Sign up to receive email updates, action alerts, health tips, promotions to support our work and more from EWG. You can opt-out at any time. [Privacy]


Mercury in Seafood

U.S. Fish Advice May Expose Babies to Too Much Mercury

March 16, 2016

Mercury in Seafood: What does science say about seafood safety?

In high doses mercury causes serious nerve damage, seizures and birth defects. In the body, mercury creates tissue-damaging free radicals – highly charged, short-lived reactive atoms – and inhibits cellular repairs, making it harmful to the heart, kidney and other body organs. A number of genes appear to affect a person’s ability to metabolize and excrete mercury, and others appear to increase vulnerability to mercury’s toxic effects.12

But of greatest concern for pregnant women, low doses of mercury can provoke subtle and lasting changes to the developing brain and nervous system. There is strong evidence that mercury exposure during pregnancy and childhood causes lifelong deficits in learning, memory and reaction times.

During pregnancy the fetus' brain and nervous system are developing rapidly. Mercury disrupts the normal process by which brain and nerve cells form, connect and organize.

Because the placenta allows mercury from the mother's blood to pass into the fetus, mercury levels in the fetus and umbilical cord blood are consistently higher than in the mother.

Newborns are also highly vulnerable to mercury. However, mercury levels in breast milk are relatively low, and infants and young children are not typically fed a high seafood diet. As a result, public health experts have focused on pre-conception and pregnancy as key times to limit mercury ingestion.

In 2000 the EPA determined that mercury levels in pregnant women's blood should be below 5.8 micrograms per liter of cord blood to ensure that the fetus was not harmed.13 The guideline of 5.8 micrograms per liter in cord blood corresponds to 3.5 micrograms per liter in the mother’s blood14 and roughly one part per million mercury in hair.15

In the 16 years since EPA set its mercury exposure recommendations, new studies have shown that mercury is harmful at lower doses than previously thought. One reason is that harmful effects of mercury in earlier studies were to some degree masked by the benefits of omega-3s and other nutrients in seafood.16 Newer research accounts for this through isolating and independently examining the benefits of omega-3s and the harm of mercury exposure to early childhood cognition, memory and attention.

Most studies find negative effects of mercury exposure in utero, and positive outcomes for children whose mothers eat more high omega-3 seafood or seafood in general. The task is complicated by the fact that each study includes women from different regions of the world, eating different amounts and varieties of seafood that vary in contaminants and beneficial nutrients.

The primary benefits of seafood are thought to come from the omega-3 fatty acids, DHA and EPA. They are a major component of cell membranes in the brain, nerve cells, retina and heart. A diet rich in omega-3s can lower blood triglycerides, and lessens the risk of cardiac disease. Omega-3s also reduce inflammation. Researchers are looking at them to treat diseases like arthritis, macular degeneration of the retina and some mental disorders.

One of the most relevant studies of the effects of seafood consumption for American babies was Project Viva, headed by Dr. Emily Oken of the Harvard Medical School and Harvard Pilgrim Health Care.17 It looked at the relationship between women’s seafood intake and mercury exposure during pregnancy and their children’s intelligence, memory, and motor development at six months and three years.

Oken’s team found a benefit to children whose mothers ate seafood frequently during pregnancy, as long as they kept their mercury levels low. Women who ate more than two seafood meals weekly had kids with better neurological development, while children whose mothers had the highest mercury levels showed subtle neurological deficits.

In Oken’s study, the benefits of eating enough seafood and the harm of ingesting too much mercury during pregnancy were roughly equivalent, with each causing about a 10 percent increase or decrease in children’s memory and visual-motor skills. The research team concluded that children benefit most when mothers eat a substantial amount of seafood, but of species with low mercury. Nearly a dozen studies published since have confirmed and reinforced that conclusion.18

Similar findings were reported in a study with more than 400 women and children from New Bedford, Mass. Women with hair mercury concentrations greater than one part per million when they gave birth had children with increased risk of inattentive and impulsive or hyperactive behavior at age 8. Children whose mothers ate more than two servings of fish a week during pregnancy had a much lower risk of ADHD.19

Another study of children’s visual memory and learning found that every part per million increase of mercury in their mothers’ hair during pregnancy was associated with about a 2.8 point decline in visual memory and 2.2 point decline in verbal memory, after adjusting for estimated omega-3 ingestion.20

However, not all studies report the same effects.

A 2016 study by Jordi Julvez and colleagues, of the ISGlobal Center for Research in Environmental Epidemiology, examined Spanish mothers and children with high seafood-high mercury diets. The study found that greater seafood consumption was generally associated with kids’ better scores on neurodevelopment tests at 14 months and 5 years. The benefits were mixed and declined somewhat for women who ate the most fish. Yet this relationship remained when the authors looked at women who primarily eat higher mercury fatty fish like tuna and swordfish.

However, the researchers noted that this finding doesn’t invalidate previous studies that find mercury to be harmful. Spanish women with higher socioeconomic status tend to eat more seafood, exposing them to more mercury, so other factors could contribute to their kids’ higher scores.21

In 2012 Julvez and his colleagues reviewed results from 27 large epidemiological studies. Many were of people in cultures that eat much more seafood than Americans, but four were of communities with diets comparable with the U.S. The authors concluded that the “majority of the publications describe neurodevelopment impairments, particularly when the exposure was measured during pregnancy.” They also noted that “in some populations, nutrients from fish and seafood seemed to counterbalance the real extent of developmental neurotoxicity due to methylmercury.”22

Phillipe Grandjean of Harvard, described how this balance works:

When you eat seafood during pregnancy, you get the benefits from omega-3s but from mercury you have the risk of toxicity If you get a little bit of mercury it can be offset by the omega-3s. But that means you don't get the full benefit of the omega-3s and other nutrients in seafood. So women should minimize mercury exposure because only then will they get the maximum benefit of seafood.

Numerous researchers have identified fish species that women should limit or avoid due to high mercury or a poor ratio of omega-3s to mercury:

  • Gary Ginsberg of the Yale School of Public Health and the Connecticut public health department drew on Oken's work to examine 16 common species. He estimated that nine species would, on balance, harm infants if their mothers ate six ounces a week during pregnancy: shark, swordfish, yellowfin tuna, canned albacore tuna, lobster, sea bass, halibut, cod and canned light tuna.23
  • Katrina Smith and Jane Guentzel of Coastal Carolina University identified ahi tuna, sea bass, halibut, snapper, cod and canned mackerel as species with mercury risks that outweigh omega-3 benefits.24
  • Kathryn Mahaffey of EPA used data developed by Amy Tsuchiya of the University of Washington to identify additional species – carp, monkfish, sturgeon, marlin, flounder and canned light tuna – as fish too high in mercury with too little omega-3s to be healthy during pregnancy.25
  • Marco Zeilmaker and his colleagues at the National Institute for Public Health and the Environment in the Netherlands concluded that for people who eat fish frequently, the negative effects of mercury outweighed the benefits of omega-3s for 70 percent of the species they studied.26 They suggested that women should avoid high mercury fish for up to a year before conceiving.

Clearly the science supports nutritional policies that encourage consumption of high omega-3 seafood. Research that aims to identify and promote consumption of the healthiest species is more scarce.

Emily Oken developed a simple handout to advise women about the amount and species of fish they should eat during pregnancy.27 She found that pregnant women who got the handout raised their level of consumption of the essential omega-3 DHA and kept their level of mercury low. Participants ate even more DHA when they were given a gift card to a grocery store that could be used weekly to buy healthy seafood.

Some researchers and advocates, concerned about contaminants in seafood and the sustainability of fisheries, suggest that people seek out vegetarian sources of omega-3 fatty acids. However DHA and EPA are only found only in fish, shellfish and some algae-derived supplements. Nuts and vegetables, including canola oil, soybean oil, flax seeds, chia seeds and walnuts contain a third type of omega-3 fatty acid known as ALA (alpha-linolenic acid). The body can convert this fatty acid to EPA and then DHA, but in most people only a small part is converted, making these foods a less effective source of the omega-3s essential to a healthy pregnancy.