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Mercury in Tuna and Sardines

Q: I eat tuna fish and sardines for lunch at least 4 times per week. Am I at risk for mercury poisoning, and if so, what will happen? How much fish is safe to eat?

Dr. Donnica:
Mercury contamination in fish has existed for centuries, although it has been exacerbated in the past centuries by pollution and other industrial activities. Mercury poisoning is an important, but controversial, issue.  It also varies from state to state and even depends on the body of water.  The groups who should be most concerned are women who are pregnant, breastfeeding, or trying to become pregnant.  According to a US government advisory panel, pregnant women who eat "too much" tuna may expose their unborn babies' brains to possibly harmful levels of mercury.  How much is too much?  The panel advised a limit of two 6-ounce cans of tuna each week if tuna was the only fish eaten or one can if other seafood was eaten that might also contain mercury.  Small children (under 7) are also highly susceptible to mercury toxicity.

Mercury toxicity mainly affects the nervous system. In adults, mild symptoms can include loss of or abnormal sensation in the hands and feet, tiredness, or blurred vision. Severe poisoning involves vision, hearing and speech impairment, and may later result in coma and death.  Long-term exposure to methyl mercury may cause kidney damage.  In children, mercury toxicity may result in decreased brain size, delayed physical development, mental impairment, abnormal muscle tone, and coordination problems.

According to estimates from the Center for Disease Control and Prevention, approximately 8% of American women of childbearing age have enough blood mercury to put their babies at risk for learning difficulties.  Eating contaminated seafood is the main source of mercury exposure.  The extent of contamination not only depends upon the source of the fish, but its size and age: Bigger and older fish eat or have eaten more smaller fish and thus have accumulated more mercury. 

Despite this, fish is considered a highly nutritious food.  Women are advised to eat up to 12 ounces a week of cooked fish, including canned tuna, shellfish and other ocean fish.

Created: 3/8/2003  -  Donnica Moore, M.D.

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