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Prenatal mercury intake linked to ADHD



Attention deficit hyperactivity disorder (ADHD) appears to be on the rise in the United States, and in the search for explanations researchers have begun to scrutinize fetal exposure to a wide range of toxins, including lead, tobacco, pesticides, and chemicals such as PCBs.
ADHD meanwhile affects around ten percent of children worldwide. Symptoms include having a hard time paying attention, daydreaming a lot, problems listening, being easily distracted, forgetting things, needing to be in constant motion, acting and speaking without thinking and interrupting others.

Mercury, and especially methylmercury, the prominent mercury species in fish, is a strong neurotoxin. Most people are most likely to be exposed to mercury by eating fish that have ingested the metal in contaminated waters. This is why the U.S. Food and Drug Administration recommends that pregnant women eat no more than two six-ounce servings of low-mercury fish per week.

The new study:
Led by Susan Korrick, MD, MPH, of Brigham and Women's Hospital (BWH), and Sharon Sagiv, PhD, MPH, of Boston University School of Public Health, the study published  Oct. 8 in the Archives of Pediatric and Adolescent Medicine, shows that fish consumption for pregnant women can increase and decrease the risk of ADHD-related behaviors in children.

The study examined 788 children born in New Bedford, Mass. between 1993 and 1998. About 400 pregnant mothers agreed to give a hair sample to help researchers determine their mercury levels and about 500 pregnant mothers completed a questionnaire which asked questions about their fish consumption. Researchers performed a follow-up eight years later with the children, conducting standardized tests to check if they exhibited behaviors related to ADHD. When looking at cases beyond a certain exposure  threshold (1 microgram per gram of mercury in hair), child's risk of ADHD symptoms increased by 40 percent to 70 percent.

In the study, however, fish consumption was not independently related to ADHD symptoms. In fact, when the authors conducted a second analysis among the same group of mothers and children, they found that the offspring of mothers who reported eating more than two servings of fish per week while pregnant actually had a 60 percent lower risk of ADHD symptoms.

"These findings underscore the difficulties pregnant women face when trying to balance the nutritional benefits of fish intake with the potential detriments of low-level mercury exposure," Dr. Susan Korrick, assistant professor of Medicine Harvard Medical School and associate physician at Brigham and Women's Hospital, said in a press release.

This twofold possibility can occur because several types of fish have low levels of mercury, allowing pregnant women to consume fish without being exposed to a large amount of mercury. Previous studies have even indicated that the benefits from  omega-3 fatty acids resulting from fish consumption outweigh the risks due to mercury contamination.

Not examined in this study were the types of fish that are best for a pregnant woman to eat, however, earlier studies have shown women should stay away from fish high in mercury, such as fresh tuna, swordfish, shark, and king mackerel. Good sources of nutrition can be found in fish with low levels of mercury such as haddock, flounder, and salmon. Appropriate fish consumption can be beneficial both to the mother and baby.

The potential link between mercury and ADHD will need to be explored further in future studies. The new study has some weak points, since it looks only at ADHD symptoms, rather than official diagnoses and it shows only an association, not a cause-and-effect relationship. However, such weakness is not easy to circumvent, since for ethical reasons pregnant women could not be selectively exposed to high levels of mercury.

The original study

Sharon K. Sagiv, Sally W. Thurston, David C. Bellinger, Chitra Amarasiriwardena, Susan A. Korrick, Prenatal Exposure to Mercury and Fish Consumption During Pregnancy and Attention-Deficit/Hyperactivity Disorder–Related Behavior in Children, Arch. Pediatr. Adolesc. Med., Published online October 8, 2012. doi:10.1001/archpediatrics.2012.1286

Related studies

Olivier Boucher, Sandra W. Jacobson, Pierrich Plusquellec, Éric Dewailly, Pierre Ayotte, Nadine Forget-Dubois, Joseph L. Jacobson, Gina Muckle, Prenatal Methylmercury, Postnatal Lead Exposure, and Evidence of Attention Deficit/Hyperactivity Disorder among Inuit Children in Arctic Québec, Environ. Health Perspect., 120 (2012) 1456–1461.  doi: 10.1289/ehp.1204976

Yeni Kim, Soo-Churl Cho, Bung-Nyun Kim, Yun-Chul Hong, Min-Sup Shin, Hee-Jeong Yoo, Jae-Won Kim, Soo-Young Bhang, Association between blood lead levels (< 5 µg/dL) and inattention-hyperactivity and neurocognitive profiles in school-aged Korean children, Sci. Total Environ., 408 (2010) 5737–5743.  doi:10.1016/j.scitotenv.2010.07.070

Mina Ha, Ho-Jang Kwon, Myung-Ho Lim, Young-Koo Jee, Yun-Chul Hong, Jong-Han Leem, June Sakong, Jong-Myun Bae, Soo-Jong Hong, Young-Man Roh, Seong-Joon Jo, Low blood levels of lead and mercury and symptoms of attention deficit hyperactivity in children: A report of the children’s health and environment research (CHEER), NeuroToxicol., 30 (2009) 31–36. doi: 10.1016/j.neuro.2008.11.011

Michael Aschner, Neurotoxic mechanisms of fish-borne methylmercury, Environ.  Toxicol. Pharmacol., 12 (2002) 101 -104. doi: 10.1016/S1382-6689(02)00028-5

Donna R. Palumbo, Christopher Cox, Philip W. Davidson, Gary J. Myers, Anna Choi, Conrad Shamlaye, Jean Sloane-Reeves, Elsa Cernichiari, Thomas W. Clarkson, Association between Prenatal Exposure to Methylmercury and Cognitive Functioning in Seychellois Children: A Reanalysis of the McCarthy Scales of Children’s Ability from the Main Cohort Study, Environ. Res. Section A, 84 (2000)  81-88. doi:10.1006/enrs.2000.4095

Philip W. Davidson, Gary J. Myers, Christopher Cox, Catherine Axtell, Conrad Shamlaye, Jean Sloane-Reeves, Elsa Cernichiari, Larry Needham, Anna Choi, Yining Wang, Maths Berlin, Thomas W. Clarkson, Effects of Prenatal and Postnatal Methylmercury Exposure From Fish Consumption on Neurodevelopment Outcomes at 66 Months of Age in the Seychelles Child Development Study, JAMA, 280/8 1998) 701-707. doi: 10.1001/jama.280.8.701

D. Bellinger, A. Leviton, E. Allred. M. Rabinowitz, Pre- and postnatal lead exposure and behaviour problems in school-aged children, Environ. Res., 66/1 (1994) 12-30. doi: 10.1006/enrs.1994.1041,

Related EVISA Resources

Link database: Mercury exposure through the diet
Link database: Environmental cycling of mercury
Link database: Toxicity of Organo-mercury compounds
Link database: Research projects related to organo-mercury compounds

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last time modified: November 4, 2012


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