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“You can’t eat too much fish when you are pregnant.”

The next time you hear someone say that, you can point out that what they are saying is true, but not in the way they may think. We now know that the more ocean fish that women eat during pregnancy, the better it is for them and their baby. So it is true; you really can’t eat too much fish when you are pregnant! The more ocean fish you eat during pregnancy, the better!

Ocean fish are rich in omega-3 fatty acids, selenium, vitamin D, vitamin E, and iodine that are important for the health of the mother as well as the child. For this reason, the FDA recommends pregnant women should eat up to 12 ounces of seafood per week, an amount equal to two portions that are about the size of a woman’s palm. The biggest and best designed studies have found that children whose mothers ate more than this amount of ocean fish did better on intelligence tests and have greater social and scholastic aptitudes than children whose mothers ate less than two fish meals per week, and children whose mothers ate no fish did the poorest.

But what about the mercury in ocean fish?

The current warnings regarding potential risks of mercury exposures arose because negative effects that were reported in a study conducted in the Faroe Islands. The Faroe Islands lie between Iceland and Scotland with a population that is mainly of Danish descent. Because their mercury exposures are among the highest in the world, this population was one of the first that was studied. However, only a fraction of their mercury exposure comes from eating fish. The majority of their mercury exposure comes from eating pilot whale meat. This is an important distinction since pilot whale meats contain high concentrations of mercury, cadmium, PCBs, dioxins, and various other organic pollutants, but they do not contain the same beneficial nutrients as fish.

Pilot whales are slightly smaller than their killer whale cousins and they occupy the top of the food chain in their part of the ocean. Studies have found that mercury exposures from eating the meats of pilot whales and large sharks can be harmful, but beneficial effects are observed in children whose mothers eat more commonly consumed types of ocean fish. The differences in their effects depend on how much mercury is present in relation to the amount of selenium also present in fish.

Selenium’s role in preventing mercury toxicity was first shown in 1967 and has been under continual study ever since. In fact, to understand any aspect of the seafood mercury issue, you must first understand selenium’s role in the body and the interactions between mercury and selenium.

Selenium is an essential trace element that we need in our diets for numerous processes including brain development and protection from oxidative damage, especially in our brain. Importantly, a pregnant mother needs to supply selenium to her baby in order to meet the needs of her child’s growing brain. Under normal conditions, selenium is very effectively delivered from the mother to her baby. However, high mercury exposures directly interfere with this essential process. We now know that the way mercury causes harm in the body is by binding selenium and preventing it from performing its necessary functions. Since one atom of mercury binds one atom of selenium, it is important to eat foods that contain more selenium than mercury. Fortunately, virtually all foods including almost all seafoods contain much more selenium than mercury. The only seafoods that contain significantly more mercury than selenium are pilot whale and shark meats. These are also the mercury-containing foods that were the basis for the fish consumption advisories.

Most people are aware that whales are not a type of fish, but few realize that our mercury concerns are based on the adverse effects associated with eating whale meat, not ocean fish. In the Faroes study, the more ocean fish that mothers ate during pregnancy, the better protected their children were from the bad effects of eating whale meat. This is because ocean fish are rich in selenium. Although this sounds counterintuitive, because they contain far more selenium than mercury, the more ocean fish a mother eats, the better protected against mercury she and her baby will be.

So, “You can’t eat too much fish when you are pregnant” turns out to be true, but not in the way most people currently think. It is now clear that the more fish you eat during pregnancy, the better it is for you and your child!

Where can I find out more information about this?

Check out the PBS documentary; “Fish, Mercury, and Nutrition: The Net Effects”, available at this website:

Dr. Nicholas Ralston worked in nutrition research for many years and was trained in biomedical research at the Mayo Graduate School in Rochester, MN. For the past 12 years, he has led many research studies to examine mercury–selenium interactions that have been primarily funded by the National Oceanic and Atmospheric Administration (N.O.A.A.) and the U.S. Environmental Protection Agency (E.P.A.).