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Mind your seafood: mercury in cooked fish and sushi

By Jane Harrison, R.D., Staff Nutritionist, myOptumHealth

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Lisa enjoys eating lunch at the local sushi place every Monday, Wednesday and Friday. But aside from filling her quota of fish for the week, Lisa may unknowingly be getting an unhealthy dose of mercury along with her omega-3s.

With advice to eat more fish for heart health, it's more important than ever to get the facts straight. The number one source of mercury contamination in our diet comes from seafood.

How mercury gets into fish

Mercury is a metal that is naturally found in low levels in soil and rock. It is also released into the air, water and land from pollution, unsafe waste disposal or erosion.

When mercury gets into water, bacteria can change it into a toxic form called methylmercury. This is then absorbed by tiny organisms. When freshwater and ocean fish eat these organisms, the mercury builds up in their bodies. When larger fish eat smaller fish, mercury can build up to even higher levels.

Why is mercury dangerous?

Mercury mostly affects nerve cells in the brain and spinal cord. Unborn babies and young children are especially at risk.

• Mercury passes easily through the placenta. It can cause birth defects, brain damage, blindness, seizures and miscarriages.
• Mercury is also passed through breast milk, raising the risk of delays in brain development.
• Children who eat too much mercury may develop problems of their nervous and digestive systems, as well as kidney damage.

Adults can suffer headaches, fatigue, numbness in the hands and feet, and a lack of concentration. Some studies suggest that men also face an increased risk of heart attacks.

Limited data

There is no federal testing program for mercury. And numbers can vary greatly based on size, age, location and type of fish. Scientists can only estimate the contamination levels based on limited samplings.

The FDA has set a maximum permissible level of 1.0 part of methylmercury in a million parts of seafood (1 ppm).They have issued warnings for canned albacore tuna, which averages about 0.35 ppm in the agency's testing. Though no other warnings were issued, fresh tuna (bigeye), grouper and mackerel all had higher levels of mercury than albacore tuna.

Guidelines for eating fish

FDA scientists responsible for seafood safety say that large predatory fish are safe to eat, as long as you don't eat them too frequently - not more than once a week. These include:

• Shark
• Tilefish
• King mackerel
• Swordfish

Sushi guidelines

Researchers found the types of tuna used for sushi contain three times the mercury of fresh tuna sold as steaks or fillets. With that in mind:

• Avoid or limit tuna to once a week. Tuna routinely ranked the highest in contamination out of all the sushi fish, especially bluefin and bigeye.
• Choose low-mercury sushi sources. This includes squid, trout, flatfish, octopus, sea urchin roe, salmon, scallop, crab, eel and clam.

Guidelines for small children, pregnant and nursing women

The FDA advises this group to avoid all high-mercury fish. In addition:

• Albacore ("white") tuna has more mercury than canned light tuna. Experts advise up to, but no more than, six ounces of albacore tuna per week.
• You may safely eat up to 12 ounces (about two servings) of seafood that is low in mercury per week. This includes salmon, cod, cooked shellfish, canned light tuna, pollock, haddock, tilapia and catfish.
• Check before eating fish caught in local waters. If you're not sure about the safety of a local fish, check with your local state health department.

Surf's up

Here's the good news. Experts still conclude that the benefits of eating seafood outweigh the harm if you limit your intake of the riskiest choices. Though all fish have trace levels of mercury, most do not contain enough to pose a hazard to human health.

The following FDA advisory for women and children contains more detailed information on mercury levels in most species of fish: Advisory on Mercury in Seafood, found at http://www.fda.gov/Food/FoodSafety/Product-SpecificInformation/Seafood/FoodbornePathogensContaminants/Methylmercury/ucm115662.htm.

View the original Mind your seafood: mercury in cooked fish and sushi article on myOptumHealth.com 

SOURCES:

• Agency for Toxic Substances and Disease Registry. ToxFaqs for mercury. Accessed: 06/03/2009

• Consumer Reports. Safer sushi. Accessed: 06/03/2009

• U.S. Environmental Protection Agency. What you need to know about mercury in fish and shellfish. Accessed: 06/03/2009

• Institute of Medicine. Seafood choices: balancing benefits and risks. Accessed: 06/03/2009