Women who eat as much seafood as the FDA recommends for people who are pregnant or who eat slightly more may be exposing themselves to unsafe levels of mercury depending on the kinds of fish they’re eating, says a new study just published by the Environmental Working Group (EWG). The report calls for more detailed federal guidelines on what types of fish are safe, and in what quantities.

But an industry group has already criticized the study. The National Fisheries Institute, a trade organization representing the seafood industry, released a statement Tuesday decrying the report’s “fear-mongering,” even as other academics supported its basic conclusions.

Mercury contamination in the environment comes from a variety of sources, mainly industrial pollution. Mercury that makes it into water systems and eventually into the ocean can be consumed by small organisms and work its way up the food chain in larger and larger amounts, which is why it tends to exist in the highest levels in large, predatory fish — often the kinds of fish that people like to eat, such as certain species of tuna.

In previous decades, nutritionists have recommended that pregnant women abstain from seafood entirely to avoid exposing their developing babies to harmful mercury. But in the past decade or so, “we’ve seen the nutritional science shift to say that there are benefits to eating seafood,” said the new report’s lead author Sonya Lunder, a senior analyst at the EWG, a nonprofit environmental group with a long history of working on the mercury issue.

The most widely touted of these benefits is the prevalence of omega-3 fatty acids, which are considered essential for human health but can’t be made naturally by the body. There are three types of omega-3s, two of which are found mainly in seafood. Research has suggested that consuming these omega-3s during pregnancy can aid in a fetus’s development, which is the major reason nutritionists now generally give pregnant women a complex recommendation: consume a moderate amount of seafood, adhering to certain federal guidelines to create a safe level of mercury exposure.  

In 2014, the FDA and EPA jointly released a new draft set of guidelines to aid in just that. Overall, for pregnant women and some other groups, the guidelines recommend eating 8 to 12 ounces of a variety of fish each week, and list a number of healthy, low-mercury examples, including salmon, shrimp and light canned tuna, as well as four types of fish to avoid entirely: tilefish from the Gulf of Mexico, shark, swordfish and king mackerel. They also recommend limiting the consumption of albacore tuna to 6 ounces or less per week.

But the EWG’s report suggests that may not be specific enough. A study of more than 250 women of childbearing age who ate approximately the amount of seafood recommended by the federal guidelines found that around 30 percent of them had higher mercury levels in their bodies than is considered safe by the EPA. On average, the participants were found to have mercury levels 11 times higher than those of a control group of women who ate seafood rarely or not at all (though the control group consisted of only 29 individuals).

The results suggest that study participants may not be choosing the most optimal fish for low mercury and high omega-3 intake. The study estimated, for instance, that tuna accounted for about 40 percent of all the participants’ mercury intake — a result that may have been caused in part by the guidelines’ incomplete recommendations when it comes to tuna consumption, Lunder pointed out.

The government recommends light canned tuna — which is usually composed of skipjack tuna — as a healthy seafood choice that’s low in mercury. However, canned tuna comes in many other varieties, some of which include different species with generally higher mercury concentrations. Canned white tuna, for instance, is usually made from albacore tuna, which can have mercury concentrations several times higher than skipjack. 

The importance of differentiating between the different types of canned tuna is not articulated in the guidelines. In fact, Lunder noted, when surveyed many of the study’s participants were unsure exactly what type of canned tuna they’d been eating. Additionally, participants reported eating many other forms of tuna, including tuna steaks and tuna sushi, which often are made from species with relatively high mercury contents. None of these are specifically addressed in the guidelines, either.

In general, Lunder said, the EWG continues to support the recommendation that pregnant women consume more seafood. But, she added, “We think that those recommendations need to be paired with much more detailed information about moderate- and high-mercury species that would pose a risk if you eat them.”

Other experts agree that better information needs to be included in the guidelines — it just needs to be done carefully.

“I think that the recommendations that this group make are reasonable — the challenge is that there’s a trade-off in providing more information,” said Roxanne Karimi, a research scientist at Stony Brook University who has conducted similar research. “Overall, more information is good so that consumers can make decisions on their own, but it can also be confusing, and there’s a concern that consumers will be discouraged from eating fish altogether, even when there’s an overall benefit.”

Sharon Sagiv, an assistant adjunct professor of epidemiology at the University of California Berkeley, noted that the FDA/EPA guidelines already list some specific recommendations when it comes to which fish to avoid and which fish might be better choices. So one question is whether the participants in the study were unclear about some of these recommendations (for example, the recommendations on tuna) or did not strictly heed them. Lunder pointed out that strict adherence to the types of fish recommended was not a requirement for participation in the study, and indeed some women did report eating fish that the guidelines specifically warn against, such as swordfish.

So the issue is not that the existing recommendations are wrong. Rather, the report urges more specific and detailed instructions to consumers that may make it less likely for women to misunderstand the guidelines.

FDA and EPA can put out these recommendations, but if they’re at all complicated in terms of their message that’s a problem because it means that women aren’t necessarily getting effective risk communication,” Sagiv said.

Sagiv has conducted research on the effects of prenatal exposure to both mercury and fish consumption. A 2012 study she co-authored found that low-level prenatal mercury exposure was associated with a greater risk for ADHD behaviors in children, but fish consumption during pregnancy can actually protect against these behaviors. “These findings underscore the difficulties of balancing the benefits of fish with the detriments of low-level mercury in developing dietary recommendations in pregnancy,” she and her colleagues wrote in the paper — a conclusion that aligns closely with the EWG’s new report.

The EWG’s study has not been received favorably by all, however. “Published peer-reviewed science that takes into account the befits of omega 3’s and the risks of mercury together…is accepted and understood as the gold standard,” the National Fisheries Institute’s statement says. “Consumers don’t eat fish with a side of mercury, studying it that way only works to further EWG’s agenda when they don’t agree with the avalanche of research that stands in contrast to the narrative they are pushing to the press.”

Aside from the value of revamping the seafood guidelines, Lunder noted that the report highlights the continued need for policies aimed at reducing mercury pollution. 

In 2013, the U.S. was one of nearly 150 countries to ratify the Minamata Convention on Mercury, an international treaty aimed at reducing mercury emissions worldwide. The report calls for strong and effective implementation of the treaty. Such steps will be necessary to protect both the environment and human health, Lunden noted.

“Since we’ve polluted nature’s perfect food, we now have to look to changing human habits and patterns in order to protect ourselves from these known toxins,” she said. And Sagiv echoed her sentiments.

“If we didn’t have contaminated seafood, we wouldn’t have to advise women not to eat [certain types of] fish,” Sagiv said. “Unfortunately, we’re in an environment where we do have to worry about that, and that risk communication is really very important.”