NEW YORK (Reuters Health) - Preschoolers whose mothers
regularly ate low-mercury fish during pregnancy may have
sharper minds than their peers, a study suggests.
Researchers found that among 341 3-year-olds, those whose
mothers ate more than two servings of fish per week during
pregnancy generally performed better on tests of verbal, visual
and motor development.
On the other hand, tests scores were lower among
preschoolers whose mothers had relatively high mercury levels
in their blood during pregnancy.
And mothers who regularly ate fish during pregnancy were
more likely to have such mercury levels than non-fish-eaters
were, the researchers report in the American Journal of
The findings add to evidence that fish can be brain-food,
but underscore the importance of choosing lower-mercury fish
"Recommendations for fish consumption during pregnancy
should take into account the nutritional benefits of fish as
well as the potential harms from mercury exposure," write the
researchers, led by Dr. Emily Oken of Harvard Medical School in
Oily fish such as tuna, salmon and sardines contain omega-3
fatty acids, which are important in fetal and child brain
development. The problem is that fatty fish are more likely to
be contaminated with mercury, a metal that is toxic to brain
cells, particularly in fetuses and young children.
Because of this, pregnant women are advised to avoid
certain fish altogether: shark, swordfish, king mackerel and
tilefish. These fish are particularly high in mercury because
they eat other fish and are long-lived, over time accumulating
mercury in their fat tissue.
Less clear is how the benefits of other omega-3-containing
fish stack up against the potential risks. Currently, U.S.
health officials recommend that pregnant women eat no more than
12 ounces, or roughly two servings, of fish per week.
For the current study, Oken's team collected blood samples
from 341 women during their second trimester and asked them how
often they ate various foods, including fish. When their
children were 3 years old, they took standard tests of
vocabulary, visual-spatial skills and fine-motor coordination
of the hands and fingers.
Overall, the researchers found, children whose mothers ate
fish more than twice a week had higher test scores.
However, children whose mothers had mercury levels in the
top 10 percent of the study scored more poorly than those whose
mothers had lower mercury levels.
Only 2 percent of mothers who never ate fish during
pregnancy had blood mercury levels that high, versus 23 percent
of those who ate fish more than twice weekly.
According to Oken's team, the bottom line is that eating
fish lower in mercury could "allow for stronger benefits of
Fish that are high in omega-3 but relatively lower in
mercury include canned light tuna, which has less mercury than
albacore tuna, and smaller oily fish like salmon. White-meat
fish such as cod and haddock tend to be low in mercury, but
have less omega-3 than fattier fish.
The study was funded by the National Institutes of Health
and Harvard. Some of Oken's co-researchers have received
funding from the food and supplement industry.
SOURCE: American Journal of Epidemiology, April 2008.