Mercury is a natural heavy metal that is sometimes released into the environment through industrial processes and that can be incredibly detrimental to human health. Lately, the fact that fish and shellfish can sometimes absorb large amounts of this mercury from the water has become something of a public health scare.

Mercury first builds up in the bodies of sea plants, which are in turn eaten by small fish. Larger fish then eat the smaller ones, thus absorbing their mercury concentrations. This process is called bio-magnification.

Thus, us humans, who will eat pretty much anything that we can catch, are at an even greater risk for mercury build-up, which has led the FDA to suggest that we limit our fish intake to 2-3 servings a week. Unfortunately, it's much harder to remove mercury from a fish than contaminated peel from an apple, since the mercury is dispersed throughout the fish's body.

Of course, that's no invitation to swear fish off. Because of the many health benefits of fish, such as its high protein and omega-3 content, not eating enough fish could be as dangerous as eating too much!

This situation offers a particular paradox for pregnant women. While too much mercury can harm their fetus's developing nervous system, a lack of healthy omega-3s might also produce nervous system disturbances in their infants.

Luckily, all fish are not created equal. Though practically all seafood will contain some amount of mercury, the smaller the animal, the smaller the build-up. Your safest options may be anchovies, sardines, scallops, and shrimp.

On the other hand, larger and longer-lived fish like bluefish, shark, swordfish, fresh tuna, marlin, king mackerel, and tilefish have been found to have the highest mercury concentrations, and should thus be strictly limited or avoided entirely in a healthy diet.

If you do have a tuna craving, a light canned or skipjack version is probably a better choice than ahi or even albacore. Other relatively safe choices include salmon, pollock, cod, catfish, grouper, and snapper.

Farmed fish may be lower in mercury than wild-caught fish, with one study on salmon indicating that wild-caught fish contained up to 3 times more. However, since the risk of contamination isn't eliminated entirely in farmed fish and they can have a slightly different overall nutritional profile than wild-caught, doing a little more research to find out your best option may not be a bad idea.

You should also choose the eel, salmon, crab, and clam over tuna, mackerel, sea bass, and yellowtail when ordering sushi. Finally, be sure to do some research about mercury levels in your area before eating any fish that you or your fishing pals catch yourself.

You might also want to look into a fish's ratio of selenium to mercury, since mercury poisoning works primarily by blocking the effect of selenium. Children and infants are at much higher risk in part because they have yet to build up sufficient selenium stores.

Of course, there are many sources of mercury besides the fishy ones; other potential causes of mercury poisoning include broken thermometers or CFL bulbs, silver dental fillings, certain kinds of jewelry, and imported cosmetics.

Signs of mercury exposure in infants include vision changes and deficits in cognition, fine motor skills, speech and language development and visual-spatial awareness. In older children, they include anxiety, depression, irritability, memory problems, numbness, pathological shyness, and tremors. The sudden onset of any of these symptoms may indicate acute mercury toxicity, which could have permanent developmental consequences.

Adults who have been poisoned by mercury may experience hearing and speech difficulties, lack of coordination, muscle weakness, nerve loss in the hands and face, trouble walking, and permanent brain or kidney damage. Mercury levels that are higher than average but not high enough to constitute an overdose have also been tied to higher blood pressure, an increased risk of heart attacks, and higher “bad” LDL cholesterol.

So if you're a fish fanatic who is experiencing mysterious symptoms, it could be worth looking into a blood test. You should also stay on the safe side of any mercury concerns if you are pregnant, trying to conceive, or breastfeeding.

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