The current recommendation is to eat fatty fish ≥ 2 times a week because of the beneficial polyunsaturated n-3 fatty acids that it contains. These fatty acids (eicosapentanoic acid and docosahexanoic acid) are beneficial to health because they inhibit the production of pro-inflammatory, carcinogenic, platelet activating eicosanoids. Research has shown that consumption of fatty fish is associated with protection from cancer, heart disease, diabetes and dementias, on account of their anti-inflammatory effects. Consumption of fatty fish may increase insulin sensitivity and prevent obesity. However, fatty fish may also contain mercury, which accumulates in the food chain in the larger predators. This has resulted in some countries recommending that pregnant women avoid or limit fatty fish until after child birth or attempt to select fish that may have a lower concentrations of mercury pollution.
Research published in the American Journal of Nutrition in 20091, investigated the association between consumption of different types of fish and prenatal mercury exposure with birth weight and birth length. They measured the umbilical cord blood of 554 newborn for mercury and recorded the fish consumed by the mother in a food frequency questionnaire. The researchers reported that newborns exposed to the highest amounts of mercury weighed 143.7 grams less than newborns exposed to the least amount of mercury. In addition, those consuming ≥ 2 portions of large oily fish had a higher chance of having a low birth weight baby than those consuming < 1 portion a month, and those consuming ≥ 2 portions of lean fish had a higher chance of having a low birth length baby than those consuming < 1 portion a month.
These results suggest that there are potential deleterious effects from consumption of certain types of fish during pregnancy. Indeed, farmed fish stocks are now known to be heavily polluted with mercury, polychlorinated biphenyls and dioxins, as well as various drugs such as antibiotics that are given to the fish to prevent parasites and infections. Evidence does suggest that wild fish is less polluted, but this is subjects to regional variation. Mercury is found not only in fish, but is also used as a preservative in vaccinations and is also present in amalgam fillings, and both of these routes can increase exposure to mercury compounds. The solution to attaining adequate long chain fatty acids from fish without the pollution is to select organic trusted sources of mercury free fish, or to consume supplements that have been processed to remove the mercury.