The Scary Reason Why Pregnant Women Should Avoid Fish

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Why Pregnant Women Should Avoid Tuna

All fish contain small amounts of methylmercury, the most toxic form of mercury, and “fish consumption represents the main source of methylmercury exposure.” In my videos Nerves of Mercury and Fish Fog, I discuss how mercury exposure through fish consumption, even within the government’s safety limits, can have adverse neurological and behavioral effects on child development. Severe exposure can cause overt structural brain abnormalities like microcephaly, a shrunken brain disorder. But we didn’t know whether low exposure could also affect brain size until recently.

Autopsy studies suggest mercury preferentially affects the developing cerebellum, so researchers used ultrasound to measure cerebellum size in newborns of mothers who had high body levels of mercury.

Let’s put those levels into practical terms. In the video, Fish Intake Associated with Brain Shrinkage, you can see the results of a study measuring mercury concentration in human hair. Just one can of tuna a week raises human hair mercury concentration to levels nearly three times as high as the “high” group in the ultrasound study. So the bodies of the women suffering high mercury contamination were considered heavily contaminated, but even just a little canned tuna once in a while could bump our levels even higher. So the high really wasn’t that high. But what did they find?

The researchers demonstrated that babies born to mothers with higher hair mercury levels had cerebellums up to 14% shorter than those born to mothers with lower hair mercury levels. They conclude that prenatal exposure to what may be considered low-levels of methylmercury does indeed influence fetal brain development as evidenced “by decreased size of a newborn’s brain.”

But what about the long chain omega-3 DHA in fish—isn’t that necessary for healthy brain development? That’s the topic of my video Mercury vs. Omega-3s for Brain Development.

For more on canned tuna in particular, check out:

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my videos for free by clicking here and watch my full 2012 – 2015 presentations Uprooting the Leading Causes of Death, More than an Apple a Day, From Table to Able, and Food as Medicine.

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Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous "meat defamation" trial.


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