HPH NOW, March 3, 2006, Risks and Benefits of Eating Fish Debated

To eat fish or not to eat fish? Today's consumers may be confused about seemingly contradictory research findings and advice about how much fish to eat, what kinds, and whether farm-bred or wild-caught fish are healthier. To make sense of the situation, the HSPH Department of Environmental Health and the Harvard NIEHS Center for Environmental Health convened a mini-symposium on February 2 in Snyder Auditorium. Speakers presented the current evidence and discussed the difficulties in translating possible risks and benefits into dietary advisories and clear health advice. The gathering was prompted by recent health advisories on how to approach the risk of consuming fish that contain mercury.

Epidemiological studies at HSPH and elsewhere have framed some of the pluses and minuses of consuming fish, but so far the data do not provide unambiguous guidelines. Some papers show that eating fish or fish oil supplements may help protect the heart and sharpen the mind. Other reports have warned that ingesting too much mercury in seafood may damage developing brains and bodies of fetuses and children.

"In many ways, it's a classic example of the trade-offs of the modern world," said Douglas Dockery, chair of the Department of Environmental Health. "The power plants that are producing electricity emit mercury, which deposits in aquatic environments. The mercury bioaccumulates through the food chain and reaches the predator species. If we ingest them, it can lead to deleterious effects. On the other hand, seafood is a source of nutrients with effective benefits."

The speakers addressed the role of seafood in a balanced diet, the environmental determinants of seafood contamination, the adverse effects of seafood contaminants, and methods for evaluating risks and benefits of seafood diets.

The most influential recent health warning about mercury in fish came from a joint 2001 advisory by the U.S. Food and Drug Administration and the Environmental Protection Agency, which advised pregnant women and those who might become pregnant to avoid certain fish thought to be high in methyl mercury, said HSPH Associate Professor of Epidemiology and Nutrition Eric Rimm. In 2004, the advisory was updated with a recommendation for one to two servings per week of fish that were thought to have lower mercury concentrations.

On the other hand, many observational studies have found a cardiovascular benefit to eating fish, Rimm said. A 2004 meta-analysis by a graduate of the HSPH Department of Nutrition found strong evidence for cardiovascular benefit from n3 polyunsaturated fatty acids, from fish and supplements, as well as a beneficial effect on blood lipids.

Heart disease is the number one killer of men and women in this country. Several fish meals a week or fish oil supplements may reduce heart rate and blood pressure in the short term, Rimm speculated, and have anti-inflammatory benefits that extend to other chronic inflammatory diseases, such as rheumatoid arthritis.

To understand how fish become contaminated, it helps to look at fish in their environments, said James Shine, HSPH associate professor of aquatic chemistry. Fish absorb contaminants from surrounding waters, and they can ingest greater quantities from the other plants and fish they eat. "We're putting the fish and contaminants together," Shine said. "The ultimate sink is the coastal marine environment. Twenty-two percent of all the PCBs ever produced are now in estuarine and coastal sediments. That's also where 95 percent of the fish productivity is."

Philippe Grandjean, adjunct professor in the Department of Environmental Health, worries about mercury in particular. In an ongoing study of 1,000 children on the Faroe Islands in the North Atlantic, he and his colleagues have reported a dose-response relationship between signs of impaired brain function in children and their mercury exposure in the womb and in their own diets. The islanders' traditional diet includes pilot whales, whose meat contains mercury and whose blubber contains PCBs. People on some of the islands eat a lot, and others just a little, depending on how often pilot whales swim into the inlets where they are caught.

Grandjean and his colleagues published two papers in the Journal of Pediatrics last year on the Faroe Islands research. They have continued to study the population and plan to release their dataset upon receiving clearance from island authorities, said Grandjean. Responding to studies that have found less risk to children, Grandjean asserted that the research suffers from methodological imprecision that causes substantial underestimation of the mercury toxicity.

Changing diets to low-mercury fish is a quick fix because mercury only lingers in people for about 45 days, he said. "The omega 3 fatty acid content varies widely, independent of the mercury content," he said. "Choose fish high in essential nutrients."

Joshua Cohen, senior research associate at the Institute for Clinical Research and Health Policy Studies at Tufts New England Medical Center, is concerned that fish consumption advisories be issued with care. Fish advisories have had to walk a fine line between describing the risks and benefits of eating fish.

In 2005, Cohen worked at the Harvard Center for Risk Analysis and led a study in the American Journal of Preventive Medicine that warned substantial nutritional benefits could be lost if the general public decreased the amount of fish they eat. "If we are contemplating giving out advice, we need to know the public health impact of that advice," Cohen said at the symposium. "If we scare people from eating fish, and that means more cardiovascular deaths, we have to account for that."

Part of the problem is that the federal government has not rigorously studied the influence of official advisories. (A group from Harvard Medical School has reported that pregnant women reduced their fish consumption by 17 percent in response to the 2001 federal advisory.) To address this need, Cohen developed a methodology for measuring the public health impact of advice when actual data on changes in fish consumption become available, he said.

A panel discussion chaired by Joseph Brain, Cecil K. and Philip Drinker Professor of Environmental Physiology in the Department of Environmental Health, followed the individual presentations. The panel included the speakers and David Bellinger, professor of environmental health at Children's Hospital Boston, who described how insufficient data make balancing the risks and benefits of eating fish problematic.

—CCM