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Food, Health, and Eco-news
What You Need to Know About Seafood and Mercury
Despite the common conception, seafood is generally quite low in mercury. And there are few better sources of wild protein and essential nutrients. 04/15/2021 by Nathaniel Scharping

Seafood, for all of its health benefits, has been dogged for decades by the reputation that it is a significant source of the toxin mercury. Wild-caught fish contain a host of essential nutrients for our bodies, and it’s more environmentally friendly than many other protein sources. However, some consumers still stay away for fear of mercury.

It’s a situation where a small kernel of truth has cast a shadow much larger than is actually warranted. Mercury is indeed toxic, but scientists have shown for years that mercury levels in most fish are too low to cause us harm. Meanwhile, fortunately, official diet recommendations have come to include hefty helpings of seafood, even (and especially) for pregnant women and children.

Recent studies of mercury have also found that the majority of the metal found in our bodies may not come from seafood (Golding J et al., 2013). That aligns with the fact that most of the fish we eat have very little mercury in their bodies in the first place.

Furthermore, research shows that the many health benefits of eating seafood are more than enough to offset inaccurate concerns about mercury levels. Seafood-rich diets are linked to better cardiovascular health, longer lifespans and much more. The omega-3 fatty acids from fish such as salmon, sardines and sablefish are good for brains, joints and eyes, among other things, and fish contain additional nutrients like vitamin D and selenium. In fact, most dietary advice holds that everyone should be eating more fish, not less.

The Truth About Mercury

Mercury exists at very low levels all around us. Volcanoes were once the main source of environmental mercury, but today, human activities like fossil fuel burning and mining operations are the major contributors. Bacteria turn that mercury into methylmercury, which is the form of organic mercury that humans encounter most often.

When ingested in large enough quantities, mercury is bad for our health. It’s challenging to establish the levels at which it becomes toxic for an individual, as variables are many – type of mercury, rate and amount of exposure, and individual susceptibility can all vary. For example, inhaling mercury in the air is far worse for our health than ingesting it through our digestive system.

The effects of mercury poisoning are most commonly seen in gold mine workers and others who are exposed to high levels of mercury through their jobs. And the side effects can be serious, including both mental and physical harms.

Mercury also makes it into our oceans, through both volcanoes and pollution. From there, the metal works its way up the food chain. Mercury bioaccumulates in animals as they eat one another (Ward et al., 2010). That’s why animals such as sharks and whales typically have the highest mercury concentrations. Very long-lived fish have spent more years ingesting and accumulating the element (Burger and Gochfeld, 2004).

It’s worth noting that sharks, whales and very long-lived fish are seldom eaten in the U.S., eliminating a major source of potential mercury ingestion.

Fish Low in Mercury

Smaller, shorter-lived species usually have very low mercury levels, as they don’t live long enough or sit high enough on the food chain to ingest much. This includes the U.S. Food and Drug Administration’s (FDA) “best choice” fish such as cod, sardines and mackerel, as well as scallops and shrimp.

Wild salmon, another best choice, have especially low mercury levels, as their lifespans are relatively short. Salmon typically live between two and seven years, and they spend part of that time in freshwater snacking on small invertebrates. Specific studies back those findings up, including surveys of wild-caught salmon from British Columbia that showed mercury levels well below recommended intake limits (Kelly et al., 2008).

In real-world numbers, this means a pregnant woman would need to eat 53 pounds of salmon or and even more absurd 110 pounds of shrimp in one week before the I.Q. of her child was more negatively affected by mercury than it was improved by seafood intake.

And studies have also failed to find a strong relationship between eating fish and having high levels of mercury in the body. One 2013 study used surveys of dietary habits among over 15,000 pregnant mothers, and compared them to blood mercury levels to assess what foods contributed the most to levels of mercury. The researchers found that just seven percent of the mercury in mothers' bodies actually came from fish (Golding J et al., 2013). While the researchers don’t provide much insight into where else mercury in our bodies come from, they do note that other foods, like herbal teas and rice, can carry mercury.

In addition, we’re all exposed to mercury in the environment around us as well. Most people will test positive for very low levels of mercury in their blood — anywhere from 1 to 8 micrograms per liter. By contrast, clinical interventions aren’t usually recommended until mercury levels are around 40 micrograms per liter or higher (Ye et al., 2016).

Our bodies also expel mercury naturally as our digestive systems process waste. That means that the very low levels of mercury we encounter naturally aren’t typically an issue because our bodies can clear the metal out.

Crystalline selenium
Selenium, abundant in seafood and especially salmon, offers protection against mercury – and wild salmon has little mercury in the first place.

More recent research indicates there’s also another reason we shouldn’t fear mercury in seafood: Fish are high in selenium. Selenium is a lesser-known element that’s commonly found in fish, especially salmon. Our bodies use it for DNA synthesis and protection from oxidative damage and infection (Sunde RA, 2012).

Selenium and mercury have an uncommon attraction to each other — they’ll bind together very readily. Studies show that selenium can effectively “soak up” mercury in our bodies, rendering it harmless (Berry and Ralston, 2008). So, even though most fish don’t contain enough mercury to harm us, their selenium content offers another level of protection anyway.

Health Benefits of Seafood

Meanwhile, studies looking at the health of people who eat a lot of fish also indicate that more fish brings better, not worse outcomes for our bodies. One study examined the cardiovascular health of people who ate higher levels of seafood, and found that their heart health was better than those who didn’t eat much seafood (Mozaffarian et al., 2011). Mercury is known to increase the risk of heart disease — though by a very small amount at low levels. But this research should put our minds at ease, as it confirms that fish intake is positively correlated with heart health. Meanwhile, the heart-healthy omega-3s and other nutrients in fish make them an ideal ingredient for prime cardiovascular health.

In that vein, the FDA recently updated its guidance for pregnant mothers and children concerning seafood. Most moms and kids should actually eat more seafood, the agency concluded, up to 12 ounces a week for pregnant women and eight ounces a week for children. The majority of people in the U.S. fall below those guidelines, the FDA says.

Based on studies showing that omega-3 fatty acids and other minerals and nutrients in fish are essential for children’s developmental health, the federal agency has come out staunchly in favor of seafood for young children and mothers (Hibbeln at al., 2007).

The one warning the FDA does provide for pregnant women and young children concerns four species known to have high levels of mercury. Pregnant women and children should stay away from tilefish from the Gulf of Mexico, as well as shark, swordfish and king mackerel, the FDA says. They should also limit their intake of albacore tuna. (Vital Choice sells albacore tuna. Our supplier provides us - cherry picks, really! - smaller, younger tuna likely to have less bioaccumulation). Instead, these groups should prioritize smaller and younger light tuna, which includes skipjack and yellowfin.

As our understanding about mercury and health has progressed, dietary advice has steadily shifted in favor of more seafood. In the balance, the scales are now firmly placed on fish being not just safe, but essential for robust physical and mental health.

 

Sources:

  • Berry MJ, Ralston NVC. Mercury Toxicity and the Mitigating Role of Selenium. EcoHealth. 2008;5(4):456-459. doi:10.1007/s10393-008-0204-y
  • Burger J, Gochfeld M. Mercury in canned tuna: white versus light and temporal variation. Environmental Research. 2004;96(3):239-249. doi:10.1016/j.envres.2003.12.001
  • Hibbeln JR, Davis JM, Steer C, Emmett P, Rogers I, Williams C, Golding J. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet. 2007 Feb 17;369(9561):578-85.
  • Golding J, Steer CD, Hibbeln JR, Emmett PM, Lowery T, Jones R. Dietary Predictors of Maternal Prenatal Blood Mercury Levels in the ALSPAC Birth Cohort Study. Environ Health Perspect. 2013 Oct;121(10):1214-1218. Epub 2013 Jun 26. doi.org/10.1289/ehp.1206115
  • Kelly B, Ikonomou M, Higgs D, Oakes J, Dubetz C, Canada O. Mercury and Other Trace Elements in Farmed and Wild Salmon from British Columbia, Canada. Environmental Toxicology and Chemistry. 2008;27(6):1361-1370. doi:10.1897/07-527.S1)
  • Mozaffarian D et al. Mercury Exposure and Risk of Cardiovascular Disease in Two U.S. Cohorts. N Engl J Med 2011; 364:1116-1125 March 24, 2011
  • Spiller HA. Rethinking mercury: the role of selenium in the pathophysiology of mercury toxicity. Clinical Toxicology. 2017;56(5):313-326. doi:10.1080/15563650.2017.1400555
  • Sunde RA. Selenium. In: Ross AC, Caballero B, Cousins RJ, Tucker KL, Ziegler TR, eds. Modern Nutrition in Health and Disease. 11th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:225-37
  • Ward DM, Nislow KH, Folt CL. Bioaccumulation syndrome: identifying factors that make some stream food webs prone to elevated mercury bioaccumulation. Annals of the New York Academy of Sciences. 2010;1195(1):62-83. doi:10.1111/j.1749-6632.2010.05456.x
  • Ye B-J, Kim B-G, Jeon M-J, et al. Evaluation of mercury exposure level, clinical diagnosis and treatment for mercury intoxication. Annals of Occupational and Environmental Medicine. 2016;28(1). doi:10.1186/s40557-015-0086-8
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