."

The paper is entitled, "Relationships between seafood consumption during pregnancy and childhood and neurocognitive development: Two systematic reviews." It is published in the journal, Prostaglandins, Leukotrienes and Essential Fatty Acids. Its authors come from some of the most prestigious medical and scholarly institutions in three countries, including the Center for Food Safety and Applied Nutrition, American Society for Nutrition, National Institutes of Health and others.

So whatʻs going on? The authors say thereʻs something in seafood that counteracts the impacts of mercury, and makes it even healthier for kids to eat seafood than not to eat it.

Here is the technical way they say that: "This evaluation of seafood consumption inherently integrates any adverse effects from neurotoxicants, and benefits to neurocognition from omega-3 fats, as well as other nutrients critical to optimal neurological development."

Even small amounts of seafood have a beneficial effect, and the study found no downside to large amounts: 

"Benefits to neurocognitive development began at the lowest amounts of seafood consumed in pregnancy (∼4 oz/wk) and up to >100 oz/wk, with benefits to age appropriate measures of neurocognitive development including an average increase of 7.7 IQ points, in evaluating 44 publications reporting on 102, 944 mother-offspring pairs, no adverse effects on neurocognitive development were found."

It is not that the mothers and children arenʻt exposed to methyl mercury. They are, but there appear to be no negative impacts from that exposure from seafood, the paper says: "No net adverse neurocognitive outcomes were reported in offspring at the highest ranges of seafood intakes despite associated increases in mercury exposures."

The authors are aware that this is controversial stuff, and they urge the scientific community to do more research. There needs to be work, they say, that follows the children into older age, research into whether fatty or oily fish like tuna are healthier than white-fleshed fish, on making sure the IQ tests in studies are comparable, and research on differences based on species of fish and of how it is prepared.

But how is it possible that mercury exposure in kids is dangerous, except when it comes from fish? 

The authors of this paper donʻt say in the publication, but others have suggested that seafood contains something else that protects against mercury- namely, selenium.

This study from 2010 argues that selenium protects against mercury poisoning, and it cites studies indicating selenium can actually reverse some of the effects of methylmercury toxicity. 

"Studies of populations exposed to MeHg (methyl mercury) by eating Se-(selenium) rich ocean fish observe improved child IQs instead of harm."

Tuna and most billfish tend to have high levels of selenium, which may help explain things. Hereʻs a useful report from NOAA and other agencies.

© Jan TenBruggencate 2019