Millions of people make fish -- a low-fat, vitamin-rich source of protein -- part of their diet for nutritional reasons, or simply because they enjoy the taste. But eating a diet high in fish is becoming increasingly controversial, as some studies tout the benefits of seafood while others argue that the toxic mercury found in some fish outweighs any potential benefits.
Last week, there was good news for fish lovers. Researchers at Rush University Medical Center in Chicago reported that eating fish once a week was associated with slower cognitive decline in adults over 65.
Compare that good news with the findings of a 2002 study by the Research Institute of Public Health at the University of Kuopio in Finland, which found that Finnish men with the highest concentrations of mercury also had the highest death rates from cardiovascular disease.
Larger, longer-living fish, such as tuna and swordfish, tend to have the highest levels of mercury because it accumulates over time. Fish absorb mercury from polluted water and from eating algae and other contaminated fish. People with elevated mercury levels can experience what's called mercury poisoning or mercury toxicity, which can cause physical and neurological problems. The U.S. govenrment does not track cases of mercury poisoning, so it's difficult to determine the breadth of the problem.
Mercury is particularly worrisome during pregnancy. A high level of mercury is thought to affect the neurological development of a fetus. Children with mercury poisioning are at higher risk for learning disabilities, poor motor function and seizures. The Center for Children's Health and the Environment at Mount Sinai School of Medicine warned, in an article published in Environmental Health Perspectives this February, that the projected 300,000 to 600,000 American children born each year with a reduced IQ due to mercury poisoning will end up costing the U.S. $8.7 billion in lost earnings.
The U.S. Food and Drug Administration and Environmental Protection Agency advise women and young children not to eat shark, swordfish, king mackerel or tilefish -- fish all high in mercury. Federal guidelines further advise that women and young children can eat up to 12 ounces a week of fish and shellfish that are lower in mercury, such as canned light tuna (which is lower than canned white albacore or tuna steaks), salmon and shrimp.
These guidelines were developed based on several studies, but especially what is referred to as the Faroe Islands study, conducted by researchers from the Harvard School of Public Health and other institutions. Published in February 2004, the study examined 1,022 mothers and children in these North Atlantic islands and found that the electrical signals in the brains of mercury-exposed children aren't transmitted as quickly as in unexposed children. It also appeared that mercury weakened the children's heartbeats.
But research on mothers who eat a lot of fish and their children, conducted in the Seychelles, has not associated fish consumption with any adverse effects in the children.
Dr. David Perlmutter, a neurologist in Naples, Fla., said he thinks women of childbearing age should not eat any fish.
"We discover three or four people each week with mercury toxicity -- everything from kids with hyperactivity to elderly people with cognitive difficulties," he said.
Perlmutter, medical director of the Perlmutter Health Center in Naples, and author of "The Better Brain Book" (Riverhead, 2004), said most people don't even think of mercury when they see health problems.
"Most of the people we end up treating are those in whom the idea of mercury hasn't been explored," he said. He diagnoses his patients by, first, blood test, and then confirms it by giving them a chelating agent and testing their urine.
"Usually if people had issues relating to mercury, they improve right away," he said. "Generally, people in their 20s and 30s have wonderful recovery, and we see it every day."
Even though mercury-based tooth fillings and vapors from industrial pollution can be culprits for mercury toxicity, Perlmutter said most of his patients get it from fish.
"I think more and more it's from fish, because the levels are getting higher and higher, and dentists are using less and less mercury -- not because they're concerned about the mercury (although they should be), but because aesthetically people prefer white fillings," Perlmutter said.
He still thinks people should eat fish "but much less than they do." He recommended people eat fish that are low in mercury, such as tilapia and wild salmon, about twice a month.
Women in their childbearing years, he added, shouldn't eat any fish.
But Philip W. Davidson, who has spent 15 years studying children in the Seychelles born to mothers who consume high amounts of fish, hasn't seen any adverse effects, and thinks people shouldn't be so quick to cut fish from their diet.
Davidson, a professor of pediatrics at the University of Rochester, said he thinks there is not enough evidence for Americans to be cutting fish entirely out of their diet based on mercury fears.
"Our own feeling is that warning people not to eat fish has adverse effects of its own because fish is very nutritious," he said.
Davidson has been working on a study conducted by the University of Rochester Medical Center that suggests eating fish -- and therefore consuming mercury -- during childbearing years and pregnancy may not be problematic after all. In 1990, Davidson along with other researchers began studying children in the Seychelles, an archipelago nation off Africa's eastern coast, where women typically eat 12 meals of fish per week -- even when they're pregnant.
"The hypotheisis has always been that there are adverse effects from consuming fish during pregnancy," Davidson said.
The mothers -- who predominantly eat karang or jack but also regularly consume tuna, red snapper, parrotfish, mackerel, kordonye and grouper -- have mercury levels far higher than the average American. The Seychelles women in the study averaged six parts per million, while the average American has one part per million.
The researchers have measured the functioning of 700 children based on 64 different "endpoints," including cognitive abilities, perception, social skills and memory. The oldest children in the study are 15, as they were born when the study began.
"Up to now, six times we've seen these kids, and we've been unable to confirm adverse effects," Davidson said.
The study recently received funding to visit the Seychelles and measure the children twice more.
Perlmutter cautions against people thinking they are safe to eat all the fish they want based on this study.
"To make a global statement that high levels of mercury are safe based upon this study would be unfair," he said. "Some people are affected at lower levels."
"Lots of people are walking around with high levels of mercury," he added. "People tolerate different levels."
But even those who tolerate higher levels would be better off without it, Perlmutter contends.
"Where could they be in terms of functioning if they didn't have the mercury?" he says.
But so far, the children of the Seychelles appear to be functioning just fine. And Davidson said that he didn't believe the entire group he studied could be immune to effects of mercury because they are a diverse population.
"It's not likely," he said. "People have immigrated from all over -- it's a fairly young country."
"At this time, EPA and FDA have no current plans to revise the advisory," said Jim Pendergast of the EPA's Office of Water. "However, as science continues to advance, there is always a possibility to revisit the advisory."