The guidelines for maternal fish intake during pregnancy need rethinking

Fish contains vitamin D, omega-3 fatty acids, selenium, and iodine, all of which are important for the fetus’ brain and development. However, fish also contains mercury and that discourages many pregnant women from eating fish. According to a new study that is published in NeuroToxicology, eating fish during pregnancy is actually not harmful at all, on the contrary, and the scientists behind the study argue that the precautionary guidelines need to be revised. Apparently, the selenium content in fish and seafood determines if mercury is dangerous or not.

Scientists from the University of Bristol looked closer at two studies that included around 1,200 pregnant women. One study was conducted on pregnant women who took part in the “Children of the 90s” study in Great Britain. The women in this study came from the southwestern part of England where fish consumption is comparatively limited. The other study included women from the Seychelles where nearly all pregnant women eat quite a lot of fish. The scientists measured levels of mercury in the women’s blood and in the umbilical cords of the fetuses. They then monitored the children’s development with particular focus on birth weight, IQ, and cognitive skills.
Overall, the scientists found that it doesn’t really make much difference what type of fish pregnant women eat, because it is essential nutrients such as selenium in the fish that protect against mercury. The researchers ended up concluding that maternal fish intake is essential for fetal development no matter what. In other words, the advantages associated with eating fish during pregnancy far outweigh any disadvantages. This message stands in stark contrast to current guidelines for fish intake during pregnancy.

Fish consumption and its many benefits

It has been known for long that children reap numerous health benefits if their mothers have eaten fish during their pregnancies, especially when it comes to vision and cognitive skills. However, health authorities warn pregnant women against eating certain types of fish because they contain mercury. For that reason, many expecting mothers avoid eating fish altogether just to be on the safe side.
According to Dr. Caroline Taylor, who headed the new study, the mother’s exposure to mercury from fish during her pregnancy does not appear to have any negative effects on the child. On the contrary, mercury exposure from other environmental sources harms the child more if the mother does not eat fish. The scientists believe this is because fish contains vitamin D, omega-3 fatty acids (EPA and DHA), iodine, and selenium. All of these are important for fetal development of the brain, nervous system, thyroid function, immune system, and other functions.
The scientists encourage health authorities to change their precautionary warning against eating certain types of fish, simply because this may do more harm than good. Instead, they suggest new guidelines for fish intake that advise pregnant women to eat at least two weekly servings of fish, and one of them should be with oily fish.
The new study is published in NeuroToxicology.

More information about mercury in shellfish, fish, seals, and whales

It is common knowledge that mercury accumulates on its way up through the food chain. For that simple reason, shellfish, herring, anchovies, plaice, pollock, salmon, and mackerel, all of which are in the lower part of the food chain, contain less mercury than fish at the top. Also, the selenium-mercury ratio in the listed fish is more favorable, as the selenium content is higher than the mercury content. This means that there is plenty of selenium in the fish to effectively bind the mercury. However, the selenium that is bound to mercury is no longer free to support the different selenoproteins.
Conversely, large sea animals like whales, marlin, certain types of sharks, and seals contain far more mercury than selenium. A study of pilot whale meat, a common food in the Faroe Islands, showed a negative health effect of eating this type of fish meat because these whales accumulate enormous amounts of mercury.
The British study by Dr. Taylor therefore confirms that the selenium content in most consumed fish is sufficiently high to make it safe for pregnant women to eat them. Still, it is worth mentioning that fish from the upper part of the food chain also accumulate other environmental toxins, and many fish caught in the Baltic Sea have a mercury content that is higher than EU’s official threshold level.
The Danish Veterinary and Food Administration still recommends that pregnant and breastfeeding women avoid eating large predatory fish like tuna. The take-home message in any case is that it is healthy for both the pregnant woman and her child if she eats fish.

• Mercury’s harmful potential is determined by the amount of selenium in the body
• This connection between selenium and mercury has generally been ignored in studies

References:

Jean Golding, Caroline Taylor, Yasmin Iles-Caven, Steven Gregory. The benefits of fish intake: Results concerning prenatal mercury exposure and child outcomes from the ALSPAC prebirth cohort. NeuroToxicology, 2022; 91: 22 DOI: 10.1016/j.neuro.2022.04.01

University of Bristol. Study calls for change in guidance about eating fish during pregnancy. ScienceDaily. 2022

Michael Gochfeld, Joanna Burger. Mercury interactions with selenium and sulfur and the relevance of SE: HG molar ratio to fish consumption advice. Environ Sci Pollut res Int. 2021

Nicholas V.C. Ralston, Laura J. Raymond. Mercury´s neurotoxicity is characterized by its disruption of selenium biochemistry. 2018