The adverse effects of mercury on brain development and the nervous system have been thoroughly documented. Now, a new study reveals that children exposed to low levels of mercury in the womb are at higher risks of developing attention deficit hyperactivity disorder (ADHD) -related behaviours.

However, in the same study, researchers reported that fish consumption during pregnancy may actually protect the child from developing ADHD behaviours. The study, published in the journal Archives of Pediatrics & Adolescent Medicine, was led by Susan Korrick of Brigham and Women’s Hospital in Boston and Sharon Sagiv from Boston University School of Public Health, America.

‘One explanation for the inconsistency is that nutrients in fish that promote neurodevelopment, such as omega-3 fatty acids, may offset the neurotoxicant effects of mercury exposure’, Dr Sagiv observed. Another probable reason is that there are a wide variety of fish that are low in mercury — like salmon, Pollock, trout, catfish and herring — but rich in nutrients that support brain and nervous system development.

ADHD is one of the most widespread neurodevelopmental disorders of childhood affecting 8 to 12 percent of children worldwide. Characteristics of ADHD include inattentive, hyperactive and impulsive behaviours. Mood swings, sleep disorders, hostility and anxiety are also common features of this condition whose cause is not well understood.

Study details

The scientists examined data from the New Bedford birth cohort, a group of infants born between 1993 and 1998. Hair samples of 421 mothers were collected shortly after delivery and 515 mothers filled in a questionnaire aiming to assess their fish intake during pregnancy. The association between maternal hair mercury levels or maternal report of pregnancy fish consumption and ADHD-related behaviours at age eight were investigated. To evaluate behaviours attributed to ADHD, the children were requested to complete a standardised neuropsychological test. A teacher rating scale for these behaviours was also assessed. This study did not differentiate between types of fish to eat and those to avoid during pregnancy.

The scientists reported that children of women, whose hair sample contained mercury levels of 1 microgram per gram, had a 40 to 70 percent higher rate of inattention and hyperactivity compared to those whose mothers had lower levels of mercury.

‘It would be an unfortunate public health message if people stopped eating fish because of the concern over mercury’, commented Dr Korrick since the researchers also found that children of women who ate more than two servings of fish per week were 60 percent less likely to develop ADHD than kids whose mothers consumed less fish.

Take home message

‘Understanding and appreciating that mercury avoidance is a prudent thing to do during pregnancy is important, but understanding and appreciating that fish consumption is important is worth remembering as well. Eating fish isn’t equivalent to getting exposed to mercury’, said Dr Korrick.

Based on previous studies, pregnant women and those planning to conceive should avoid shark, marlin, swordfish, king mackerel and tile fish and should not eat more than four medium-sized cans or two fresh tuna steaks per week.