Web edition: January 15, 2007
Two recent reports urge more fish consumption, including farmed salmon. But previous studies had warned against farmed salmon because of high levels of organic pollutants. So, do we eat it or don't we?
A report published in Science 3 years ago warned that some kinds of salmon should be eaten at most once every 5 months. But in October 2006, an article in the Journal of the American Medical Association and a report by the National Academy of Sciences advised most people to eat two servings of fish a week, regardless of the species. Which advice should you follow when you go to the grocery store?
Everyone agrees that fish in general and salmon in particular have considerable health benefits. Salmon is one of the richest sources of omega-3 fatty acids, which protect the heart and support fetal brain development. Furthermore, all fish are an excellent source of protein and a wide variety of micronutrients. And salmon tends to be low in mercury, one of the most serious contaminants of many kinds of fish.
But salmon is contaminated with persistent organic pollutants such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT). Although PCBs and DDT have been banned in the United States for 30 years, the compounds still linger in the water, soil, and the flesh of animals because they biodegrade slowly. In 2003, an international accord banned a dozen of the organic pollutants from participating countries, but the chemicals are still in use elsewhere. When organic pollutants accumulate in the body, they increase risk of cancer and immune system dysfunction.
The conflicting advisories about fish consumption arise because scientists disagree about how serious the health risks from these pollutants are. The controversy focuses on the safety of farmed salmon in particular because the concentration of organic pollutants there tends to be several times higher than in wild salmon. Even by the most conservative estimates, it's safe to eat wild Alaskan salmon once or twice a week.
The two most recent reports say the benefits of eating salmonwild or farmedgreatly outweigh the risks. "It's much more dangerous not to eat the salmon," says Dariush Mozaffarian of the Harvard School of Public Health in Boston, an author of the JAMA article. "I'm bothered that people might be scared away from eating what might be the best food for their hearts."
Mozaffarian points out that most animal products and even vegetables around the world are contaminated to some degree by organic pollutants. A 2003 U.S. Food and Drug Administration study analyzed roasted chicken, for example, and found that it had the same concentration of PCBs as salmon farmed in British Columbia.
But David O. Carpenter of the State University of New York at Albany in Rensselaer, an author of the 2004 Science article, says, "One should avoid farmed salmon like the plague." In his analysis, he considered contamination to be unacceptable if it raises the risk of cancer by more than 1 in 100,000. Using U.S. Environmental Protection Agency formulas, he found that staying below that level of risk required limiting farmed salmon consumption to no more than one meal every 1 to 5 months, depending on where the salmon was raised. Salmon from Europe is far more contaminated than is salmon from western Canada, the Pacific Northwest, or Chile.
Mozaffarian says that the Science report doesn't take the positive effects of fish consumption into account. The cardiac benefits of farmed salmon, he says, prevent 100 to 400 times as many deaths as the organic pollutants cause. Furthermore, he argues that the studies showing heart benefits are stronger than the studies showing negative effects of organic pollutants. At high doses, the chemicals are known to make people sick, and animal studies suggest that lower doses can have an impact, but few large-scale human studies have been done. Mozaffarian also says that the researchers analyzed the pollutant levels of the entire fish, including the skin, which people don't ordinarily eat, thus getting artificially high values for the pollutants.
The high contamination levels in farmed salmon come from their diet. Farmed salmon are fed fish oil and fish meal, the leftover bits of fish after the meat has been removed for human consumption. These feeds give the farmed salmon a concentrated dose of pollutants because those chemicals are most abundant in fat and skin. Furthermore, farmed salmon tend to be fattier than wild salmon, also leading to higher concentrations of the pollutantsand, ironically, greater amounts of the beneficial omega-3s.
The distinction between wild and farmed is a useful one, but a new study of British Columbia salmon showed that different species and different farming practices can have a big impact on PCB concentrations. Michael Ikonomou of the Institute of Ocean Sciences in Sidney, British Columbia, and his colleagues found that farmed coho and Chinook salmon tend to be lower in PCBs than farmed Atlantic salmonand are in fact lower than some wild salmon. This makes some sense, Ikonomou says, because farmed coho and Chinook salmon have lower fat levels than Atlantic salmon. The report appeared in the Jan. 15 Environmental Science & Technology.
Fish farming practices could be changed to reduce the contamination of the fish. The salmon could be fed soybean oil instead of fish oil early in their lives. A couple of months before harvest, they would be switched to fish oil to increase the omega-3 levels. Some farming practices have already been changed, Mozaffarian says, so farmed salmon may already have lower concentrations of pollutants than the ones tested in the Science study.
The human body breaks down organic pollutants, but only very slowly. Because they are fat-soluble, the body can't excrete them through urine or sweat. The chemicals have a half-life in the body of about 10 years, Carpenter says. Because people tend to continue ingesting the pollutants, they typically build up faster than the body can break them down.
Organic pollutants are, however, secreted in breast milk, which contains fat. A woman who breastfeeds three babies can decrease her pollutant load by half, Carpenter says, but the mother passes the chemicals on to her children. Because of this, Carpenter says that it's particularly important for girls and young women to avoid farmed salmon, especially since the heart benefits of the omega-3s are less essential for young people than for older ones.
Until further studies are done, we're going to be stuck with some uncertain choices at the grocery store. The one thing all the researchers seem to agree on is that you should put some fish, in one form or another, in the cart.
David O. Carpenter
School of Public Health
University at Albany
State University of New York
One University Place, B Wing, Room B242
Rensselaer, NY 12144
Institute of Ocean Sciences
P.O. Box 6000
Sidney, BC V8L 4B2
Department of Epidemiology
Harvard School of Public Health
Building 2, Room 315
655 Huntington Avenue
Boston, MA 02115
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