Fish, mercury, and pregnancy: Good news for seafood lovers

People freak out when they hear mercury is in something and sometimes for good reasons. In vaccinations for example a very small amount of ethyl-mercury WAS used as a preservative in vaccines, people got scared so now it is not used in most vaccines. Methylmercury* however is found in seafood and larger fish in particular (in much, much higher concentrations than in vaccines mind you). They may sound the same, but the methylmercury in fish is far more toxic. That said, it turns out that fish isn’t as toxic as we thought, so all you pregnant women who love fish will be happy to hear this.

The new findings from research provide further evidence that the benefits of fish consumption on prenatal development may offset the risks associated with mercury exposure. In fact, the new study suggests that the nutrients found in fish have properties that protect the brain from the potential toxic effects of the chemical.

Three decades of research have consistently shown that high levels of fish consumption by pregnant mothers – an average of 12 meals per week – do not produce developmental problems in their children. Researchers have previously equated this phenomenon to a kind of biological horse race, with the developmental benefits of nutrients in fish outpacing the possible harmful effects of mercury also found in fish. However, the new research indicates that this relation is far more complex and that compounds present in fish – specifically polyunsaturated fatty acids (PUFA) – may also actively counteract the damage that mercury causes in the brain.

“These findings show no overall association between prenatal exposure to mercury through fish consumption and neurodevelopmental outcomes,” said Edwin van Wijngaarden, Ph.D. and a co-author of the study.

“It is also becoming increasingly clear that the benefits of fish consumption may outweigh, or even mask, any potentially adverse effects of mercury.”

The new study comes as the U.S. Food and Drug Administration and international agencies are in the process of revisiting fish consumption advisories to better reflect the health benefits of nutrients found in fish. The FDA’s current guidance — which recommends that pregnant women limit their consumption of certain fish to twice a week — was established because of the known risk of high level mercury exposure on childhood development.

Mercury is found in the environment as a result of both natural (such as volcanic emissions) and human (e.g. coal plant emissions) activity. Much of it ends up being deposited in the world’s oceans and, as a result, fish harbor the chemical in very small amounts.

This has given rise to concerns that the cumulative impact of prenatal exposure to mercury through fish consumption may have negative health outcomes, despite the fact that that a link between low-level exposure and developmental consequences in children has never been definitively established.

At the same time, fish are rich in a host of beneficial nutrients, including fatty acids, which are essential to brain development, leading to a long-standing exchange among scientists, environmentalists, and policymakers over the risk vs. benefit of fish consumption. This debate has significant consequences for global health, as billions of people across the world rely on fish as their primary source of protein.

The new study is one of the longest and largest population studies of its kind. The Seychelles, a cluster of islands in the Indian Ocean, has proven to be the ideal location to examine the potential health impact of persistent low-level mercury exposure. The nation’s 89,000 residents consume fish at a rate 10 times greater than the populations of the U.S. and Europe.

The researchers found that mercury exposure did not correlate with lower test scores. This finding tracked with the results of previous studies by the group – some of which have followed children in the Seychelles into their 20s – that have also shown no association between fish consumption and subsequent neurological development.

The researchers also measured the PUFA levels present in the pregnant women and found that the children of mothers with higher levels of fatty acids known as omega 3 – the kind found in fish – performed better on certain tests. Another common form of PUFA, called omega 6, comes from other meats and cooking oils and is found in greater abundance in the diets of residents of developed countries.

The fatty acids in fish (omega 3) are known to have anti-inflammatory properties, compared to omega 6, which can promote inflammation (which is one reason some people have a hard time digesting steak, which is typically high in omega 6). One of the mechanisms by which mercury inflicts its damage is through oxidation and inflammation and this has led the researchers to speculate that not only does omega 3 provide more benefit in terms of brain development, but that these compounds may also counteract the negative effects of mercury.

This was reflected in the study’s findings, which showed that the children of mothers with relatively higher levels of omega 6 did poorer on tests designed to measure motor skills.

• Methylmercury and ethylmercury might sound the same to anyone who hasn’t taken chemistry, but they are very different. A good analogy of this would be ethyl alcohol and methyl alcohol, one will get you drunk and the other will kill you. Ethylmercury has a shorter half life in the body so it is excreted much quicker than methylmercury and has been shown to be much safer in extreme quantities. Heck even a large dose of the right kind of mercury injected intravenously won’t kill you, as evidence by the woman who did just that trying to kill herself.

Sources:
Gutiérrez, F., & Leon, L. (2000). Elemental Mercury Embolism to the Lung New England Journal of Medicine, 342 (24), 1791-1791 DOI: 10.1056/NEJM200006153422405

JJ Strain,, Alison J Yeates,, Edwin van Wijngaarden,, Sally W Thurston,, Maria S Mulhern,, Emeir M McSorley,, Gene E Watson,, Tanzy M Love,, Tristram H Smith,, Kelley Yost,, Donald Harrington,, Conrad F Shamlaye,, Juliette Henderson,, Gary J Myers,, & Philip W Davidson (2015). Prenatal exposure to methyl mercury from fish consumption and polyunsaturated fatty acids: associations with child development at 20 mo of age in an observational study in the Republic of Seychelles The American Journal of Clinical Nutrition : 10.3945/​ajcn.114.100503