Diet books advised him that adding fish to his meals would help him reduce calories and take advantage of a hefty dose of heart-friendly fats called omega-3 fatty acids. He began replacing his red meat with fish and felt better -- and lost weight. As Duvall likes to put it, "I was hooked."
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Since 2002, the American Heart Assn. has recommended that healthy adults eat at least two servings of fish a week to protect their health. "Calorie for calorie, the benefits of fish oil exceed any other nutrient in the diet," says Dr. Dariush Mozaffarian, a cardiologist at Harvard's Brigham and Women's Hospital in Boston who researches the heart benefits of fish.
But in the last few years, a battle over safety has erupted, confusing many consumers about the benefits and risks of eating fish. All fish contain low levels of contaminants (ingested from water or by eating smaller, contaminated fish), notably polychlorinated biphenyls (PCBs), cancer-causing industrial chemicals banned in 1976; and mercury, a heavy metal found in lakes, streams and oceans.
Environmentalists and consumer groups argue that toxic contaminants in some popular types of fish, such as tuna, put the public's health at risk and that the federal government is doing little to solve the problem.
Physicians and nutrition researchers, on the other hand, fear that all the talk about the dangers of fish scares people away from a food that most of medical science generally agrees offers significant benefits.
"For adults in this country, the main problem is that people don't eat enough fish," says Dr. Eliseo Guallar, associate professor of epidemiology and medicine at the Johns Hopkins School of Public Health.
Indeed, Americans are not big fish eaters. The U.S. Department of Agriculture estimates that people eat about 3.4 ounces a week, with Californians and other Pacific coast residents eating a bit more: about 3.7 ounces.
By comparison, the USDA estimates that per capita red meat consumption totals nearly 21 ounces.
"Yet fish is healthier," Guallar says.
Benefits not without risk
Scientists now think that the healthful effects of fish are largely due to substances called omega-3 fatty acids, especially two known as DHA and EPA, which are plentiful in oily fish such as salmon, trout and herring. But the long list of possible benefits (see related story) doesn't translate to a blanket endorsement of all fish, any fish.
Fish in fish sticks, for example, is generally low in omega-3s. Fish in fast food restaurants is often high in the unhealthful trans fats used to cook it.
These factors can neutralize the benefits of fish. In an investigation known as the Cardiovascular Health Study, in which 4,775 adults older than 65 were tracked for many years, those who ate baked or broiled fish one to four times each week had 27% fewer strokes than those who ate fish less than once a month. People who ate fried fish or fish sandwiches more than once a week had 44% more strokes.
"Before I did this research, I might have considered a fish sandwich at a fast food restaurant or fish sticks a 'fish meal,' " says Harvard's Mozaffarian, lead author of this work.
Another complication is that not all of the studies on fish-eating have been positive -- possibly, scientists say, because of the effect of contaminants. A study of 1,833 men in Finland, for example, reported that those who ate mercury-contaminated freshwater fish (and who ended up with higher mercury blood levels) suffered twice the rate of heart attacks and deaths from strokes as those who did not eat contaminated fish.
This raises the possibility that the presence of mercury in fish may reduce or eliminate its cardiac benefits, says Guallar of Johns Hopkins, who has conducted studies that indicate similar results.
"It's concerning," he says.
Mercury originates from natural sources and air pollution. In water, it's transformed by bacteria into methylmercury, which is toxic to the brain, heart and nervous system and especially damaging to the neurological development of infants and young children.
It was not until 2004 that the Food and Drug Administration and the Environmental Protection Agency advised pregnant and nursing women, women of child-bearing age, and children to avoid certain fish because of the mercury content. These fish -- swordfish, king mackerel, tilefish, and shark -- are all large, long-lived predatory fish that concentrate mercury in their bodies throughout their lives through eating smaller fish that also contain mercury.
Tuna, another large, long-lived fish and the nation's most popular seafood, was not on the original list. The agencies later added a recommendation that vulnerable groups eat no more than 6 ounces, or one can, of white or albacore tuna a week. FDA tests had shown that canned albacore tuna, as well as fresh or frozen tuna steaks, contained significantly more mercury, because they came from larger tuna species, than did chunk light tuna, often canned from the smaller skipjack tuna. The advisory also recommended that vulnerable groups limit their total fish consumption to 12 ounces a week.
The advisory does not mean pregnant women should not eat fish; omega-3s are critical to an infant's developing brain. A Harvard study released last year found that pregnant women who ate fish every week during pregnancy had offspring with intelligence scores 4 points higher for every additional serving of fish over the average of one serving.
But there lies the conundrum: The finding only held if the mothers' blood mercury levels remained low. Women with the highest blood mercury levels had offspring with intelligence scores 9.3 points lower than those from mothers with an average amount of mercury, or less.
The government's fish advisory, which reflects the "reference dose" or "safe limit" of mercury for vulnerable groups set by the EPA, may not protect women who are not careful about how much fish, or what kind, they eat, says Kathryn Mahaffey, an EPA scientist. She examined the mercury levels of American women of child-bearing age recorded in a large federal health survey and found that as many as 10% of such women may be eating enough fish to have blood mercury levels over the reference dose. That, she calculated, exposed somewhere between 300,000 and 630,000 newborn infants annually to methylmercury concentrations that may put them at risk of neurodevelopmental defects.
"Some people eat a lot of fish, and many don't eat fish at all," she says. "Neither extreme is good."
People do eat a lot of tuna. Some 26% of all fish consumed in 2004 was canned tuna, according to the USDA. Environmental and consumer groups say the warnings regarding mercury in tuna are too weak. They note that canned light tuna, which is listed on the FDA's website as low in mercury, contains about 20 micrograms of mercury, whereas a 6-ounce serving of salmon has only about 1.6 micrograms.
"We're in favor of eating fish, as long as it's low-mercury fish," says Jackie Savitz, a senior scientist with Oceana, a national environmental organization concerned with the effects of ocean pollution. "But we need to solve the mercury problem."
In July, three national environmental groups led by the Defenders of Wildlife released the first comprehensive study on the amount of mercury in chunk light tuna from various countries. (Imports of canned tuna now total about 51% of all canned tuna sold in this country, though not all is labeled by country of origin.) They reported that more than a third of the cans that were tested by an independent lab had mercury levels above the level deemed safe by the EPA for vulnerable groups.
Nearly one can in 20 contained so much mercury that the FDA regulations would allow the agency to pull it from the supermarket shelves.
For its part, the tuna industry says that the mercury categories the FDA chooses for different fish, and the advisory regarding tuna, are adequate. "The health benefits of eating canned tuna far outweigh the risk from trace amounts of mercury," says Anne Forristall Luke, president of the U.S. Tuna Foundation.
Yet even physicians who are strong proponents of eating two or more servings of fish each week worry about tuna. "Tuna is just so popular and so easy for people. For elderly people, and busy people -- just open a can of tuna," says Dr. Walter Willett, professor of epidemiology and nutrition at the Harvard School of Public Health. "There really is a chance some people can get too much mercury from tuna. As a result, I think people probably should not be having tuna fish more than twice a week."
How much is too much?
Perhaps if Michelle Bekey, 48, had followed Willett's advice she would not have ended up with a high blood mercury level. "Fish was my primary source of protein," says the West L.A. communications consultant. She typically ate tuna salad for lunch and swordfish, sea bass or tuna steak for dinner. "Basically, I was eating all the higher-mercury fish. I'd been aware that mercury is a hazard for pregnant women and children. But I hadn't seen warnings for anyone else."
Indeed, there are none, nor does the FDA believe there needs to be. "Based on the science, no," says Dr. David Acheson, chief medical officer in the FDA's Center for Food Safety and Applied Nutrition. "Yes, you have to be cognizant of mercury in the type of fish you eat. If you are in those vulnerable groups of women, or for children, you have to be more cognizant of it. But with regard to the general population: eat fish."
Bekey did just that. Over time, she began to feel increasingly lethargic and to suffer from numbness in her hands. One day she came across a story in a magazine about an actress who had eaten so much fish she developed mercury poisoning -- and her symptoms sounded so familiar that Bekey had her blood tested. Her mercury level was eight times the safe reference dose set by EPA.
She's a small woman, only 110 pounds, and because the level of mercury people can tolerate is based on weight, Bekey can safely eat about 35 micrograms of mercury each week. One 4-ounce tuna steak would put her over her limit -- and she was eating several every week. Bekey is convinced that her physical problems were the result of eating too much mercury, but as with most environmentally related symptoms, cause and effect is seldom proven. She stopped eating fish a year ago and, she says, her symptoms have improved.
Based on stories like Bekey's, some nutrition researchers and activists feel the general public is not well enough informed about the amount of mercury in different types of fish and that the federal health agencies have a responsibility to provide clearer direction to all consumers. In early July, the nutrition advocacy group Center for Science in the Public Interest urged the FDA to require mercury warnings on packaged fish. (In California, supermarkets and restaurants post such warnings.)
The organization noted that although average mercury levels of many fish are posted on the websites of the FDA and EPA, only 1 in 5 consumers it surveyed could correctly identify the fish highest in mercury -- swordfish, shark, tilefish and king mackerel. More than 21% said salmon was high in mercury, when it is among the fish with the lowest mercury content.
So, to eat fish, or not? And what kind, and how much?
Nutrition researchers, federal agencies and consumer groups all agree that most adult Americans need to eat low-mercury fish, and a lot more of it. When people add fish to their diet, they tend to cut down on the extra saturated fat and calories found in beef, pork, lamb and even chicken. That's one key reason the American Heart Assn. and the government's dietary guidelines say healthy adults should eat two servings of low-mercury fish a week.
For people who've been diagnosed with heart disease, the AHA recommends eating more -- enough fish or fish oil capsules to total 1 gram of omega-3s a day.
But knowing which fish offers the most rewards may seem like something of a puzzle to consumers. To help parse the puzzle, Cathy Levenson, associate professor of nutrition at Florida State University, recently coauthored a research paper weighing the costs and benefits of eating different types of fish. "It may not be enough to simply tell people to eat fish low in mercury," she says.
For one thing, mercury is only part of the equation: The omega-3 content in each type of fish is also important. This, like the level of mercury, varies widely depending upon the fish.
Shellfish, for example, tends to be very low in mercury, but it's also, with the exception of oysters, low in omega-3s.
Shrimp, second only to tuna in popularity, has no detectable mercury. But to consume 1 gram of omega-3s, you'll need to eat 11 ounces of shrimp.
Luckily, many popular commercial fish -- including salmon, trout, herring, flounder -- are rich in omega-3s and do not have high levels of mercury. Only about 4.5 ounces of salmon, 3 ounces of rainbow trout and 2 ounces of herring contain a full gram of omega-3s.
Topping the list in Levenson's analysis (in the March issue of the journal Nutrition Reviews) was wild-caught salmon, a very low-mercury fish with among the highest amount of omega-3s.
Wild salmon trumped farmed salmon because of the risk of another kind of contamination, that of PCBs. In 2004, a study published in Science reported that farmed Atlantic salmon, particularly fish from Iceland, Norway and Scotland, contained PCB levels as much as seven times higher than those found in wild, caught salmon. (More than 80% of the fresh salmon eaten in the U.S. is farmed and imported from the north Atlantic, Canada, Chile and elsewhere.)
Although the levels didn't exceed the FDA standards for PCBs in commercial fish, they were above the safe limits set by the EPA for sport fish caught and eaten by anglers.
Farmed fish contain higher levels of PCBs because the fish meal and fish oil they're fed is high in these contaminants. The global salmon farming industry is investigating ways to reduce PCBs in farmed fish by substituting soybean oil in place of some of the fish oil in the feed.
In the meantime, country-of-origin labeling, required for two years now on fish sold in supermarkets and other retail stores, may help guide careful consumers. The least-contaminated fish among the 2 metric tons of salmon purchased by the Science researchers came from farms in Chile. Salmon from Washington state and Canada was also cleaner than that from the north Atlantic.
Another tip: In farmed salmon, the chemicals tend to concentrate under the skin and the layer of fat beneath the skin -- and thus, removing the skin and underlying fat before or after cooking will reduce the PCB content of the fish.
Figuring out how to weigh the mercury and PCB risks versus the benefits of omega-3s may become clearer with the release of a study by the National Academy of Science's Institute of Medicine, due out in the next month or so. But for some eager fish consumers, just knowing a little more about the mercury content in fish can lead to safer choices.
Weems W. Duvall, for example, ate tuna every day when he first switched to fish. Today, he's a great deal more discerning. "I'm limiting tuna now, because of the mercury," he says. "I still eat tuna, but I've come to really like that imitation crab.... And my favorite food now is salmon."
Duvall is the first to admit that he can't prove that eating fish is enhancing his health. "But I've lost some weight, my cholesterol is lower and I feel better," the Maryland attorney says. "For me, that's all the proof I need."