Mercury and fish: concerns in context

Recent studies that found higher-than-expected levels of methylmercury in fresh tuna bought in grocery stores and at sushi bars provoked a lot of questions. I'd like to put that information in context.

Mercury levels are reversible

According to Marion Nestle, chairwoman of New York University's Department of Nutrition and Food Studies and an expert on food safety, mercury has a "very fast" half-life of 3 months. "Half-life" is the time it takes for a given substance in the body to decrease by 50 percent. If you refrain from ingesting mercury, after three months your level will be halved. Three months later, it will be halved again. In a year, it will have decreased by about 94 percent. "If you don't eat this stuff for a year," Nestle said, "you're clean enough."

Don't stop eating fish

Fish is an important part of any healthy diet, a part that too many Americans leave out. Most fish provides a lean source of protein, and fish is one of the best sources of omega-3 fatty acids, a substance thought to reduce the risk of heart disease and stroke. Cold-water fatty fish, such as salmon, herring, mackerel, anchovies and sardines, tend to have high levels of omega-3. (Outside of fish, flax seed is a potent source of omega-3, as are the eggs, milk and meat of grass-fed animals.)

Pay attention to fish size

Larger fish tend to have higher levels of mercury for two reasons: Since they are older, they have had more time to accumulate mercury; larger fish are usually predators, and by eating smaller fish, they ingest their prey's mercury, too. Small fish, such as sardines, anchovies, herring, catfish and pollock — not to mention shellfish such as shrimp, clams, oysters, scallops and squid — have very low levels of mercury.

Eat a wide variety of fish

Every fish — every food — has benefits and risks. Farmed salmon, for example, has very high levels of omega-3 and low levels of mercury. But it also can contain relatively high levels of contaminants because, instead of eating a wild diet, farmed fish are fed processed food that can contain pesticide residues and industrial byproducts. Wild salmon has much lower contaminant levels, but it also is more expensive and only seasonally available.

The best diets balance benefits and risks by eating many species.