Julie Daniels, an epidemiologist at the University of North Carolina at Chapel Hill School of Public Health, has found in a population of English children that consumption of fish by mothers during pregnanacy is positively correlated with cognitive development after controlling for educational levels of the mothers and some other factors.

CHAPEL HILL -- When fish is not contaminated, moderate consumption of the protein-rich food source by pregnant women and young children appears to boost the children’s neurological development, a new study shows.

"Our research adds to the literature suggesting that fish contains nutrients that may enhance early brain development," said Dr. Julie Daniels, assistant professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health. "We can not say that we have proven that eating fish will have long-lasting effects in making people smarter since we have only looked at early development markers through an observational study."

More research is needed to corroborate the findings, Daniels said.

A report on the study appears in the July issue of the journal Epidemiology. Besides Daniels, authors are Drs. Matthew P. Longnecker of the National Institute of Environmental Health Sciences, Andrew S. Rowland of the University of New Mexico’s family and community medicine department and Jean Golding of the University of Bristol Institute of Child Health’s ALSPAC Study Team.

Conducted in Bristol, England, the research involved evaluating the association between mothers’ fish intake during pregnancy and their offspring’s early development of language and communication skills, Daniels said.

The team evaluated 7,421 English children born in 1991 and 1992. They studied the children since much has been learned about contaminants in fish, but little research has been done on the potential developmental benefits of eating fish, she said.

"We measured mothers’ and children’s fish intake by questionnaire," Daniels said. "Later, we assessed each child’s cognitive development using adaptations of the MacArthur Communicative Development Inventory at 15 months and the Denver Developmental Screening Test at 18 months."

Researchers also measured mercury levels in umbilical cord tissue for a subset of 1,054 children.

"We found total mercury concentrations to be low and not associated with neurodevelopment," she said. "Fish intake by mothers during pregnancy, and by infants after birth, was associated with higher mean developmental scores. For example, the adjusted mean MacArthur comprehension score for children whose mothers consumed fish four or more times a week was 72 compared with 68 among those whose mothers did not consume fish. While this may not be a major difference clinically, but the statistically significant results were consistent across related subtests that could be important across a large population."

Scientists found that there was a subtle but consistent link between eating fish during pregnancy and children’s subsequent test scores, even after adjusting for factors such as the age and education of the mother, whether she breastfed and the quality of the home environment.

The largest effect was seen in a test of the children’s understanding of words at age 15 months. Children whose mothers ate fish at least once a week scored 7 percent higher than those whose mothers never ate fish.

A similar pattern, although less marked, was seen in tests measuring social activity and language development. Developmental scores were also higher among children who also ate fish at least once a week before their first birthdays.

The study suggests that if a woman eats moderate quantities of fish -- about two to three servings per week, or 12 ounces, of non-contaminated species -- her child might benefit, the scientist said. There is no evidence that the more fish a woman eats, the higher that benefit would be.

"Women should definitely avoid shark, swordfish, king mackerel and tilefish, according to the U.S. Environmental Protection Agency and Food and Drug Administration," Daniels said. "Those fish are higher on the food chain and have greater accumulation of pollutants."

Depending on the region where they are caught, many of the most commonly eaten fish are low in pollutants while still being high in critical long-chain fatty acids and other nutrients, she said. They include salmon, herring, pollock, canned light tuna and sardines.

Daniels said she is pursuing similar work in a group of U.S. children to confirm the results in other populations.

"We also need to follow the children longer to determine whether any benefits from fish intake are permanent or transient," she said.

Fish intake during pregnancy has the potential to improve fetal development because it is a good source of iron and long chain omega fatty acids, which are necessary for proper development and function of the nervous system, Daniels said. Fish, especially oily fish, is a dietary source of eicosapentaenoic and docosahexaenoic acids (DHA), which are important in the structural and functional development of the brain before birth and through a child’s first year. The concentration of DHA in fetal brain increases rapidly during the last three months in the womb.

The Avon Longitudinal Study of Parents and Children, or ALSPAC, (also known as Children of the 90s) is a continuing research project based at the University of Bristol. It enrolled 14,000 mothers during pregnancy in 1991-2 and has followed the children and parents in minute detail ever since.

Support for the study came from the Medical Research Council, the Wellcome Trust, the Department of Health, the Department of the Environment, DfEE, Nutricia and other companies, all in the United Kingdom.

The US Food and Drug Admnistration has a very handy page of mercury levels in fish. See the chart Mercury Levels in Commercial Fish and Shellfish and follow down the column labelled "MEAN" to see how the various types of fish compare. Women especially would do well to avoid the higher scoring fish. You have to decide for yourself where you want to draw the line. Personally, I do not eat anything above 0.10 PPM and usually choose fish well below even that limit.

Some commentators recommend avoiding fatty fish because more PCBs, DDT, dioxin, and other chemical compounds will be found in the fat. But the big health benefit from fish comes from the omega-3 fatty acids found in fish. So the advice to avoid fatty fish ends up defeating the purpose of eating fish in the first place.

I think a smarter approach is to avoid the types of fish that have been shown to have the most chemical contamination and to eat fish which have omega-3 fatty acids as a high fraction of total fats. That way you can limit the total amount of fish fat you need to consume in order to get the omega-3 fats. I prefer ocean fish to fresh water fish since the chemical pollution has become far more concentrated in lakes and rivers than in the oceans.

Salmon has very low levels of mercury and at the same time wild salmon has lower levels of PCBs than beef. It is also has a relatively good ratio of omega-3 fatty acids to omega-6 fatty acids. However, the Environmental Working Group has found that most farmed salmon has much higher levels of PCBs along with greater amounts of non-omega-3 fats.

Seven of ten farmed salmon purchased at grocery stores in Washington DC, San Francisco, and Portland, Oregon were contaminated with polychlorinated biphenyls (PCBs) at levels that raise health concerns, according to independent laboratory tests commissioned by Environmental Working Group.

These first-ever tests of farmed salmon from U.S. grocery stores show that farmed salmon are likely the most PCB-contaminated protein source in the U.S. food supply. On average farmed salmon have 16 times the dioxin-like PCBs found in wild salmon, 4 times the levels in beef, and 3.4 times the dioxin-like PCBs found in other seafood. The levels found in these tests track previous studies of farmed salmon contamination by scientists from Canada, Ireland, and the U.K. In total, these studies support the conclusion that American consumers nationwide are exposed to elevated PCB levels by eating farmed salmon.

However, not all farmed salmon has higher chemical contamination. Environmental Working Group did find 2 farmed salmon companies which use feeds that keep PCB levels down as low as those found in wild salmon.

Some farmed salmon companies -- Black Pearl and Clare Island Sea Farm -- are producing salmon that have very low PCB levels similar to those of wild salmon, Green said. These producers use herring and sardine fish meal, canola oil, soya and other uncontaminated ingredients.

Seek out the Black Pearl and Clare Island Sea Farm brands if you can find them.

Whether the higher levels of PCBs pose a health risk is not clear. Contaminant risks are typically greater for developing fetuses than for adults because the fetuses are going through complex changes which, if disrupted by chemical toxins, can cause improper development with very lasting and even permanent results. Pregnant women therefore need to be more conservative when evaluating risks.

Keep in mind when evaluating risks of fish that fish consumption probably will reduce your risks of heart disease and other diseases in which inflammation mechanisms are implicated (e.g. arthritis and perhaps some of the neurodegenerative diseases). So the risks of chemical contaminants has to be weighed against the health benefits of eating high omega-3 fatty acid foods. In the case of wild salmon the mercury and chemical contaminant contaminations are so low that a strong argument can be made for eating the wild salmon. My guess is that even farmed salmon is a strong net health benefit for the vast majority and in the case of the farm salmon that is given feed that has low levels of contaminants the benefit from eating it almost as big as the benefit from eating wild salmon.