By Christopher Radlicz
Currently, only one-quarter of pregnant women in the United States are eating the amount of fish recommended for optimal maternal and child health (1). On the other end of the spectrum, about 10% of women of childbearing age have higher than recommended blood mercury concentrations (2).
A qualitative study done in 2010 has helped explain why women may be consuming less than the recommended two-servings of fish per week (1). Pregnant women in the Boston area, who under-consumed fish according to these guidelines, were broken into focus groups and discussed what was keeping them from eating more fish. These groups revealed that many women knew that fish might contain mercury, but were naïve of the fact that fish contained beneficial fatty acids, such as docosahexaenoic acid (DHA), which is essential for optimal fetal brain development (3). The women admitted that they hadn’t received advice to eat more fish or more specifically to eat fish that contain lower amounts of mercury and higher amounts of beneficial fatty acids. Instead, the women confessed that they were advised to limit fish intake due to possible mercury exposures.
Are these women misguided in keeping their fish consumption below the recommended two-servings per/week during pregnancy?
This past February, The 2015 Dietary Guidelines Advisory Committee reiterated that women who are pregnant, nursing, or are planning to become pregnant should still consume fish because neither the risk of mercury nor other organic pollutants outweigh the benefits (4). In a recent New York Times article on the issue, Dr. Steve Abrams, medical director of the Neonatal Nutrition Program at Baylor College of Medicine and panel member on the advisory committee commented on tuna consumption, explained, “The benefit of having (omega-3 fatty acids) in your diet really exceeds the likely risk of contamination. The point is that you should have a variety of types of seafood and not limit yourself to one type, and variety includes canned tuna.”(5).
The intake of fish during pregnancy is certainly more nuanced than simple elimination from the diet due to potential mercury exposure. Fish contains essential nutrients proven to have beneficial effects on brain neurodevelopment and may prevent cardiovascular disease (6). These benefits have been attributed in part to the long chain polyunsaturated fatty acids (LCPUFAs), DHA and eicosapentaenoic acid (EPA), but in addition seafood is a good source of protein, selenium, iron, iodine, choline, and vitamins E and D (4,7). These LCPUFAs are essential throughout pregnancy but are critical from the beginning of the third trimester until about 18 months after birth when the human brain is growing the fastest. Neglecting to supply LCPUFA among other nutrients during this period may result in deficits in brain development (9).
That being said, mercury exposure is a real possibility. Methylmercury is the organic form of mercury that we are exposed to when consuming fish in our diet. This form of mercury is relatively stable, mobile within the body, and exhibits a high potential to damage the brain (8). Mercury poisoning outbreaks in Japan are a testament to the wide spread damage that can occur from exposure to methylmercury, resulting in infants born with serious neurological damage, even when mothers were seemingly unaffected (10).
Extreme exposure to mercury in the food chain as in the case of Japan is a rather isolated instance though. But what is the balance? Do the beneficial effects of fish counteract the adverse effects of toxicants?
In the literature, detection of methylmercury exposure in infants is typically done by measuring levels in maternal hair, maternal toenails, cord blood, and maternal blood. These methods are variable in measuring biomarkers and on certain occasion, can be imprecise (5,7). Additionally, many observational studies do not statically control for negative confounders. This means that majority of the cohort studies done in this field have focused on either the risk of methylmercury or on nutrient benefits but not both (7).
All of the complications in study design and analysis show the complexity of establishing recommendations from these studies. Even with good study designs, clear-cut recommendations as to how much fish pregnant women should eat may still not be feasible, due to the variability of toxicants in seafood species. Empowering pregnant women by informing them of seafood that is lower in mercury levels and higher in LCPUFAs is a practical solution. A pertinent rule to apply when making seafood choices is that small sea creatures, which live shorter lives and are lower on the food chain, tend to be the lowest in methylmercury levels and exponentially higher in LCPUFAs. Larger seafood, on the other hand, tends to have higher levels of methylmercury due to a longer lifespan and their higher settlement on the food chain allowing for an accumulation of more methylmercury. The Dietary Guidelines Advisory Committee is in consensuses with the FDA and EPA in advising pregnant women to avoid eating these larger fish, such as swordfish, tilefish, shark, and king mackerel due to their high levels of methylmercury (4,7).
References:
1.Bloomingdale A, Guthrie LB, Price S, Wright RO, Platek D, Haines J, Oken E. A qualitative study of fish consumption during pregnancy. Am J Clin Nutr 2010; 92: 1234-40. doi:10.3945/ajcn.2010.30070
2.Mahaffey KR, Clickner RP, Bodurow CC. Blood organic mercury and
dietary mercury intake: National Health and Nutrition Examination
Survey, 1999 and 2000. Environ Health Perspect 2004 ;112:562–70.
3.Koletzko B, Cetin I, Thomas Brenna J. Dietary fat intakes for pregnant
and lactating women. Br J Nutr 2007;98:873–7.
4.United States Department of Agriculture. Scientific Report of the 2015 Dietary guidelines Advisory Committee. Washington, DC. 2015.
5.Parker-Pope T. (2015, March 2). Should Pregnant Women Eat More Tuna. The New York Times. Retrieved from https://www.nytimes.com. https://well.blogs.nytimes.com/2015/03/02/should-pregnant-women-eat-more-tuna/?r=0
6.Anon. The Madison Declaration on Mercury Pollution. Ambio 2007;36:62–65. [PubMed: 17408191]
7.Choi AL, Cordier S, Weihe P, Grandjean G. Negative Confounding in the Evaluation of Toxicity: The Case of Methylmercury in Fish and Seafood. Crit Rev Roxicol. 2008; 38: 877-893. doi:10.1080/10408440802273164.
8.Clarkson CW, Vyas JB, Ballatori N. Mechanisms of Mercury Disposition in the Body. American Journal of Industrial Medicine 2007;50:757-764.
9.Innis SM. Essential fatty acids in growth and development. Prog Lip Res 1991; 30: 39.
10.Harada M. Minamata Disease: Methylmercury Poisoning in Japan Caused by Environmental Pollution. Crit Rev Toxicol 1995;25:1–24.
11.Hibbeln JR, Davis JM, Steer C, Emmett P, Rogers I, Williams C, Golding J. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet 2007;369:578–585.
