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“All fish contain small amounts of [methylmercury], the most toxic form of mercury, [and] fish consumption represents the main source.” We’ve seen that mercury exposure through fish consumption, even within the governmental safety limits, can have adverse neurological and behavioral effects on child development. And, severe exposure can cause overt structural brain abnormalities like microcephaly, which is a shrunken brain disorder. But we didn’t know whether low exposure could also affect brain size, until this new study.

Autopsy studies suggest mercury preferentially affects the developing cerebellum, and so researchers used ultrasound to measure the brain size in newborns of mothers who had high body levels of mercury, compared to a control group of women who had low levels of mercury. Let’s put that into practical terms.

Compared to the low-level control group, here’s where the high-level mercury women were. How much canned tuna consumption is that equivalent to? Here’s what your body mercury burden is if you eat one serving of canned tuna a day—about half a can. Here’s what two cans a week will do to you. And, this is just one can a week.

So, the bodies of the women suffering high mercury contamination in the ultrasound brain study were considered heavily contaminated, but even just a little canned tuna once in a while could bump your levels even higher. So, the high really wasn’t that high. But still, what did they find?

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

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