Should I eat fish if I'm breastfeeding?

Many nursing moms wonder about this.

Even if you don't care for seafood, you've probably heard that fish is a major source of omega-3 fatty acids (specifically DHA and EPA), which are important for a baby's brain and eye development. Fish is also low in saturated fat and high in protein, vitamin D, and other nutrients that are crucial for a baby's health.

On the other hand, you've probably also heard that some types of fish contain contaminants such as mercury. In high doses, this metal is harmful to a baby's developing brain and nervous system.

Because omega-3s are passed to a baby in breast milk, most experts agree that it's a good idea for nursing women to eat some fish. But mercury can also be passed along, so it's important to figure out which fish are safe and how much to eat.

Below, we offer some guidelines to help you limit your exposure to mercury while getting the nutrients you and your baby need.

How does mercury get into fish?

Mercury is everywhere, even in the air we breathe. Some of the sources (such as volcanoes and forest fires) are natural. It's also released into the air by power plants, cement plants, and certain chemical and industrial manufacturers. It's used in making thermometers and thermostats and may be released when those products end up at the dump.

When mercury settles into water, bacteria convert it into a form called methylmercury. Fish absorb methylmercury from the water they swim in and the food they eat. Methylmercury binds tightly to the proteins in fish muscle and remains there even after the fish is cooked.

Almost all fish and shellfish contain some mercury, but large predator fish accumulate the most. That's because predator fish eat other fish – fish that have absorbed mercury themselves. And the bigger the predator fish, the more fish it eats. Larger fish also tend to live longer than smaller fish, so there's simply more time for mercury to build up in their bodies.

What could happen if I eat fish that's high in mercury while I'm breastfeeding?

Your body absorbs methylmercury from fish. While the metal doesn't typically enter breast milk in large amounts, the mercury that does make its way into your milk is absorbed by your nursing baby's body at a time when she's especially susceptible to its effects.

Methylmercury acts as a neurotoxin, which means that it can affect the brain and nervous system. Babies (including those in utero) and young children are most vulnerable to high levels of mercury because their brain and nervous system is still developing.

Experts are still debating exactly how much mercury is harmful, but most agree it's a good idea for breastfeeding women and young children to avoid fish that are high in mercury and to limit (but not eliminate) other fish in the diet.

Why not just stop eating fish?

Fish is too good a nutritional choice to give up, and most experts agree that the benefits of eating fish usually outweigh the risks.

"It's not necessary – or even possible – to avoid all exposure to mercury. There are low amounts that are not harmful," says Charles Santerre, a professor of food toxicology at Purdue University and an expert on contaminants in fish. "If a nursing woman stops eating fish to avoid mercury, her baby may miss out on the healthy nutrients that fish provides."

Studies have shown that getting enough omega-3s in utero and during infancy (either through breastfeeding or supplemented formula) has a positive effect on a child's vision and cognitive development.

Which fish are highest in mercury?

In 2004 the U.S. Food and Drug Administration (FDA) and the EPA released a joint advisory on mercury in fish. In it, they identified four types of fish that women of childbearing age and young children should not eat because they contain high levels of mercury: shark, swordfish, king mackerel, and tilefish.

Other experts and advocacy groups would like to expand this list. Purdue toxicologist Santerre recommends that you also avoid eating all fresh or frozen tuna, striped bass, bluefish, Chilean sea bass, golden snapper, marlin, orange roughy, amberjack, Crevalle jack, Spanish mackerel from the Gulf of Mexico, and walleye from the Great Lakes.

So which fish can I eat?

Different groups have different opinions. The FDA/EPA advisory says it's okay to eat up to 12 ounces a week (two servings) of any fish and shellfish, other than the four you shouldn't eat at all: shark, swordfish, king mackerel, and tilefish. (See special restrictions on canned tuna below.) Other experts take a more cautious approach.

Santerre's group at Purdue publishes a handy wallet card that tells you where most commercial fish falls on the mercury contamination scale and how much of each it's safe to eat.

The card highlights eight fish that are both low in mercury and high in healthy fats: anchovy, herring, mackerel (Atlantic, jack, chub), rainbow trout (farm raised), salmon (wild or farm raised), sardines, shad (American), and whitefish.

Purdue also has free iPhone/iPod apps that can help you track your seafood consumption and estimate your intake of omega-3s, mercury, and PCBs (another contaminant).

Editor's Note: As of June 2014, the FDA and EPA recommend that women who are pregnant or nursing and young children eat between two and three servings each week of low-mercury seafood, such as salmon, shrimp, cod, tilapia, and light canned tuna.

What can I eat, other than fish, to get my omega-3s?

Many foods – such as eggs, milk, soy beverages, juice, yogurt, bread, cereal, and margarine – are now fortified with omega-3s. Some don't contain very much DHA or EPA, but small amounts can add up. (Look for products that contain at least 50 milligrams of DHA per serving.)

You may have heard that flaxseed is a good source, but the only omega-3 fatty acid that plant foods contain is ALA, which has not been proven to provide the health benefits of DHA and EPA.

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