We are often warned not to consume too much seafood over fears of the mercury contamination in these foods building up in our systems and leading to mercury poisoning. Pregnant women are advised to limit seafood consumption to just two 6oz-servings per
We are often warned not to consume too much seafood over fears of the mercury contamination in these foods building up in our systems and leading to mercury poisoning. Pregnant women are advised to limit seafood consumption to just two 6oz-servings per week over fears that mercury will cause brain damage to the developing fetus. It certainly sounds scary, but is this concern well founded?
Mercury in Foods
Mercury is present in all foods. Concentrations are quite low in fruits and vegetables because mercury uptake by plants from soil is low. In contrast, mercury levels can be quite high in certain types of fish because fish absorb mercury from the water and from the organisms that they consume. Methylmercury, an organic form of mercury, is the predominant form of mercury in fish. It is concentrated in the muscle of the fish (not in the fat, which is why paying for low-mercury fish oil is quite pointless) and, because it binds so tightly to certain proteins in fish, it accumulates over time. Fish at the lower end of the food chain tend to contain very low levels of methylmercury; but fish that eat other fish tend to have a higher concentration of methylmercury (this process is called biomagnification).
The concern over consumption of methylmercury is that, when ingested, it is almost completely absorbed from the gastrointestinal tract and distributed to all tissues (elemental or inorganic forms of mercury are not easily absorbed and some methylmercury is converted into elemental forms by your gut microflora). Methylmercury also readily crosses both the blood-brain barrier and the placenta. High levels of methylmercury are known to cause damage to the central and peripheral nervous system (the term “mad as a hatter” comes from felt makers in the 18th and 19thcenturies eventually going crazy due to chronic mercury exposure; mercury was used in felt production which was a common material in hats). Yep, this definitely all sounds scary.
However, studies evaluating the effects of seafood consumption on the developing fetus have varied tremendously in their results. On one end of the spectrum is brain damage (albeit subtle) from methylmercury exposure and on the other end of the spectrum is enhanced cognitive ability attributed to a maternal diet rich in omega-3 fatty acids (DHA, in particular). So, should we eat more seafood or less? The answer lies is in the selenium content of the seafood being eaten.
Selenium vs. Mercury
Selenium is a mineral that is required for activity of 25–30 different enzymes (selenoenzymes), whose job it is to protect the brain from oxidative damage. Methylmercury irreversibly binds to selenium. This is bad if you are exposed to methylmercury because it renders selenoenzymes inactive. In fact, this is the mechanism through which methylmercury is believed to damage the brain and nervous system, by inhibiting the ability of selenoenzymes to protect these tissues from oxidants. Very importantly, most typically consumed varieties of ocean fish contain much more selenium than meth