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Question: Some types of canned tuna contain pollutants such as heavy metals. Which types of tuna are safe to eat?

Answer: The issue with tuna is mercury, a chemical that naturally occurs 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. Mercury in lakes and oceans is absorbed by fish and stored in their muscles. Larger, longer-living predators, such as tuna have higher concentrations of mercury than smaller fish such as salmon.

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 their pregnancy, it may increase the risk of birth defects and learning disabilities in children. Some, but not all, studies have found associations between a woman's mercury exposure during pregnancy and neurologic test scores during childhood.

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Fish that are higher in mercury include fresh and frozen tuna, shark, swordfish, marlin, orange roughy and escolar. Health Canada advises women of childbearing age and kids, aged one to 11, to limit their intake of these fish to a small portion once per month. Other individuals should consume no more than five ounces once per week.

So back to your question, what about canned tuna? If you eat canned tuna often, choose canned light tuna over canned albacore (white) tuna. Albacore tuna is generally larger, older fish that has accumulated more mercury from its environment. Light canned tuna contains smaller species of tuna such as skipjack, yellowfin and tongol which are lower in mercury.

Health Canada recommends that women limit their intake of canned albacore tuna to no more than 10 ounces or 2.25 large cans per week. Children aged one to four years can eat up to 75 grams or ½ cup per week while older children, aged five to 11, can eat up to 150 grams or one cup of albacore tuna each week.

Send dietitian Leslie Beck your questions at dietitian@globeandmail.com. He will answer select questions, which could appear in The Globe and Mail and/or on The Globe and Mail website. Your name will not be published if your question is chosen.

The content provided in The Globe and Mail's Ask a Health Expert centre is for information purposes only and is neither intended to be relied upon nor to be a substitute for professional medical advice, diagnosis or treatment.

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