04/10/2012 04:07 pm ET Updated Dec 06, 2017

Is Mercury in Fish as Harmful as You Thought?

I'm sure you've heard you should be eating seafood twice a week because it's low in calories and fat, and packed with protein, and certain oily varieties -- such as tuna, salmon and sardines -- are a good source of healthy omega-3 fats, which have been shown to improve heart health and your mood.

Related: 5 Top Food Sources of Omega-3 Fats

Healthy Recipes: 5-Ingredient Fish and Seafood Dinners

When it comes to which fish to choose, as a nutritionist and woman "of childbearing age," I've always heeded the advice the Environmental Protection Agency (EPA) gives to kids, women who are, or could become, pregnant and nursing women, to avoid eating shark, swordfish, king mackerel and tilefish, and to limit albacore tuna to 6 ounces a week.

Why? A common concern when choosing seafood is mercury. And for good reason: in unborn babies, infants and children, mercury can impair neurological development. Here's how: mercury binds up selenium, which is an essential mineral that's vital to developing brains and nervous system. In adults, mercury poisoning can hinder neurological function.

Don't Miss: 7 Simple Ways to Detox Your Diet and Your Home

Turns out, though, that eating ocean fish that contain more selenium than mercury protects against mercury toxicity. Ocean fish (i.e., halibut, salmon) and shellfish (i.e., lobster, crab) are chock-full of the mineral: 17 of the top 25 selenium food sources are seafood. (Selenium is also found in red meat, eggs and chicken.) The fact that most ocean fish are so high in selenium explains why more and more research suggests the benefits of eating seafood outweigh any risks mercury exposure could pose.

But do I (and you) really need to be avoiding those fish altogether?

The EPA and the FDA don't have an advisory message on mercury in ocean fish and shellfish for the general population. But kids, women who are, or could become, pregnant and nursing women should still follow the EPA's advice to avoid shark, swordfish, king mackerel and tilefish (they contain mercury levels that can be higher than or equal to selenium). But it appears to be unnecessary for these populations to limit albacore tuna to 6 ounces a week, says Nicholas Ralston, Ph.D., health effects research program leader at the University of North Dakota: "Like most varieties of ocean fish, tuna contain mercury, but provide far more selenium."

Don't Miss: 6 of the Healthiest Fish to Eat (and 6 to Avoid)

3 Tips for Buying the Healthiest Canned Tuna

How often do you eat seafood?

Brierley Wright, M.S., R.D.
Brierley Wright

Brierley's interest in nutrition and food come together in her position as nutrition editor at EatingWell. Brierley holds a master's degree in Nutrition Communication from the Friedman School of Nutrition Science and Policy at Tufts University. A Registered Dietitian, she completed her undergraduate degree at the University of Vermont.

More from EatingWell: