After Andrea from Rookie Cookery asked me to comment on the subject, I set out on a research project to separate fact from media hype. I even got my husband, a research scientist well-versed in statistical methods, to aid in the investigation.
The event that made mercury into a “hot button” issue was the Minamata Disaster in Japan. From 1932 to 1968, Chisso Corporation dumped an estimated 27 tons of mercury compounds into Minamata Bay . By eating the fish from the local waters, many inhabitants of that area (including many unborn babies) got severe mercury poisoning resulting in brain damage and sometimes even death. There is no dispute that high amounts of mercury are dangerous to human health. The question is how much is too much?
To find the answer, we dug through a good number of articles. Most just made big scary statements like 1 in 6 American women puts her baby at risk, and 630,000 infants are born every year with unsafe mercury levels . But it wasn’t until we got to the FDA’s site that we started getting some real facts.
Here is a diagram showing the level of mercury in mother's blood in 3 recent studies .
You see the first two large bars. That’s the average amount of mercury in mother’s blood in parts per million (ppm) of two large-scale studies. That’s a lot of mercury compared to the study done in US. But even at those high levels of mercury, the results from the two island studies were inconclusive. Faroe Island study found that this level of mercury had a negative effect on infant cognition; the Seychelles Island study found no effect.
The study in US was done by Harvard Medical School on 135 mothers and their babies in Boston area. The average level of mercury in this group was only 0.55ppm. Did those women eat fish? Yes they did! The average fish consumption during pregnancy was 1.2 servings per week. Since I eat way more than 1.2 servings of fish per week, my next question was “How much does each weekly serving raise my mercury level?” According to the HMS study, each additional weekly serving of fish increases your mercury level by 0.17ppm. So even if you were to eat fish every day, it would be practically impossible for you to reach the mercury levels of the two island studies.
The Boston study found that mother’s fish intake was positively correlated with infant cognition. While fish contains mercury, which is negatively correlated with infant cognition, it also contains “nutrients like iron, vitamin E, selenium, and long-chain n-3 polyunsaturated fatty acids that may benefit brain development.” Here is an excerpt from the study that explains risk benefit trade-off.
Mercury levels vary among different fish species. In general, white meat fish such as cod and haddock tend to have lower mercury levels but also lower levels of long-chain n-3 fatty acid, whereas dark meat fish such as swordfish, mackerel, and other large long-lived predatory fish, tend to contain both more mercury and more n-3 fatty acids. Because mercury and n-3 fatty acids often travel together, it may be difficult to isolate the opposing influences of the two on child cognition.Small fatty fish, such as sardines contain relatively more fatty acids with less mercury. But since no data is currently available on how different fish effect infant cognition, the study suggests that women eat a variety of fish to maximize the benefits and minimize the risks.
Keep in mind that I am not a doctor or a scientist, but here are my 2 cents on this polarizing debate. Fish does not make a good sacrifice to the goddess of fertility. In fact, avoiding fish may be detrimental to unborn children. If you are really looking to ban something, consider fast food. Meanwhile, if you want to do something good for your health and your palate, go to your local fish market and start exploring beyond salmon.
Mercury Levels in Commercial Fish and Shellfish
 Minamata Disaster
 Siera Club -- Overview of Mercury
 Maternal Fish Consumption, Hair Mercury, and Infant Cognition in a U.S. Cohort
Note: this post was updated on Feb. 12, 06.