It's a food that's associated with a long list of health benefits: protection from heart attack, Type 2 diabetes, dementia, Alzheimer's disease, macular degeneration, rheumatoid arthritis, even prostate cancer.
Now, a new study suggests there's one more reason to eat fish a few times each week: Doing so may lower your risk of stroke.
It's estimated there are 50,000 strokes in Canada each year, or about one every 10 minutes. The majority – 80 per cent – are ischemic strokes caused when a clot interrupts blood flow to the brain. Risk factors include high blood pressure, high cholesterol, diabetes, smoking, excess alcohol, being overweight and stress.
The remaining 20 per cent are hemorrhagic strokes caused by uncontrolled bleeding in the brain.
The analysis, published online this month in Stroke (a journal of the American Heart Association), combined the results of 15 studies conducted among 383,838 adults. Participants were asked how often they ate fish and were then followed for anywhere from four to 30 years to see who suffered a stroke.
The conclusion: Eating fish three times per week lowered the risk of stroke by 6 per cent. That may not sound like much, but according to the researchers it translates into one less stroke among 100 people who eat fish over a lifetime.
Participants with the highest fish intake – which ranged from twice weekly to every day, depending on the study – were 12 per cent less likely to have a stroke than those who ate the least (less than once per week).
Eating fish a few times a week reduced the risk of ischemic stroke by 10 per cent; fish eaters were also less likely to suffer a hemorrhagic stroke, but this finding was not statistically significant (it could have been a chance finding).
The protective effect of fish is largely attributed to omega-3 fatty acids – eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) – which make the blood less likely to form clots, and help keep blood triglycerides (fat) in check. There's also some evidence that omega-3 fats in fish may have a positive effect on blood pressure.
Other nutrients in fish such as vitamin D, selenium and certain proteins may also play a role.
Not all fish dishes are created equal. One of the studies included in the analysis found that people who ate more fried fish and fish sandwiches did not get any stroke benefit.
Health Canada and the Heart and Stroke Foundation of Canada advise people to consume fish, especially fatty fish, at least twice a week. Fatty fish such as salmon, trout, sardines, herring and mackerel are good sources of omega-3 fats.
Yet concerns over chemical contaminants in fish have left many people confused about what types are safe to eat. Of concern to many people, especially women of child-bearing age, is the mercury content.
Mercury occurs naturally at very low levels in the air, soil, lakes and oceans. It also makes its way into the environment from industries such as pulp-and-paper processing and mining. When mercury enters the water, it's absorbed by fish.
Larger, longer-living fish that eat other fish (such as shark, swordfish) have higher concentrations of mercury in their flesh than smaller fish like shellfish and salmon do. Cooking has little impact on mercury content.
The concern is that mercury can accumulate in the body and may affect the developing nervous system, especially the brain, of infants and young children. Some, but not all, studies have found associations between a woman's mercury exposure during pregnancy and neurological test scores during childhood.
Health Canada advises women who are pregnant or breastfeeding, or who could become pregnant, to limit their intake of high-mercury fish – fresh and frozen tuna, shark, swordfish, marlin, orange roughy and escolar – to 5 ounces per month. Kids aged five to 11 should consume no more than 4 ounces, and one- to four-year-olds should eat no more than 2.5 ounces per month.
Women and children should also limit their intake of canned albacore – also called white tuna – which has a medium mercury content. Canned light tuna – skipjack, yellowfin, tongol – is relatively low in mercury.
That said, it's important for women who are pregnant and breastfeeding to include low-mercury fish in their diet to get omega-3 fatty acids. DHA helps the brain, eyes and nerves develop.
If you don't like fish, consider taking a fish-oil supplement to get omega-3 fatty acids. Note, though, that fish liver oil capsules may not be a good source of DHA and EPA. Check labels for at least 300 milligrams of DHA and EPA combined. Long-term intake of fish liver oil may lead to toxicity due to high amounts of vitamin A.
If you're a vegetarian or allergic to fish, plant-based DHA supplements are available (they're made from algae).
WHICH FISH IS BEST?
If you enjoy fish, use the following guide to help you add fish and seafood to your diet at least twice a week (fish high in mercury have been excluded).
High in omega-3 fatty acids
Anchovy
Herring
Mackerel
Sablefish (black cod)*
Salmon – Atlantic, chinook, coho, pink, sockeye
Oysters, Pacific
Whitefish*
Moderate in omega-3 fatty acids
Bluefish
Blue mussels, chum salmon
Sardines
Striped bass
Trout*
Tuna, white, canned*
Sea bass
Low in omega-3 fatty acids
Catfish*
Clams
Cod, Atlantic
Crab, Dungeness
Flounder
Frozen fish sticks
Grouper
Haddock
Halibut, Atlantic and Pacific*
Lobster
Octopus
Oyster, eastern
Perch*
Pollock, Atlantic
Scallops
Shrimp
Snapper*
Sole
Tilapia
Tuna, light, canned
Turbot
* These fish have a medium mercury content and intake should be limited by young children and women who are pregnant or breastfeeding, or who could become pregnant.
Source: USDA, EPA and DHA Content of Fish Species
Leslie Beck