The official US advisory warning women to eat less seafood during pregnancy to prevent neurological deficits associated with mercury exposure in the womb could have just the opposite effect — depriving infants of long-chain omega 3 fatty acids and other nutrients in fish that promote healthy brain development, according to researchers in the US and Great Britain.
In 2004, the US FDA cautioned expectant mothers to limit seafood to under 340 grams per week — 12 ounces or around three servings. But a longitudinal outcome study of nearly 9,000 children, published in the Feb. 17 Lancet, revealed that infants born to mothers who consumed more than that amount had more robust cognitive development throughout early childhood, and had higher IQ scores at age eight (2007;369:578–585).
The study involved women in Bristol, England, who were pregnant between April 1991 and December 1992. Dietary questionnaires were sent four times during the pregnancy, and then periodically afterward. Child development was monitored through questionnaires sent to the parents and with IQ testing when the boys and girls were 8-years-old.
Among findings, toddlers of mothers who ate less than 340 grams per week had significantly slower social and behavioral development and less developed fine motor and communication skills than mothers who ate more seafood. By age 8, children of mothers who ate little seafood scored in the lowest 25 percent of verbal IQ.
“The findings suggest that advice to limit seafood consumption could actually be detrimental,” said lead researcher Joseph L. Hibbeln, MD, chief of the Laboratory of Membrane Biophysics and Biochemistry at the National Institute on Alcohol Abuse and Alcoholism at the NIH.
The data were adjusted for 28 different possible confounders and maternal seafood intake was divided into three categories: none, 340 grams per week, or more than that amount. Without adjustment, there was a six-point difference in IQ scores between children of mothers in the highest and lowest categories. When adjusted for the 28 variables, the difference was a statistically significant three to four IQ points, Dr. Hibbeln told Neurology Today in a telephone interview. Although they did not include maternal or paternal IQ scores, the researchers did consider education and job status as potential confounders, as well as ethnicity.
“This was the world's largest and most comprehensive longitudinal study on pregnancy and childhood outcomes. At 12 ounces of seafood during pregnancy, the beneficial effects on cognitive development far outweigh the risks.”
Children born to mothers who ate no fish were 28 percent more likely to have poor communication skills at age 18 months and 35 percent more likely to have poor fine motor coordination by 3.5 years, compared with children of women who ate more than 340 grams; by age 7 they were 44 percent more likely to have poor social skills and 48 percent more likely to have lower verbal IQ scores.
SCIENCE BEHIND GRANDMOTHER'S OLD SAYING'
Seafood, including fish and shellfish, is rich in long-chain omega 3 fatty acids and other nutrients that are very important for neonatal neurological development, observed Gary J. Myers, MD, professor of neurology, pediatrics, and environmental medicine at the University of Rochester Medical Center in New York, who co-authored an accompanying editorial.
“It has been known for years that polyunsaturated fatty acids are essential in the development of healthy brains,” he told Neurology Today in a telephone interview, “but the researchers have put some science behind grandmother's old saying that fish is brain food.”
Nonetheless, he said, it is important to “have an informed and balanced view” of the benefits and potential risks, and to limit seafood to fish “low on the food chain” and from fresh water, where contaminant levels are lowest.
Most major studies of impaired cognition from potential seafood contaminants have been based on epidemiological studies in normal children in communities where the diet is rich in seafood with high mercury levels such as whale meat, Dr. Myers noted.
In a 2004 study, published in The Journal of Pediatrics, researchers at Harvard University School of Public Health and institutions in Japan, Denmark, and the Faroe Islands in Northern Europe, reported that high levels of mercury, primarily from eating whale meat, passed from mothers to child in utero, produced irreversible cognitive impairment affecting different targets in the brain (2004;144:177–183).
The study of more than 800 children showed that the toxic effects of prenatal mercury exposure could occur, even at low levels. The findings were based on auditory electrical signal responses when children were tested at age 14. The teenagers also had impaired neurological regulation of their heart rate, a sign of mercury toxicity in the brain stem. Neuropsychological tests conducted at ages 7 and 14 years revealed similar deficits associated with prenatal mercury exposure.
“But remember that this evidence was in normal children and based on finger-tapping responses to auditory signals,” said Dr. Meyers. ”A one finger-tap difference might be statistically significant, but you have to ask yourself whether it is clinically significant.”
Although the researchers cited the 2004 Faroe study in their paper, Dr. Hibbeln stressed that their research was not designed to evaluate mercury exposure, but to look at maternal seafood consumption and neurocognitive development in infants and young children.
“We simply wanted to test the efficacy of the 2004 FDA guidelines,” he told Neurology Today. “These results show that risks from the loss of nutrients were greater than the risks of harm from exposure to trace contaminants in 340 grams of seafood eaten weekly, especially with regard to verbal development in the older children.”
Philippe Grandjean, MD, adjunct professor of environmental health at Harvard University School of Public Health in Boston, was one of the authors of the 2004 study. In a commentary published in the March 2007 issue of the journal Environmental Health Perspectives, Dr. Grandjean and colleagues argue that unadjusted observational studies evaluating the risks and benefits of dietary seafood during pregnancy and cognitive development in children tend to underestimate the opposite effects of mercury and nutrients.
Mercury-associated [cognitive] deficits increased by up to two-fold, after adjustment, when compared with risk estimates based on observational analysis alone, they wrote in critiquing such studies.
Dr. Grandjean told Neurology Today in a telephone interview that the latest study findings should be viewed with caution.
“Because the data are limited to seafood consumption and did not include assessment of actual mercury exposure levels, the correlations may be misleading,” he said.
The true benefits of seafood diets are likely to be even greater than those reported, he said. “In blood and hair samples, we found that both prenatal and postnatal mercury exposure affects brain function and that they seem to affect different targets in the brain. The adverse effects became greater when we took into account the benefits from fish diets. Even though the children performed within normal ranges, none of us would like to give up IQ points on behalf of our children, so that they will not enjoy the full benefit from seafood nutrients.”
He said that a more accurate analysis would have examined both seafood consumption and mercury exposure in order to “dissociate” the effects of the two.
“If the results [of the current study] had been adjusted for mercury, I think the cognitive gains in infants whose mothers ate high seafood/low mercury diets would have been even greater,” he told Neurology Today.
He also noted that the children in the 2004 Faroe study had average exposure levels that were similar to the current exposure limit set by the US Environmental Protection Agency, and 95 percent of them were below the limit used by the FDA. Even then, mercury caused an adverse effect on the speed of electrical signals in the brain. “The same is probably true in the UK population, but unfortunately the new study is silent on this matter,” he noted.
“It is important that mothers understand that there are significant differences in mercury risk depending on the type and origin of fish or seafood they eat, not just how much seafood. I would let people have all of the benefits of seafood in their diet while minimizing the risk from pollutants.”
ARTICLE IN BRIEF
A new study reported that toddlers of mothers who ate less than 340 grams of seafood per week during pregnancy had significantly slower social and behavioral development and less developed fine motor and communication skills than mothers who ate more seafood. By age 8, children of mothers who ate little seafood scored in the lowest 25 percent of verbal IQ.