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.
Mercury, a metal that affects the nervous system, is among the latest
suspects to be investigated. And in a new study, researchers report that
children who are exposed to higher levels of mercury in the womb are more
likely to exhibit attention problems, hyperactivity, and other ADHD symptoms
when they're eight years old.
The study included roughly 600 mothers and children from New Bedford,
Mass. The researchers measured prenatal mercury exposure by analyzing samples
of the mothers' hair shortly after birth, and found that a child's risk of ADHD
symptoms increased by 40 percent to 70 percent past a certain exposure
threshold (1 microgram per gram).
The association was seen primarily in boys, which wasn't unexpected,
since previous research has shown that boys seem to respond differently than
girls to chemicals that disrupt the endocrine system. "This study and one
other recent study -- which both implicate prenatal mercury exposure [in] the
development of ADHD -- suggest that the impact of mercury is much greater than
previously recognized," says Bruce P. Lanphear, M.D., a professor of
children's environmental health at Simon Fraser University, in Vancouver,
British Columbia, and the author of an editorial accompanying the new study.
The other recent study, which was conducted in Inuit children in Québec
and published earlier this fall, produced similar results: Children who were
exposed to higher prenatal levels of mercury (as measured by samples of
umbilical-cord blood) were more likely to exhibit ADHD symptoms between the
ages of 8 and 14.
The new study, which appears in the Archives of Pediatrics &
Adolescent Medicine, looks only at ADHD symptoms, rather than official
diagnoses. And it shows only an association, not a cause-and-effect
relationship. That said, it would be difficult to design a mercury-exposure
study more rigorous than this one, since for ethical reasons pregnant women
could not be selectively exposed to high levels of mercury, which is known to
be toxic to the developing fetus, says Elza Vasconcellos, M.D., a pediatric
neurologist at Miami Children's Hospital.
Aside from on-the-job exposure (which is common in mining and certain
types of manufacturing), 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. 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.
How to reconcile these seemingly contradictory findings? One possibility
is that the nutritional benefits of fish may offset the harmful effects of
mercury, the authors suggest. Fish is a good source of omega-3 fatty acids,
healthy fats that are essential to brain development, Vasconcellos says.
"It's possible to eat fish low in mercury and high in nutritional
value, and it's possible to eat fish high in mercury and low in nutritional
value," says Susan Korrick, M.D., the senior author of the study and an
assistant professor of medicine at Brigham & Women's Hospital, in Boston.
"What really matters is the kind of fish you're eating."
But Korrick and her coauthors had no information on what type of fish
these women ate. And even if they did, there's no reliable way to estimate
which fish are high in mercury and which aren't. As a general rule, a fish's
mercury content depends on the size of the fish (larger fish tend to contain
more mercury), where it was caught, and how far along it is in its lifespan.
Smaller, oily fishes such as salmon, herring, mackerel, and sardines
tend to be low in mercury and yet high in omega-3s. Shark, swordfish, and fresh
tuna, on the other hand, are more likely to contain high levels of mercury and
a relatively modest amount of omega-3s, though there are exceptions to all of these
rules. The potential link between mercury and ADHD will need to be explored
further in future studies. In the meantime, Lanphear says, avoiding
mercury-laden fish is only a short-term precaution. A more permanent solution
to the contamination problem, he says, will require societies to reduce mercury
emissions across the board.
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