Photo taken in Kirchen, Germany

That's Metal

Is too much seafood bad for you? New study challenges mercury link

Pass the oysters.

Ana Maria Perez Leal / EyeEm/EyeEm/Getty Images

Rich in omega-3 fatty acids, vitamin D, calcium, protein, and “good fats,” fish can boost our brains and bodies. Still, tuna and other beloved fish are often maligned for the supposed mercury hazard they pose — the idea here is that too much fish could mean ingesting too much mercury, which is in turn linked to potentially fatal heart issues. But a new study calls this widely-accepted public health wisdom into question.

What’s new — In a recent study published Monday in JAMA Network Open, scientists find little correlation between a person’s seafood consumption and blood mercury levels. Further, they find no correlation between high blood mercury levels and cardiovascular disease-related mortality. The study is based on a trove of biobank data involving 17,294 participants in total.

The authors claim this is the first study that estimates the risk of mortality associated with mercury exposure in the general U.S. population.

The study is based on data from a survey begun in the 1960s called the National Health and Nutrition Examination Survey. It is a large-scale, ongoing health study run by the Centers for Disease Control. Participants periodically answer questions about habits and health and medical outcomes. A nationally representative study group, it gives researchers in the U.S. countless health data points to examine.

Tuna is a wildly popular fish dish, but it also may raise blood mercury levels if eaten to excess.Justin Sullivan/Getty Images News/Getty Images

Periodically, the scientists running the survey collect blood samples from the participants that can then be paired with their survey question answers. The blood samples, paired with the participants self-reported dietary information, gave the researchers of this study their dataset.

Why it matters — There are seemingly countless studies showing fish consumption is good for cardiovascular health. The link is partly to do with fish’s aforementioned nutritional qualities, but it is also to do with the fact that fish can be a substitute protein for more harmful meats, like beef.

But mercury exposure is a real danger associated with developmental issues, particularly in the earliest periods of development. To try and limit mercury exposure, the U.S. Food and Drug Administration recommends children, pregnant women, and breastfeeding women avoid eating large predatory fish,including:

  • Swordfish
  • Tilefish
  • Tuna
  • King mackerel
  • Marlin
  • Shark

Small fish and crustaceans are fair game, however. The reason why is to do with how mercury gets in the environment and up the food chain in the first place.

Is mercury in fish toxic?

Mercury is a liquid metal used in several industrial products, including thermostats, gas appliances with pilot lights, fluorescent lamps, and barometers. Coal-combustion electrical power generation also releases mercury as a byproduct.

Once mercury gets into the environment, it has a particularly toxifying presence in aquatic ecosystems. Microorganisms digest mercury into a biologically toxic form of methylmercury, which then spreads through the food chain: Plankton consume the methylmercury; small fish eat the plankton; large fish consume the smaller fish; we consume all the fish.

Mercury poisoning is bioaccumulative, so the bigger the fish, the more mercury that collects within the fish. People living in coastal and oceanic regions with high seafood consumption tend to have higher levels of mercury in their blood.

Eating crustaceans may be both safer in terms of mercury exposure as well as delicious.Melissa Tse/Moment/Getty Images

Like it is in fish, mercury also bioaccumulative in humans. One plate of swordfish probably won’t poison someone, but eating mercury-rich fish over long periods could be harmful. But this study shows the science is not so clear cut.

What’s next — Ultimately, the results don’t rebutt the body of evidence suggesting consuming too much mercury harms heart health, and that one way we consume too much mercury is via fish. But they are interesting when thinking about how to direct public-health efforts around eating meat and heart health.

Eating red meat is strongly linked to cardiovascular problems, like high blood pressure, stroke, and heart attack. Fish, on the other hand, is a key component of well-studied diets, like the Mediterranean diet, which emphasizes eating fish, eggs, and nuts as alternative sources of protein, and is also tied with better long-term health outcomes.

If nothing else, the study highlights the amount of conflicting evidence out there about environmental hazards, food, and health — and the issue of drawing conclusions from correlations.

IMPORTANCE: Although seafood is known to contain heart-healthy omega-3 fatty acids, many people choose to limit their seafood consumption because of fear of mercury exposure from seafood. It is imperative to clarify the potential health effects of current mercury exposure in contemporary populations.
OBJECTIVE: To examine the association of seafood consumption and mercury exposure with all-cause and cardiovascular disease (CVD)–related mortality in the US general population.
DESIGN, SETTING, AND PARTICIPANTS: This prospective cohort study included adults 20 years or older who participated in the 2003 to 2012 cycles of the National Health and Nutrition Examination Survey; data were linked to mortality records through December 31, 2015. Data analysis was performed from January to March 10, 2021.
EXPOSURES: Seafood consumption was assessed through two 24-hour dietary recalls, and mercury exposure was assessed by blood mercury levels.
MAIN OUTCOMES AND MEASURES: All-cause and CVD-related mortality. Multivariable Cox proportional hazards regression was used to estimate hazard ratios (HRs) and 95% CIs of mortality associated with usual seafood consumption and blood mercury concentration quartiles.
RESULTS: This study included 17 294 participants (mean [SD] age, 45.9 [17.1] years; 9217 [53.3%] female) with a mean (SD) blood mercury concentration of 1.62 (2.46) μg/L. During 131 276 personyears of follow-up, 1076 deaths occurred, including 181 deaths from CVD. The multivariable-adjusted HR for an increase in seafood consumption of 1 oz equivalent per day and all-cause mortality was 0.84 (95% CI, 0.66-1.07) and for CVD-related mortality was 0.89 (95% CI, 0.54-1.47). Blood mercury level was not associated with all-cause or CVD-related mortality. Comparing the highest with the lowest quartile of blood mercury concentration, the multivariable-adjusted HRs were 0.82 (95% CI, 0.66-1.05) for all-cause mortality and 0.90 (95% CI, 0.53-1.52) for CVD-related mortality.
CONCLUSIONS AND RELEVANCE: In this cohort study of US adults, seafood consumption and mercury exposure with the current seafood consumption level were not significantly associated with the risk of all-cause or CVD-related mortality. These findings may inform future public health guidelines regarding mercury exposure, seafood consumption, and cardiovascular health promotion.