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Prenatal mercury exposure linked with ADHD

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A new study suggests exposure to even low levels of mercury while pregnant can increase the risk of ADHD symptoms in children.

But, conversely, children of mothers who ate more fish– which is considered to be the main source of mercury exposure for many people –during pregnancy appeared to have a lower risk of exhibiting ADHD symptoms, highlighting the confusion many mothers-to-be face in planning their diets.

Researchers from Brigham and Women’s Hospital took hair samples from mothers shortly after giving birth and analyzed them for mercury.  They also gave the mothers a questionnaire to determine fish consumption during pregnancy.

Eight years later, the researchers administered standardized tests to the children to find whether they exhibited any ADHD-related behaviors.  The analysis included approximately 400 children in New Bedford, Mass., born between 1993 and 1998.

The results indicated low levels of prenatal mercury exposure could drastically increase a child’s risk of ADHD, according to researchers.

“A hair-mercury level in the mother of about 1 part per million (ppm) or more was associated with a 60 percent increased risk of ADHD-related behavior,” senior study author Dr. Susan Korrick, an assistant professor of medicine at Brigham and Women’s Hospital and Harvard Medical School, told FoxNews.com.

Additionally, the risk increased in accordance with the mother’s level of mercury – meaning, the higher the level of mercury measured in the mother’s hair, the more likely the child was to exhibit ADHD symptoms – including trouble concentrating or an inability to sit still, Korrick added.

However, the researchers clarified, though certain types of fish, including fresh tuna, shark, mackerel and swordfish, typically have high levels of mercury -- the study results don’t necessarily mean pregnant women should avoid eating seafood entirely.  

In fact, the study indicated eating other types of fish may actually decrease the risk of ADHD symptoms in children.  In some cases, children of mothers who ate more than two servings of fish a week were less likely to exhibit ADHD behaviors.

This amount exceeds the guidelines of the U.S. Food and Drug Administration (FDA), which recommend that pregnant women only consume up to 12 ounces (approximately two servings) of fish low in mercury per week.  

Korrick explained that this is because fish such as shrimp, canned light tuna, salmon, pollock and catfish have such low levels of mercury that it is unlikely that mothers would accumulate a ‘body burden’ of 1 ppm.

“It may have been due to the nutritional value of fish essentially having a more prominent, beneficial effect than the adverse effect of the low levels of mercury in the fish,” Korrick said.  “…The message is, it’s a good thing to eat fish while pregnant but mothers should be focused on eating fish that’s low in mercury.”

Pregnant mothers should talk to their doctors before making any adjustments in their diet.

The study was published Monday in the journal Archives of Pediatrics & Adolescent Medicine.