Fish is good food--a rich source of protein and omega-3 fatty acids, important to early brain development and cardiovascular health. But some Americans became confused about fish's place in their diet after the U.S. Food and Drug Administration (FDA) issued an advisory in March warning pregnant and lactating women to avoid certain types of fish.

The FDA's goal is to limit the exposure of unborn children to mercury, a neurotoxin found at high concentrations in some species. Unfortunately, public health officials contend, more than a few adults--not just new and expectant mothers--stopped eating fish, or cut back, to avoid mercury altogether.

By reducing one risk (mercury), people sometimes incur others (not enough omega-3s and fish-derived nutrients). It's a phenomenon decision scientists call "risk-risk tradeoffs." While the FDA makes this tradeoff clear, the news media have sometimes failed to do so.

According to James Hammitt, director of the Harvard Center for Risk Analysis (HCRA), the center has convened a group of experts to study the health implications of environmental mercury, including the FDA's recommen-dations. Currently, women of childbearing age are discouraged from eating more than two to three servings a week of shark, swordfish, king mackerel, tile fish, or albacore "white" tuna.

TRAWLING FOR ANSWERS

Humans get "almost all" of their mercury from eating fish, Hammitt notes. Where fish get this pollutant is more complex.

Small amounts of environmental mercury come from natural sources, including volcanic rock. Another source is the burning of municipal waste. But much of it, Hammitt says, comes from coal-burning power plant emissions, which fall to earth, then flow into lakes, streams, and oceans, where it is converted into a form called methylmercury. Tiny zooplankton at the base of the marine food chain consume this heavy metal. When these creatures are consumed in turn, the dose is magnified. Big predators at the top of the food chain, including tuna and swordfish, are the most contaminated (see chart).

Even at 10 times the average level of fish consumption in the U.S., some studies show no major ill effects of mercury in humans. Others point to subtle adverse effects in the unborn and very young.

In the Department of Environmental Health at HSPH, adjunct professor Philippe Grandjean has documented heart and brain function damage in children. Early this year, his two studies in the Journal of Pediatrics examined mercury's impact on women and their children in the Faroe Islands, a fishing community that consumes large quantities of fish, whale meat, and shellfish.

Checking pregnant women's hair and umbilical cord blood, Grandjean's team documented how much mercury from the mother's diet had found its way to the fetus. About 1,000 children participated in the study, and about 90 percent were examined again at ages 7 and 14. Measuring heart rates and testing variables such as motor speed, language skills, and attention span, the researchers found subtle but distinct signs of neurological damage. Effects were first apparent at or slightly above mercury levels of 1 microgram per gram of hair, the level of exposure considered safe by the U.S. Environmental Protection Agency.

CHEAP AND NUTRITIOUS

Meanwhile, fish is an excellent food, rich in protein and those omega-3 fatty acids. According to Eric Rimm, associate professor of epidemiology and nutrition at HPSH, some people are overreacting by steering clear of fish. He fears they may instead substitute red meat, laden with heart-damaging saturated fat.

In one large, recent study, Rimm notes, omega-3 fatty acid supplements were associated with a 50 percent reduction in sudden death from heart attack--a benefit which has also been attributed to weekly consumption of tuna, salmon, and other fatty fish. Omega-3's may work, Rimm says, by reducing heart rhythm irregularities as well as inflammatory molecules and triglycerides associated with cardiovascular disease. In a 2002 study, Rimm and colleague Alberto Ascherio showed that men who eat fish just twice a month cut their risk of ischemic stroke by up to 40 percent.

Now Rimm is exploring the nutrients and contaminants contained in wild and farmed salmon. He is measuring omega-3 levels, which depend on what salmon eat and their age at harvest. He will also test for PCBs, dioxin, and pesticides.
HCRA senior researcher Joshua Cohen has built a mathematical model to help Americans fine-tune their fish-eating choices. He will publish a cluster of papers this year on how changes in fish consumption (and both omega-3 and mercury intake) affect cognitive function in children, and stroke risk and cardiovascular disease in adults.

Until more research further clarifies all these risks and benefits, what's a consumer to do? Enjoy fish, HSPH researchers urge. For pregnant and breastfeeding women, that means up to 12 ounces per week of certain species (see the EPA's website at www.epa.gov/ost/fish, and state health department websites for local advisories). Others need not be concerned as long as they are eating fish in moderation. Bear in mind, HSPH investigators say, that research to date has found no significant ill effects from mercury at the lifetime levels to which most humans are exposed.

Karin Kiewra is Editor of the Harvard Public Health Review and Associate Director of Development Communications at the Harvard School of Public Health

Illustration: Ida Floreak