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The question: My seven-year-old son loves tuna sandwiches in his lunch. How much tuna is safe for him to eat?

The answer: For many parents, the tuna sandwich is a lunchbox staple. Nutritionally speaking, tuna is a healthy food. It's an excellent source of high-quality protein, low in saturated fat and a good source of niacin and selenium. It also delivers beneficial omega-3 fatty acids, which have been linked to a lower the risk of heart disease, stroke, arthritis, macular degeneration and Alzheimer's.

The downside, however, is that tuna contains mercury, a contaminant that occurs naturally in the environment and is also released in the air and water from industrial pollution. Once in the water, bacteria convert mercury to methyl mercury, which is then absorbed by fish.

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Larger, longer-living predatory fish such as Chilean seabass, grouper, swordfish and some types of tuna accumulate the most mercury. Since the contaminant is bound to the protein in fish, cooking has little impact on its mercury content.

Albacore (or white) tuna contains more mercury than light tuna (skipjack, yellowfin, tongol) because it's a larger fish that lives longer and, as a result, accumulates more mercury.

It's known that exposure to mercury during pregnancy can lead to learning disabilities in children. Mercury can cross the placenta and build up in the brain of a developing fetus. It can also be passed to infants through breast milk.

That's why Health Canada advises women of childbearing age and pregnant and breastfeeding women limit their intake of fresh or frozen tuna (e.g. tuna steaks, sushi tuna) to 150 grams per month and canned albacore tuna to 300 grams per week, or about two tins worth. (One regular-sized tin of tuna, once drained, contains 133 grams of tuna.)

When it comes to kids, though, there's little data on the health risks of eating high-mercury fish. The question hasn't been well studied. Even so, experts concur that developing children are especially vulnerable to the harmful effects of mercury in the brain and nervous system, and high-mercury fish like tuna should be limited.

But this doesn't mean your tuna-loving son has to give up tuna sandwiches altogether. It all depends on how much and which type he eats. Health Canada's limits for canned tuna apply only to albacore tuna: Kids aged 5 to 11 should eat no more than 150 grams (about one tin) per week; younger children should not eat more than 75 grams (about half a tin) per week.

Canned light tuna is a safer choice. However, since it's considered a medium-mercury fish, some agencies recommend moderation. Toronto Public Health, for example, advises young children eat no more than one tin per week.

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An even safer choice is canned salmon. It's low in mercury and higher in omega-3 fatty acids.

My advice: If you currently make your son's sandwiches with canned albacore tuna, then switch to light tuna. Let him enjoy a tuna sandwich twice a week (about one tin in total). Rotate other healthy proteins into his lunch box, such as salmon, chicken, turkey and lean roast beef.

Leslie Beck, a registered dietitian, is based at the Medisys clinic in Toronto. She can be seen every Thursday at noon on CTV News Channel's Direct; lesliebeck.com.

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