Seafood and brain development - Harvard Health
Pregnant and nursing women concerned about consuming too much mercury from seafood can safely eat fish if they follow certain guidelines regarding the quantity and kinds of fish they eat.
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Seafood and brain development
Balancing mercury exposure against the nutritional benefits of fish.
Periodically a public debate flares about how much fish a woman should consume if she is pregnant or nursing. In the fall of 2007, for example, some members of the National Healthy Mothers, Healthy Babies Coalition received a great deal of press attention after urging these women to eat more fish in order to nurture and protect their children's developing brains.
Almost immediately, other high-profile coalition members, such as the U.S. Department of Health and Human Services, criticized the advice, which contradicted existing guidelines from the FDA and the U.S. Environmental Protection Agency (EPA). Those federal agencies had advised pregnant and nursing women to limit their consumption of certain fish because of the risk of exposing their babies to mercury. The controversy grew more heated after National Public Radio revealed that the coalition had accepted funding from the National Fisheries Institute, an advocacy organization for the seafood industry.
What was lost in the hubbub, however, was that most of the nation's experts agree on several key points when it comes to fish consumption by women who are pregnant, planning to become pregnant, or nursing children. Their advice aims to balance the risks of mercury exposure and the benefits of omega-3 fatty acids and other nutrients.
The growing brain
During fetal development, the brain grows rapidly from a bit of tissue roughly the size of a grain of salt in the first month to a sophisticated organ that is about 350 grams (nearly 1 pound) at birth. At the height of fetal development, a quarter of a million new brain cells are formed every minute.
Many nutrients are needed to ensure the healthy prenatal development of the brain and nervous system. Probably the best known is folic acid, which promotes growth of new cells; deficiencies of this nutrient increase the risk of neurologic birth defects such as spina bifida. But an omega-3 fatty acid known as DHA is also important, both for forming new brain cells and for enabling them to communicate efficiently. During fetal development and the first two years of infancy, DHA concentrates in the eyes and the gray matter of the brain, and studies suggest that this nutrient may enhance the child's vision and cognitive development.
People do not make adequate amounts of DHA or other omega-3 fatty acids on their own. In the American diet, most DHA is obtained by eating seafood. But seafood also contains a number of contaminants, including methylmercury, which can be toxic to the brain and nervous system. The developing fetus is at greatest risk from methylmercury exposure.
Hence the dilemma: how to balance the benefits of DHA against the risks of methylmercury?
Where the experts agree
It's possible for a woman who is pregnant or nursing to consume enough fish to provide adequate levels of DHA while protecting the developing fetus. The best advice is to follow a few guidelines from the FDA, EPA, and Institute of Medicine: Monitor consumption. Women can safely consume up to 12 ounces of seafood per week, although they are advised to limit albacore or "white" tuna to six ounces total per week, since this contains more mercury than "light" tuna.
Choose low-mercury fish. Among seafood commonly eaten in the United States, canned light tuna, catfish, pollock, salmon, and shrimp are all relatively low in mercury.
Avoid high-mercury fish. Swordfish, shark, king mackerel, and tilefish all contain high levels of mercury, and so should be avoided.
Unresolved issues
Some evidence suggests that selenium, another nutrient found in fish and other food sources, may somehow interact with mercury in a way that prevents it from accumulating in tissue. But more research is needed before the experts will be able to say for sure, or make any recommendations about selenium intake.
It's also not clear what to advise women who don't like fish. Some experts think that it may be helpful to consume alternative sources of DHA, such as fish oil pills or cereals, eggs, and other foods fortified with omega-3 fatty acids. Flaxseed, canola oil, and soybeans all contain ALA, a plant version of omega-3, but this is not easily converted into DHA, and it's not clear that ALA benefits the developing brain.
For now, the evidence remains strongest that fish truly is "brain" food for a developing child — but only when a mother consumes it within the accepted guidelines.
Chavarro J, et al. The Fertility Diet (McGraw-Hill, 2007).
Hibbeln JR, et al. "Maternal Seafood Consumption in Pregnancy and Neurodevelopmental Outcomes in Childhood (ALSPAC Study): An Observational Cohort Study," Lancet (Feb. 17, 2007): Vol. 369, No. 9561, pp. 578–85.
Institute of Medicine. Seafood Choices: Balancing Benefits and Risks (National Academies Press, 2006).
Mozaffarian D, et al. "Fish Intake, Contaminants, and Human Health: Evaluating the Risks and the Benefits," Journal of the American Medical Association (Oct. 18, 2006): Vol. 296, No. 15, pp. 1885–99.
Myers GJ, et al. "Maternal Fish Consumption Benefits Children's Development," Lancet (Feb. 17, 2007): Vol. 369, No. 9561, pp. 537–38.
