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Fish is great brain food. Minimize its contaminants by choosing the right kinds.

By Leslie Beck Posted Jan 28, 2009

Eat fish for your brain, but choose it wisely.

Health professionals have long praised the nutritional virtues of fish and advise Canadians to eat two servings per week to help guard against heart disease. Two new studies reported last week suggest eating fish can keep your mind sharp, too.

Fish, high in protein and low in artery-clogging saturated fat, contains nutrients that may benefit the brain such as iron, vitamin E, selenium and omega-3 fats. But fish also contains contaminants. To reap the benefits of fish and minimize exposure to harmful chemicals, choose wisely.

In one study, posted on the website of the Archives of Neurology, researchers followed 3,781 Chicago residents, 65 and older, for six years and found that those who ate at least one fish meal per week had a slower decline in mental function — about 10 per cent less per year — than their peers who didn’t eat fish as often. Eating fish at least twice a week was linked with a 13-per-cent slower rate of cognitive decline, a benefit researchers liken to being three to four years younger in age.

The two omega-3 fats in fish, DHA and EPA , are thought to keep the brain healthy by reducing blood fat levels, slowing plaque growth on artery walls, and reducing inflammation. DHA is a large component of the brain and the retina of the eye and is crucial for proper brain and eye development during fetal life and infancy.

Methylmercury

But fish also contains methylmercury, a chemical that can disrupt normal brain development and cognitive function. Mercury occurs naturally in very low levels in soil and water, but makes its way into the environment through industries such as pulp and paper processing and mining, and the burning of fossil fuels.

In water, mercury is converted to methylmercury, which fish and shellfish absorb. Larger, older fish have higher levels because they accumulate more over time.

When we eat fish, methylmercury can build up in our bodies and affect the nervous system; it’s especially harmful to infants. Consuming too much methylmercury before and during pregnancy, or during the breastfeeding period, could cause birth defects and learning disabilities. Effects can include decreased intelligence, delays walking and talking, lack of co-ordination, blindness and seizures. (Evidence for mercury’s effect on adults is less clear-cut. A recent study in the Journal of the American Medical Association found no compelling evidence higher blood-mercury levels were associated with worse cognitive performance in 474 healthy adults 50 to 70 years of age. However, extreme mercury exposure can lead to personality changes, tremors, vision changes, deafness, loss of co-ordination, memory loss, intellectual impairment and death.)

In a study published last week, researchers from the Harvard Medical School in Boston evaluated fish consumption during pregnancy and its effect on fetal brain development among 135 mothers and their infants. The higher a woman’s fish consumption during the second trimester of pregnancy, the better her six-month-old performed on a test that measured visual memory. However, greater fish consumption was also linked with higher mercury levels (measured in hair samples), and infants of mothers with higher mercury levels had lower cognition scores.

It might seem contradictory that eating fish raised mercury levels and higher mercury levels led to poorer mental capacity scores, yet at the same time higher fish consumption was linked with better cognition. The implication here is that fish can be a brain-healthy food for pregnant women, and their infants, only if the women consume fish low in mercury.

So the good news is it’s possible to reap the health benefits of fish and minimize exposure to contaminants, provided you choose the right fish.

In 2002, Health Canada issued an advisory recommending Canadians limit their intake of shark, swordfish and fresh and frozen tuna to one meal per week; once a month for pregnant women, women of child-bearing age, and young children. Health Canada says canned tuna is safe because it contains smaller and younger fish that have lower mercury levels than tuna used for steaks.

In 2004, the U.S. Food and Drug Administration and the Environmental Protection Agency jointly issued a tougher fish advisory recommending that women who may become pregnant, pregnant and breastfeeding women, and young children completely avoid king mackerel, shark, swordfish and tilefish. American women are told to eat up to 12 ounces (two average meals of 170 grams each) of low-mercury fish per week, and no more than six ounces (170 grams) should come from canned albacore (white) tuna. U.S. studies have found that white-style tuna contains higher levels of mercury than light-style (skipjack, light, or chunk light).

It’s understandable if mixed messages about fish have left you wondering whether you should add it your weekly diet.

Fish is an important part of a healthy diet and can be enjoyed safely. I encourage you to add it to your menu twice a week. And while we all should limit our intake of high-mercury fish, women and young children need to be especially vigilant.

Mercury levels in fish

High levels

  • Grouper
  • Mackerel (king)
  • Marlin
  • Orange roughy
  • Shark
  • Swordfish
  • Tilefish
  • Tuna steaks
  • Tuna, canned albacore

Low levels

  • Anchovies
  • Catfish
  • Clams
  • Cod
  • Crab
  • Haddock
  • Herring
  • Lobster
  • Oysters
  • Perch
  • Pickerel
  • Pollock
  • Salmon (canned/fresh/frozen)
  • Sardines
  • Scallops
  • Shrimp
  • Squid
  • Tilapia
  • Trout
  • Tuna, canned light
  • Whitefish

Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV’s Canada AM every Wednesday. Visit her website at lesliebeck.com.

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