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.