Fish gets sold as a health food with no upper limit, but the two federal agencies that regulate what ends up on your plate do set one. The FDA and EPA jointly recommend two to three four-ounce servings a week of lower-mercury fish, and no more than that.
They also name seven species that should not be eaten at all, and one popular fish (canned albacore tuna) that carries a stricter one-serving-a-week cap. Most people who order swordfish or ahi at a restaurant have no idea the guidance exists.
The three-tier chart most shoppers have never seen
The joint FDA and EPA advisory sorts more than 60 fish and shellfish species into three tiers based on average mercury content. The Best Choices tier covers the lower-mercury staples most Americans already buy, including salmon, shrimp, canned light tuna, catfish, cod, tilapia, sardines, pollock, scallops, and clams. This is the tier the agencies recommend eating from two or three times a week.
Good Choices sit at a middle mercury range and cap at one serving a week. That group includes yellowfin tuna, halibut, mahi mahi, snapper, sea bass, monkfish, grouper, and canned albacore tuna.
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Choices to Avoid are the seven species with the highest mercury loads. According to the joint FDA and EPA advisory, they are shark, swordfish, king mackerel, bigeye tuna, marlin, orange roughy, and tilefish.
The EPA states its own version plainly, telling the public to focus on lower-mercury species and to hold back on higher-mercury ones.
What "too much" actually looks like
A four-ounce swordfish steak counts as a single serving. Eating one twice a week is a common restaurant habit, but the advisory places swordfish in the do-not-eat tier for anyone who is pregnant, breastfeeding, or feeding young children, and recommends sharp limits for other adults too.
Albacore tuna is the everyday version of the same problem. It carries more mercury than canned light tuna, and the joint advisory limits it to one serving per week. Two albacore sandwiches in a week already puts an adult past the ceiling.
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Pregnant and breastfeeding women, and adults trying to conceive, are advised to eat two to three four-ounce servings a week from the Best Choices list only, or one serving from the Good Choices list. Children get smaller portions, and no more than one to two servings a week.
What mercury does inside the body
Mercury from fish takes a specific form called methylmercury. It accumulates in tissue over time and is not easily cleared. Its main target is the central nervous system.
A study of a fish-dependent Amazonian population found that adults with chronic methylmercury exposure developed numbness or tingling, loss of coordination, slurred speech, changes in the visual field, and hearing impairments, with cognitive changes appearing at lower exposure levels than the motor ones. In the same population, cognitive alterations were seen in 17 percent of people with hair methylmercury below 10 micrograms per gram, and in 36 percent of those at or above that level.
An NIH clinical reference on methylmercury toxicity, drawing on Japan's Minamata Bay outbreak, describes prenatally exposed children as showing intellectual disability, primitive reflexes, cerebellar ataxia, physical growth disturbances, slurred speech, and limb deformities. The developing brain is the most vulnerable target of all.
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The bottom line for readers who eat fish weekly
The two- or three-servings-a-week ceiling is not a suggestion reserved for a rare treat. It is the outer limit for the fish the FDA and EPA consider safest. Someone who eats fish daily, or who leans on high-mercury species like swordfish and tuna steak, is likely well past the guidance the agencies actually publish.
Anyone who is pregnant, breastfeeding, feeding a young child, or has unexplained neurological symptoms should raise mercury exposure with their doctor.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.