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National PostMay 28, 2001
One fish, two fish, good fish, bad fish
Some nutritionists think fish should carry a label trumpeting its health benefits. Others say the toxins in seafood require a health warning. Who's right?
Mark Stevenson National Post with files from Ottawa Citizen and wires
Call it the great tuna paradox. Tuna is rich in fatty acids that improve brain and eye development and protect against heart disease. It may also contain toxins that can kill nerve cells and cause birth defects. Every week there is a new study showing that we need to eat more fish to live longer, followed by another study showing that consuming too much fish can be dangerous.
Just last week, a report in the prestigious Proceedings of the National Academy of Sciences showed that seafood was a large component of the diet of our early ancestors. Stephen Cunnane, a professor of nutrition at the University of Toronto, called the study "an important finding" that supports the theory that the brainpower of early humans was boosted by seafood containing DHA, a fatty acid proven to enhance brain and eye development.
A few days earlier, a study by the Harvard School of Public Health showed that people who ate canned tuna more than once a week had a 40% lower risk of developing age-related macular degeneration -- a leading cause of blindness in older people -- than those who ate it less than once a month. Other studies have shown that regularly eating fatty fish significantly reduces the risk of dying of a heart attack, particularly for the elderly.
"I don't know of any other nutritional category where our intake is falling so very short of what is optimal for cardiovascular health," says Bruce Holub, a professor of nutritional sciences at the University of Guelph. "We need to eat more fish."
But recent tests done on canned tuna by the Consumers Union of the United States and on a range of fish by the Ottawa Citizen show that tuna and other popular fish contain so much methylmercury that they should be avoided by pregnant women, nursing mothers and children under the age of five.
Mercury occurs naturally in the environment. It is also a by-product of coal-fired power plants and waste incinerators. When it enters the water and reacts with bacteria, it is transformed into methylmercury. Fish absorb it through their gills, and it builds up in their tissue. Older fish, and larger fish that eat smaller fish, have higher concentrations of mercury.
The problem with mercury is that it is a neurotoxin that accumulates in the body. At high levels, it can kill nerve cells, causing blurred vision, lack of co-ordination and slurred speech. The human fetus is particularly vulnerable. Even a relatively small exposure can cause subtle changes to the developing brain. In larger doses, it can cause blindness and other birth defects.
Earlier this year, the U.S. Food and Drug Administration (FDA) warned pregnant women and those trying to become pregnant not to eat certain fish -- shark, swordfish, King mackerel and tilefish -- because of concerns about mercury levels.
The FDA considers fish with no more than 1 part per million (ppm) of mercury safe to eat. Health Canada is more restrictive. It limits mercury in fish to 0.5 ppm. Whichever measurement you choose, tuna was found to have too much mercury for comfort. The Ottawa Citizen's independent testing service found swordfish with an average 1.09 ppm; shark at 1.63ppm; and fresh tuna with 1.27 ppm.
Since 1998, Health Canada has warned pregnant women against eating more than one serving a month of shark, swordfish and tuna (fresh and frozen). Everyone else, it says, should eat no more than one serving of the same fish a week.
Last month, two U.S. environmental groups, the Environmental Working Group and the United States Public Interest Research Group, issued a report, Brain Food: What Women Should Know About Mercury in Fish, naming additional species for pregnant women and women planning to become pregnant to avoid, based on fish contamination levels in U.S. government records. The list included sea bass, tuna (in steak form), oysters from the Gulf of Mexico, halibut, marlin, pike, pickerel, white croaker and largemouth bass.
In addition, it warned women about eating more than one meal a month of canned tuna, blue mussels, mahi-mahi, Eastern oysters, cod, pollock, salmon from the Great Lakes, wild channel catfish, blue crab from the Gulf of Mexico and lake whitefish.
Some nutritionists would like fish to come with a label touting its health benefits. Some consumer groups would like to see fish with high mercury levels come with a label carrying a health warning.
Nutritionists acknowledge that it is, indeed, dangerous to eat too much fish with high concentrations of mercury. But there is also concern that warnings such as the FDA's will discourage people from eating fish that is safe.
"The irony about the FDA report is that pregnant women would be among those who would most benefit from eating fish," says Prof. Cunnane. "It's unfortunate that pregnant women are being [coaxed away] from what otherwise would be very good food for them."
Polyunsaturated fats found in fish, says Cunnane, are crucial to early brain and eye development and may also help prevent premature delivery. And the benefits continue with age. Studies have shown that fish oils may help prevent cancer, some types of stroke, and symptoms associated with adult diabetes.
"[Eating fish] is sort of a preventive step you can take if you have diabetes, much like taking an Aspirin a day," says Cunnane. "It won't prevent diabetes, but it will prevent the long-term damaging effects of diabetes on the blood vessels."
Research also suggests fish oils may help ward off senility and declining brain function that come with age.
But the heart seems to gain the most. A study released at the annual convention of the American Heart Association in February found that eating fatty fish reduced the risk of deadly heart attacks in older people by 44%.
The research, conducted by the U.S. National Heart, Lung and Blood Institute, tracked the diets of more than 4,000 men and women above the age of 65 over a seven-year period. Heart attacks were much less common in people who ate at least one serving of fatty fish a week.
The study distinguished between fatty fish, such as salmon and mackerel, which are rich in Omega-3 fatty acids, and fried fish made from leaner varieties, such as cod, catfish and snapper.
"Fatty fish are more abundant in Omega-3 fatty acids, while fried fish are typically lean fish without significant Omega-3 fatty acids," says Dr. Dariush Mozaffarian, an author of the study. Omega-3, a type of polyunsaturated fat found in fish oils, has been proven to dramatically improve the overall health of the heart in clinical studies.
"Because these Omega-3 fatty acids may protect against dying from a heart attack, eating fatty fish may be of greater benefit than eating fried fish," Dr. Mozaffarian adds.
Researchers have shown that fish oils can dramatically reduce the risk of death from sudden heart attack, above and beyond the best drugs on the market. In a landmark 1999 study published in Lancet, the British medical journal, researchers in Italy tracked more than 11,000 people for three and a half years after they suffered a first heart attack. All were put on a healthy diet and given advanced drugs to prevent future attacks. Some were given a fish-oil supplement containing Omega-3 fatty acids. Those who took the supplement had a 45% reduction in sudden cardiac death.
Not everyone sees this as a reason to take Omega-3 supplements. The Heart and Stroke Foundation of Canada suggests people eat two to three servings of fatty fish a week if they wish to increase the amount of Omega-3 in their diet. But it claims the benefits of fish oils are over-rated.
"A balanced diet rich in Omega-3 fatty acids is desirable," says Andreas Wielgosz, a cardiologist with the foundation. "But I don't think we can go to the next step and say people should actually supplement their diet or expect a certain result from it."
There may be disagreement over whether supplements are worthwhile, but one thing is clear: Not all fish are created equal when it comes to healthy fish oil. Salmon, trout, mackerel and other fatty fish are loaded with Omega-3 fatty acids. Tuna is not a fatty fish but is considered a decent source of Omega-3. Leaner fish, such as cod, catfish and snapper, have less.
"The cold-water marine fish are the best," says Prof. Cunnane. "But any fish, whether they're grown on a fish farm in southern Ontario or whether they are imported directly from the Tokyo Fish Market, you're going to get some benefits as long as there are no toxic environmental contaminants."
FISH TIPS:
So what should consumers do to limit their exposure to contaminants while getting their fair share of dietary Omega-3? Pregnant women and would-be mothers should also stay away from king mackerel and tuna, both on the high-mercury list. Instead, Guelph's Prof. Holub suggests looking at other fish high in Omega-3 but low in mercury, such as salmon, herring, sardines and rainbow trout. Other low-mercury options include flounder, haddock, mullet, scallops and shrimp.
-- Anonymous, May 28, 2001
Lots of waste incinerators in eastern and central Massachusetts. There has been an advisory for some time not to eat much locally caught fish. One answer over time would be to prevent additional mercury from entering the environment. VERY important to safely recycle or dispose of those mercury thermometers and button batteries. There are substitutes for at least the thermometers.
-- Anonymous, May 28, 2001