What’s high in protein, low in calories, and rich in nutrients?

If you guessed chocolate, you’re wrong—there’s no protein—I only wish. If you guessed ice cream, you’re wrong—there’s too many calories—I only wish. If you guessed fish or seafood, you’re right—but not exactly what I had in mind.

Fish has always been considered an essential part of a healthy diet, and a recent study conducted in Denmark and published in the September 2008 issue of the American Journal of Clinical Nutrition serves to reinforce that belief. A group of Danish researchers found that children whose mothers ate 3 or more servings of fish per week during pregnancy and who breastfed 6 months or longer were more likely to have better motor and cognitive skills compared to children whose mothers ate 2 or less servings of fish a week and breastfed fewer than 6 months.

In the past, concerns have been raised about whether it’s safe for pregnant and breastfeeding women (and young children) to eat fish, and if so how much. But given this new research, it appears that choosing the right kinds of fish may be more important than limiting the amount of fish.

Fish (like human milk) is often referred to as brain food because it contains lots of omega-3 fatty acids—nutrients that are essential for brain development, particularly in unborn babies and young children. However, fish also contains mercury and other environmental pollutants, that in high amounts, can damage the nervous system, making fish both a healthy choice and a potentially harmful choice where pregnant and breastfeeding mothers are concerned.

In 2004, the Food and Drug Administration (FDA) and the Environmental protection Agency (EPA) revised their guidelines on fish and shellfish intake in an effort to minimize mercury exposure. Recommendations include limiting intake of fish or seafood to no more than 12 ounces per week, avoiding fish known to contain high levels of mercury, and checking local advisories about the safety of fish caught in nearby lakes, rivers, and streams.

Unfortunately, the guidance is not as clear as many would like it to be. According to a 2007 study of 11,875 pregnant women published in The Lancet, children whose mothers ate no seafood during pregnancy were nearly 50 percent more likely to have a low verbal IQ score, compared to children whose mothers ate high amounts of seafood (2-3 servings per week). Researchers also found that lower intake of seafood during pregnancy did not protect children from adverse outcomes. Instead, they found beneficial effects on child development when maternal seafood intake was greater than 340 g per week (the equivalent of 2-3 servings), suggesting that any advice to limit seafood consumption during pregnancy could actually be detrimental. The theory being that the risks associated with the loss of essential nutrients may be greater than the risks associated with exposure to small amounts of mercury (assuming the amount is indeed small).

Finding the balance
Before you grab your fishing pole and head for the nearby lake or stream (or stop by the fish counter at the local grocery), careful consideration needs to be given to what lies beneath those scales.

It’s clear that seafood is an important part of a healthy diet, and that most fish contain some mercury. Depending on how much fish you eat and how often you eat it, you can consume a lot of mercury or a little. To muddy the waters even more, in addition to mercury, fish can also be contaminated with polychlorinated biphenols (PCBs)—chemicals used in the production of electrical equipment. PCBs were banned in the U.S. in the late 1970s, but they are still produced in other countries and stay in the environment for many years.

How did mercury get in the fish in the first place?
Mercury exists in several forms: metallic mercury, inorganic mercury, and organic mercury (methyl mercury, ethyl mercury, phenyl mercury). Nearly all of the research done on mercury toxicity has focused on the organic mercury—methylmercury. Methylmercury and ethylmercury are closely related but they have important differences. Methylmercury has a half life of 50 days and accumulates in the body. Ethylmercury has a half-life of 7-10 days and is less likely to accumulate in the body. Also, ethylmercury is converted in the body into inorganic mercury and excreted in the stool.

Mercury occurs naturally in air, soil, and water. However, the burning of garbage and coal has polluted many of the nation’s lakes and streams. Once the mercury gets into the water system, it is converted into methylmercury and absorbed by the fish. The amount of mercury in fish depends on three factors, (1) the level of mercury in the water, (2) whether the fish is predatory (eats other fish), and (3) how long the fish lives.

Does everyone need to be concerned?
Most adults don’t need to be concerned, because their brains have already developed (a topic for another post). However, mercury can pass from a mother to her baby during pregnancy and while breastfeeding. No one knows how much methylmercury it takes to harm a baby’s brain, but numerous cases of mercury poisoning have been reported along with significant negative health outcomes, both neurological and developmental.

Although most babies whose mothers eat fish during pregnancy are born healthy and develop normally, the less mercury a child is exposed to, the better. It’s important that women who are pregnant or planning to get pregnant and breastfeeding mothers know how much fish to eat and which types of fish are safest. In addition, parents of children 6 years and younger, need to limit the amount of fish their children eat and choose fish that contain the least amount of mercury.

Let’s talk about serving size?
You can start by putting away those dinner plates that serve only to suggest that an appropriate serving size is twice as much as it should be. For adults, a typical serving is 4 to 6 ounces (imagine a deck of cards). Obviously, a child’s serving should be smaller. If you choose to eat larger portions, then you need to eat fish less often. If you’re inclined to eat raw fish (sushi, sashimi or nigiri), know that raw fish can contain harmful bacteria and should be avoided during pregnancy.

Which types of fish are safest?
Theoretically, fish that are commonly eaten by other fish (small fish) and that live a short time are going to have the least amount of mercury, assuming the level of contamination in the water is low. Because there are so many variables and so many differing opinions, in an effort to guide consumers, the National Resources Defense Council has created a list of fish along with their mercury levels (see below).

If you want more detailed information about the fish you eat, check out the mercury thermometer, an interactive tool provided by the American Pregnancy Association.

Group 1:  Fish that contain the least amount of mercury. Eat up to 2 to 3 servings a week.

• Anchovies
• Butterfish
• Calamari
• Catfish
• Caviar
• Clams
• Crab (king)
• Crawfish/Crayfish
• Flounder
• Hake
• Herring
• Lobster (spiny/rock)
• Oysters
• Perch (ocean)
• Pollock
• Salmon (farm raised salmon can contain other contaminants)
• Sardines
• Shad
• Shrimp
• Sole
• Tilapia
• Whiting

Group 2:  Fish that contain low amounts of mercury. Eat up to 6 servings a month.

• Carp
• Cod
• Crab (dungeness, blue, snow)
• Flounder
• Haddock
• Hake
• Herring
• Lobster (spiny/rock)
• Mahi Mahi
• Monkfish
• Perch (freshwater)
• Oysters
• Snapper
• Skate
• Trout (freshwater)
• Tuna (canned chunk light)
• Tuna (fresh Pacific albacore)
• Whitefish

Group 3:  Fish that are high in mercury. Eat no more than 3 servings a month.

• Bass (saltwater)
• Bluefish
• Croaker
• Eel
• Halibut
• Lobster (American Maine)
• Sea Trout
• Skate
• Snapper
• Tuna (canned white albacore)
• Tuna (fresh bluefin or ahi)
• Sea Trout

Group 4:  Fish that are highest in mercury. Do not eat.

• Chilean Sea Bass
• Grouper
• Mackerel (king)
• Marlin
• Orange Roughy
• Shark
• Swordfish
• Tilefish
• Tuna (fresh steaks, sushi)