When to Worry About Mercury in Seafood

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Despite the health benefits of the omega-3 fatty acids and vitamins found in fish, you’ve probably hesitated before taking a bite of that tuna sandwich because of one word: mercury.

Every week seems to bring new and alarming information about the dangers lurking in the cuts of seafood that are supposed to be making us healthier. That’s because mercury, a natural element that fish absorb throughout their lifespan, is a heavy metal that’s been proven toxic to humans in excess doses (human activities, like burning coal and artisanal gold mining, have greatly increased the amount of mercury pollution). Due to its effects on the nervous system—particularly in developing children and pregnant women—restricting your diet to just two 6-ounce servings of low-mercury fish per week has been the longtime recommendation of the Food and Drug Administration (FDA).

Is that too conservative? The answer really depends on you—specifically, your genes. Geneticists believe that some people are capable of expelling mercury from the body more efficiently than others. Since it’s the cumulative effect of the metal in the bloodstream that can cause memory, learning, and behavioral problems, the body’s ability to excrete it can have a significant impact on whether a person will develop systems of exposure. This is one possible reason why a bodybuilder can eat tuna with abandon while actor Jeremy Piven allegedly developed mercury poisoning after a diet rich in sushi.

Since genetic testing is not typically part of a normal physical, a sensible and moderate approach to eating seafood is probably going to work for the majority of the population. The FDA warns that larger fish—shark, swordfish, king mackerel, tilefish—are heavy with the metal and should be avoided in any amount; shrimp, salmon, cod, and canned light tuna are generally safe to eat twice weekly.

(Tuna is, however, typically higher in mercury overall, containing 13 to 58 micrograms per four ounces, while salmon and tilapia typically contain 4 micrograms or less. That discrepancy has led some scientists to caution against anything other than very modest servings of tuna during pregnancy.)

The FDA’s rule applies for pregnant women, and similar guidance exists for children, the elderly, and heavy consumers of fish; fully-developed adults are unlikely to suffer ill effects from adding another serving or two, particularly if they maintain a balance by eating a little less fish the following week.  

While an all-tuna diet could prove harmful, making common-sense choices is your best bet for enjoying the benefits of seafood without the worry.