Nutritional scales tip in fish's favour

The Globe and Mail, October 18, 2006 by Leslie Beck

Ever since studies demonstrated lower rates of dying from heart disease among Greenland Inuit, North Americans have been told repeatedly to eat more fish. But concerns over potential harm from chemical contaminants in fish have left many people confused about what type of fish is safe to eat -- or if they should steer clear of it all together.

According to a report published today in the Journal of the American Medical Association, the benefits of eating fish (including shellfish) are far greater than the potential risks. One to two servings of fish per week is enough to reduce the risk of dying from a heart attack by 36 per cent. Researchers from Harvard School of Public Health and Harvard Medical School based their conclusions on a review of hundreds of studies about fish and health. Fish is an excellent source of protein that's low in artery-clogging saturated fat. Cold water fish, such as salmon, trout, sardines, mackerel and herring, contain two omega-3 fatty acids called DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid). Plenty of evidence suggests that DHA and EPA guard against heart attack and stroke.

A steady intake of omega-3 fats might even reduce the risk of Alzheimer's disease, dementia and rheumatoid arthritis. DHA is also needed for the development and maintenance of the brain, eye and nervous tissue throughout life, beginning in the final trimester of pregnancy.

The cardio-protective benefits of eating fish are so clear, the Heart and Stroke Foundation of Canada advises people to consume fish, especially fatty fish, at least two times per week. Omega-3 fats in oily fish make the blood less likely to form clots, lower blood fat levels and protect against irregular heartbeats that cause sudden cardiac death.

Yet concerns about harmful chemicals in fish -- methylmercury, polychlorinated biphenyls (PCBs) and dioxins -- have caused some people to question the wisdom of this advice. In May of 2002, Health Canada issued an advisory, warning pregnant women, women of childbearing age, and children 4 and younger to limit their intake of high-mercury fish -- swordfish, tuna steaks and shark -- to no more than once a month. Other people were told to consume these fish no more than once a week. (Health Canada is currently in the process of updating its fish consumption advice.)

Last month, Toronto Public Health released a report outlining stricter consumption advice. The report advises women who are pregnant or breastfeeding, women who could become pregnant, and young kids to completely lay off high-mercury fish.

Mercury is naturally occurring at very low levels in the air, soil, lakes and oceans. It also makes its way into the environment from industry, such as pulp and paper processing and mining operations. When mercury enters streams and water, it's converted by bacteria to methylmercury, which is then absorbed by fish. Larger, longer-living predators (e.g. shark, swordfish and tuna) have higher concentrations of methylmercury than smaller fish (e.g. shellfish, salmon). Cooking has little impact on mercury content.

The concern is that mercury can accumulate in the body and affect the developing nervous system, especially the brain, of infants and young children. If women consume too much mercury before and during pregnancy, it may increase the risk of birth defects and learning disabilities. Some, but not all, studies have found associations between a women's mercury exposure during pregnancy and cognitive test scores during childhood.

The link between high-mercury intakes and the risk of heart disease remains inconclusive. According to the Journal of the American Medical Association review, mercury in fish may lessen the natural benefits from fish, but the overall benefit is still positive.

The review turned up similar results for PCBs and dioxins in fish. PCBs were widely used in industry until the chemicals were banned in North America in 1977. Dioxins are byproducts of waste incineration, paper bleaching and the production of certain plastics.

Both types of chemicals persist for long periods in the environment and are present in low concentrations in many foods. Animal experiments, and some evidence in humans, have found PCBs and dioxins to be carcinogenic; some experts worry consuming low levels of them could increase the risk of cancer.

The major sources in our diets of PCBs and dioxins are beef, chicken, and pork (34 per cent); dairy products (30 per cent); vegetables (22 per cent); fish and shellfish (9 per cent); and eggs (5 per cent). Cooking methods that allow the fat to cook off (e.g. baking, broiling, grilling versus frying) will lower the amount of PCBs and dioxins in foods.

Even though major sources of these chemicals are meats, dairy products and vegetables, much attention has focused on fish. When researchers evaluated the cancer risks and heart benefits associated with eating fish, they found the heart benefits outweighed cancer risks by 100-fold to 370-fold for farmed salmon, and by 300-fold to 1000-fold for wild salmon.

There's no doubt fish is good for your heart. Experts recommend a daily intake of 250 milligrams of omega-3 fatty acids (EPA and DHA combined) to reduce the risk of dying from heart disease. Since omega-3 fats store in the body, this intake can be achieved by eating one six-ounce (170-gram) serving per week of salmon or other oily fish such as sardines, anchovies, or herring. People with heart disease are advised to consume 1,000 milligrams per day.

Based on this extensive review, the benefits of eating fish (one to two servings per week) outweigh the risks among adults, and with the exception of high-mercury fish, among women of childbearing age. Fish that are rich in omega-3 fats and low in mercury include salmon, anchovies, herring, sardines and trout.

Women who are or may become pregnant, nursing mothers and young children should avoid high mercury fish (swordfish, shark, tuna steaks, king mackerel, golden bass). Canned albacore ("white") tuna, considered moderately high in mercury, should be avoided or eaten rarely. Light canned tuna, which is smaller and younger than albacore, is considered to have medium mercury content and should be limited to one can per week. It's important, however, that women of childbearing age include 12 ounces of low mercury fish per week in order to consume reasonable amounts of DHA.

If you don't like fish, supplements are a good alternative.

Most fish-oil capsules (one gram each) provide 300 milligrams of EPA and DHA, but some contain a higher dose (check the label). Fish-oil capsules contain little or no mercury and variable levels of PCBs and dioxins. Given the small amounts of supplemental fish oil typically consumed (one to three grams a day), exposure to PCBs and dioxins is considered low.

Fish with low mercury content

Catfish
Clams
Crab
Haddock
Herring
Mackerel (except King mackerel)
Oysters
Pollock
Salmon, wild and farmed
Salmon, canned
Sardines
Scallops
Shrimp
Tilapia

TroutSource: Journal of the American Medical Association