March 2017 Issue

Seafood: Fishing for Answers
By Carrie Dennett, MPH, RDN, CD
Today's Dietitian
Vol. 19, No. 3, P. 16

Can the health halo surrounding selenium help change public perception of seafood?

For many consumers, decisions about whether to eat seafood come after weighing the relative benefits of omega-3 fatty acids and other nutrients found in fish against the risks of methyl mercury (MeHg) exposure. Unfortunately, mercury fears often win, even though the weight of scientific evidence clearly demonstrates that the benefits of fish consumption for neurological and cardiovascular health far outweigh the risks.

The official recommendation for seafood consumption from both the 2015–2020 Dietary Guidelines for Americans and the American Heart Association is to eat fish at least twice weekly—at least 8 oz total—but only 1 in 10 people do.1-3 The average American eats 3.5 to 4.3 oz per week, and that drops to an average of 2 oz during pregnancy—despite the recommendation that pregnant and breast-feeding women should increase fish intake to up to 12 oz per week.4-6

"The public is confused," says Elizabeth Ward, MS, RD, author of Expect the Best: Your Guide to Healthy Eating Before, During & After Your Pregnancy. She says that some health experts still advise pregnant women to be cautious about eating fish. "That is completely contrary to the US Dietary Guidelines' advice. If you don't encourage seafood, then you aren't following the science."

Putting Risk in Perspective
So where did this confusion come from? According to Nicholas Ralston, PhD, a research associate professor in the department of earth system science and policy at the University of North Dakota, part of the confusion about mercury and seafood stemmed from seemingly conflicting results from large studies on the effects of mercury consumption on childhood brain development.7 Ralston and Ward both spoke at the Food & Nutrition Conference & Expo™ in Boston in October 2016.

On one hand, two major studies, one from the Faroe Islands and one from New Zealand, found that children with the highest mercury levels experienced adverse neurological effects. On the other hand, studies in the United States and other countries found increased seafood consumption was associated with higher child IQ, even with similarly high mercury levels.8 Japan and the Seychelles islands consume much more seafood than we do in the United States, and long-term evaluation of study cohorts in the Seychelles finds that, despite eating an average of 12 ocean fish meals per week, there are no consistent adverse effects from mercury exposure.9,10

Ralston says that when researchers dug harder to determine what was driving the inconsistencies, they found the selenium link. Selenium is an essential trace mineral that helps prevent free radical damage to cells, but it's also an essential part of 25 enzymes (selenoenzymes) that protect the brain from damage. Mercury binds strongly to selenium, inhibiting selenoenzyme activity, which can be especially devastating during pregnancy and shortly after birth, when a child's brain is developing rapidly.8,11 On the positive side, selenium has been shown to protect against mercury toxicity in animal studies and in in vitro studies on human cells.8,12 Ralston says this makes mercury levels alone an unreliable measure of risk, and that the Selenium-Health Benefit Value he's helping to develop can better predict the risks associated with high mercury exposures.8,11

In the Faroe and New Zealand studies, the most heavily consumed types of seafood were whale and shark, which are high in mercury and very low in selenium. "That's not what most people eat," Ralston says. To top it off, both countries also were selenium-poor. Even though they also consumed selenium-rich ocean fish, it wasn't enough to compensate. "At the time of the study, New Zealand was one of the most selenium-poor nations on earth," he says. "So throw some mercury at them, and they're going to go down hard and fast."

The United States isn't a selenium-poor nation, and most Americans get the Recommended Dietary Allowance of 55 mcg per day, but even if it was, the bottom line is experts agree that it's much safer to eat fish than to not eat fish. "There's so much selenium in ocean fish that rather than falling behind in your selenium, you get enriched," Ralston says.

Health Benefits
A 2011 report from the United Nation's Food and Agriculture Organization (FAO) and the World Health Organization (WHO) found that for the majority of commercial wild and farmed species, the health benefits of seafood consumption—primarily decreased cardiovascular disease (CVD) risk and improved infant neurodevelopment—outweigh the risks of mercury and other organic pollutants. For all levels of intake, the evidence was in favor of net benefits.13

The FDA says that pregnant women who eat two seafood meals per week could provide their child with an additional 3.3 IQ points by age 9.14 Compared with no intake, eating enough fish to provide about 250 mg per day of the omega-3 fatty acids EPA and DHA lowers risk of death from coronary heart disease by 36%. That's the equivalent of one to two servings of salmon per week, two servings of sardines or albacore tuna, four servings of halibut, or six servings of skipjack tuna.15 Fish consumption appears to lower CVD risk even in populations where adverse effects of mercury exposure have been observed.12

Current Guidelines
While there are many reasons why people don't meet the seafood recommendations, including cost and environmental concerns, consumers have tended to misinterpret fish advisories, choosing to err on the side of caution by eating less fish.5,13,14 Mercury is neurotoxic, and the developing central nervous system is especially sensitive. All fish contain some MeHg thanks to industrial pollution that finds its way into lakes, rivers, and oceans. Large, predatory fish have more mercury because it bioaccumulates.9,16,17

In 2004, the US Environmental Protection Agency and FDA jointly cautioned pregnant or breast-feeding women to avoid four types of fish that are very high in mercury: tilefish, shark, swordfish, and king mackerel. They then recommended that women of childbearing age and young children limit intake to 12 oz of a variety of lower-mercury fish per week.18 These recommendations largely were based on the Faroe Island findings, even though, unlike the subjects in the study, Americans aren't eating whale.7,8 This advice may have pushed already low fish intake even lower.5,7

"We've gone for a very long time with a very negative impression of fish because the FDA and Environmental Protection Agency advisory statement came out and led with the negative," Ward says, pointing out that the four high-mercury species account for only 0.5% of the fish we eat in the United States. As it turns out, these fish aren't just higher in mercury but they're also lower in selenium.8,16

In January, the Environmental Protection Agency and FDA updated their advice, accentuating the positive and deeming the 2004 advice "superceded and no longer in effect." They set a minimum level of intake—at least 8 oz per week—and state that women who are pregnant or breast-feeding (or who may become pregnant) should eat more fish than the current national average to support fetal growth and development as well as maternal health.

In addition, the updated advice recommends that women in these categories limit consumption of certain fish—including albacore tuna—to 4 oz per week. The most recent Dietary Guidelines Advisory Committee supported the draft version of the updated Environmental Protection Agency/FDA guidelines19 but recommended that the agencies reevaluate their stance on albacore tuna, based on the most current evidence on mercury levels from the FAO and the WHO.20 Interestingly, albacore tuna has significantly more selenium than mercury, while bigeye, yellowfin, and skipjack tuna have even better margins.8

What about freshwater fish? The draft advice also encourages consumers to follow fish advisories from local authorities before eating seafood from local streams, rivers, and lakes. Ralston agrees with this guidance because of the potential for serious mercury exposures. "We find that wherever selenium is poorly available in the watershed surrounding a lake or river, more mercury will bioaccumulate in the fish." He says that selenium—and mercury—levels can vary dramatically even between adjacent areas.

Recommendations for RDs
The bottom line is that the official guidelines recommend regular seafood consumption. While dietitians may want to mention the "four to avoid," the reality is that most Americans aren't eating those types of fish—and one in five aren't eating any fish.4 The emerging role of selenium offers some clarification for the divergent findings of major epidemiologic studies regarding the neurological effects of fish intake. While some dietitians may find that detail useful mostly for their own background understanding, others may find that some of their patients are interested in—and reassured by—that information as well.

— Carrie Dennett, MPH, RDN, CD, is the nutrition columnist for The Seattle Times and speaks frequently on nutrition-related topics. She also provides nutrition counseling via the Menu for Change program in Seattle.

1. US Department of Health & Human Services. Dietary Guidelines for Americans 2015–2020: Eighth Edition. Published January 7, 2016.

2. Eating fish for heart health. American Heart Association website. Updated May 15, 2015. Accessed December 12, 2016.

3. Fish and omega-3 fatty acids. American Heart Association website.  Updated October 6, 2016. Accessed December 12, 2016.

4. Jahns L, Raatz SK, Johnson LK, Kranz S, Silverstein JT, Picklo MJ. Intake of seafood in the US varies by age, income, and education level but not by race-ethnicity. Nutrients. 2014;6(12):6060-6075.

5. US Food and Drug Administration, Office of Nutrition, Labeling, and Dietary Supplements, Center for Food Safety and Applied Nutrition. Background package for the November 3–4 2014 Risk Communication Advisory Committee meeting.
. Updated October 29, 2014.

6. US Envrironmental Protection Agency, US Food and Drug Administration. Eating fish: what pregnant women and parents should know. Updated January 18, 2017.

7. Oken E, Bellinger DC. Fish consumption, methylmercury and child neurodevelopment. Curr Opin Pediatr. 2008;20(2):178-183.

8. Ralston NVC, Azenkeng A, Ralston CR, Blackwell JL, Raymond LJ. Selenium-health benefit values as seafood safety criteria. In: Kim S, ed. Seafood Science: Advances in Chemistry, Technology and Applications. Boca Raton, FL: CRC Press; 2014:433-452.

9. Myers GJ, Davidson PW, Strain JJ. Nutrient and methyl mercury exposure from consuming fish. J Nutr. 2007;137(12):2805-2808.

10. Davidson PW, Cory-Slechta DA, Thurston SW, et al. Fish consumption and prenatal methylmercury exposure: cognitive and behavioral outcomes in the main cohort at 17 years from the Seychelles child development study. Neurotoxicology. 2011;32(6):711-717.

11. Ralston NV, Ralston CR, Raymond LJ. Selenium health benefit values: updated criteria for mercury risk assessments. Biol Trace Elem Res. 2016;171(2):262-269.

12. Mozaffarian D. Fish, mercury, selenium and cardiovascular risk: current evidence and unanswered questions. Int J Environ Res Public Health. 2009;6(6):1894-1916.

13. Food and Agriculture Organization of the United Nations, World Health Organization. Report of the Joint FAO/WHO Expert Consultation on the Risks and Benefits of Fish Consumption. Published 2011.

14. McGuire J, Kaplan J, Lapolla J, Kleiner R. The 2014 FDA assessment of commercial fish: practical considerations for improved dietary guidance. Nutr J. 2016;15(1):66.

15. Mozaffarian D, Rimm EB. Fish intake, contaminants, and human health: evaluating the risks and the benefits. JAMA. 2006;296(15):1885-1899.

16. Karimi R, Frisk M, Fisher NS. Contrasting food web factor and body size relationships with Hg and Se concentrations in marine biota. PLoS One. 2013;8(9):e74695.

17. Gribble MO, Karimi R, Feingold BJ, et al. Mercury, selenium and fish oils in marine food webs and implications for human health. J Mar Biol Assoc U.K.. 2016;96(1):43-59.

18. What you need to know about mercury in fish and shellfish. US Environmental Protection Agency website.

19. US Environmental Protection Agency, US Food and Drug Administration. Fish: what pregnant women and parents should know. Updated June 2014.

20. US Department of Agriculture, US Department of Health & Human Services. Scientific Report of the 2015 Dietary Guidelines Advisory Committee. Published February 2015.