Mercury in the food chain

October 9, 2015

Fish is nutritious and delicious, but one thing may be preventing you from consuming it more frequently: mercury levels. The following guidelines will explain safe levels of mercury and name the best fish to consume so you can still reap the benefits of this bounty from the sea.

Mercury in the food chain

1. How mercury enters food

Mercury, a heavy metal that enters the atmosphere primarily from coal-burning electric utilities, becomes more toxic when bacteria in lakes and oceans convert it into methylmercury, which fish then absorb into their fat tissues. The bigger a predatory fish — like swordfish — the more methyl­mercury it's likely to harbour.

2. Dangers and effects

Methylmercury is particularly toxic to pregnant women, nursing mothers and young children. Even low-level exposure can affect the developing brain and have neurological and behavioral effects. In adults, dietary methylmercury may also increase the risk of heart disease.

3. What are safe levels?

Seafood is nutritious — a low-saturated-fat source of high-quality protein rich in heart-healthy omega-3 fatty acids — so public health experts are eager to determine the level of mercury in seafood that can be safely consumed. In the United States, the Food and Drug Administration (FDA) considers a safe limit to be one part per million (ppm) of methylmercury.

However, the U.S. Environmental Protection Agency (EPA), which doesn't regulate food, uses a more stringent limit of 0.25 ppm, and in 2003, the FDA's Food Advisory Committee recommended that the FDA adopt that limit. Health Canada's guidelines are in the middle: 0.5 ppm.

  • Based on the current FDA standard, women of childbearing age (especially if they are already pregnant) should avoid shark, swordfish, king mackerel and tilefish, and limit themselves to 12 ounces (340 grams) of any other fish or seafood per week.
  • Health Canada advises all adults to limit consumption of swordfish, shark and fresh or frozen tuna to one meal a week — and for pregnant women, to one meal a month.
  •  In any case, it makes sense to choose seafood sources that are lowest in mercury.
  • Among the lowest in mercury are catfish, flounder, salmon, shrimp, haddock, pollock (used in frozen fish products), sardines, crab and scallops.
  • When choosing canned tuna, select "light" over "white" varieties. If you fish, check local advisories.
  • Trim the fat from swordfish and other high-fat predator fish.
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