Possible health concerns revolving around mercury levels in fish tend to come up here and there, but most people don’t really know what it really means — or they mistakenly think it’s a myth. But, recent research shows that levels of the toxic metal have continued to rise, leading the United Nations to adopt the first legally binding international treaty on reducing mercury emissions last week. In other words, mercury is a way bigger issue than you might realize.
“Methylmercury (the organic form of mercury) is a known neurotoxin," says Elsie M. Sunderland, professor of aquatic science in the department of environmental health at Harvard School of Public Health in Boston. "Exposure in adults has also been associated with negative effects on cardiovascular health, such as increased heart attacks in adult men.” Basically, that means it can pass the blood/brain barrier and interfere with your neurological system. Why is it typically more tied to moms-to-be? “The developing fetus is the most sensitive life stage for exposure to methylmercury, and that is why the safety standards like the reference dose from the Environmental Protection Agency (EPA) are based on these health outcomes,” Sunderland says.
Even if you aren’t expecting, some studies show that consuming high-mercury fish on a regular basis (as in, say, three times a week for 10 years) can have catastrophic neurological side effects over time. Celia Chen, a research professor in the department of biological sciences at Dartmouth College in Hanover, New Hampshire and the lead author of the recent report Sources to Seafood: Mercury Pollution in the Marine Environment notes that while the studies are not consistent, mercury has been shown to decrease fine motor speed and dexterity, memory and response ability, and neuropsychiatric symptoms of depression, anxiety, and compulsive behavior in women.
So, how does mercury even get in the fish in the first place? The two biggest sources of mercury globally are coal-fired power plants and artisanal gold mining (mostly occurring in developing countries). “Mercury is a byproduct of both of these — it’s cast out into the atmosphere, transported around the globe, and then ends up in the oceans (and the fish we eat), or is discharged from sources to rivers from gold mining and industrial sources,” says Chen. Next, it gets into lakes, estuaries and the ocean, where this inorganic mercury is converted into methylmercury by microorganisms.
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Why did the United Nations Environment Program step in to help curb mercury emissions? Because mercury is considered a global pollutant, and it stays around for a very long time. “It’s an international issue because these contaminants go across international boundaries when they are atmospheric,” says Chen. “And when it’s in the sediment, that raises a whole new issue because maybe you can control the sources, but how do you control what’s in the environment in contaminated areas?” Sadly, it’s a man-made problem. “Inorganic mercury occurs naturally, but human activity since antiquity has released very large amounts into the environment. We think this accounts for the majority of mercury present in most aquatic ecosystems now,” explains Sunderland.
Luckily, not all seafood has high mercury levels. “Fish that have the highest mercury levels tend to be predatory and eat other fish as opposed to plant matter or invertebrates,” explains Sunderland. The best low-mercury fish are closer to the bottom of the food chain. Think: sardines, salmon, tilapia, flounder, trout, herring, and mackerel (except king mackerel). Fish to avoid are those at the top of the ocean pecking order, such as shark, swordfish, king mackerel, tilefish, and tuna. (Tuna steak and sushi is especially high in mercury, and albacore has three times more mercury than light canned tuna).
But here's the catch-22. There’s also plenty of research that backs up the fact that eating fish as part of a balanced diet is very important, and that moms who ate two or more servings per week of low-mercury concentrated fish had children who did better developmentally than those that didn’t. Seafood is awesome from a nutrition standpoint: it’s rich in EPA (aka eicosapentaenoic acid) and DHA (aka docosahexaenoic acid), a type of long-chained omega-3 fatty acid and the building block of nerve tissues in the brain and eyes (which mostly develops in your childhood years), plus it’s high in protein and low in fat. So, all this mercury talk in no way means you should skip the surf options at your local eatery. “Since seafood is generally a health food choice — it’s high in protein and essential nutrients — the best advice is to eat mostly fish that’s low in mercury,” says Sunderland.
The bottom line: while we should all continue to keep watch on our seafood intake, it still seems that with a little planning, the benefits of a few servings of seafood a week outweigh the risks.
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