Should I Worry About Mercury in Seafood?

Should I Worry About Mercury in Seafood?

There seems to be a lot of negative press regarding health concerns with respect to high levels of mercury and fish consumption. In fact, pregnant women are sometimes even advised to limit seafood consumption to just two 6-oz servings per week over fears that mercury will cause brain damage to the developing fetus. But, are these concerns based on sound science? What’s the deal, are we all at risk of mercury poisoning with every serving of fish consumed?

The short answer is no, you don’t need to worry about eating fish!

As for the long answer, to understand why mercury in seafood isn’t a problem, it’s important to recognize what the mineral selenium does in our bodies. We have roughly 30 selenium-dependent enzymes (aka selenoenzymes).  These are enzymes that have a selenium atom as part of their molecular structure. The two most important roles for selenoenzymes in our bodies are (1) in the production of thyroid hormones and (2) as antioxidant enzymes, meaning they combat the damage of oxygen radicals (aka free radicals or reactive oxygen species). In general, antioxidants are very important for our health, and several selenoenzymes are specifically responsible for protecting our brain against oxidative damage.

What does selenium have to do with mercury?

Mercury irreversibly binds to selenium.  This is bad if we’re exposed to mercury because if it binds to one of our selenoenzymes, it renders it inactive such that it is no longer effective as an antioxidant and no longer able to protect our brains from oxidative damage. In fact, this is the mechanism through which mercury is believed to damage the brain and nervous system: by inhibiting the ability of selenoenzymes to protect these tissues from oxidants, which is why the effects of mercury can be so severe.

If the effects of mercury exposure are bad for us, why don’t we need to worry about the mercury content in seafood?

Seafood contains methylmercury, which is an organic form of mercury and a potent neurotoxin, and is produced by certain bacteria in aquatic environments when they encounter the pollutant -inorganic mercury. Methylmercury bioaccumulates up the food chain in higher concentrations and is actually a lot worse than inorganic mercury for us since it’s more readily absorbed into our bodies.

You might be thinking that this doesn’t sound good, but it just so happens that fish and shellfish are some of the best food sources of selenium (isn’t Mother Nature amazing?)!  Nearly all fish have way more selenium in their tissues than methylmercury they’re exposed to.  This is great news for the fish who don’t die from the amounts of mercury they are exposed to, but even better news for those of us eating the fish since the majority of methylmercury in the fish is already bound to selenium.  It’s harder for us to absorb selenium-bound methylmercury and the little we do is already bound to selenium and therefore unable to interfere with our own selenoenyzmes.

How is selenium and mercury in fish monitored?

The selenium health benefit value is a measurement of the ratio of selenium to mercury in fish, and is used for biomonitoring (tracking the mercury pollution problem).  It shows us that nearly all seafood is safe to consume. There is only a handful of fish species, mostly top predator larger fish, considered high-mercury fish, meaning that they build up more methylmercury than selenium.  These include shark, pilot whale, king mackerel, marlin, tarpin, tilefish, and the data is mixed on swordfish, so swordfish would ideally be consumed only occasionally to be on the safe side. 

The vast majority of ocean fish and approximately 97% of fresh water fish have lower levels of mercury and/or contain enough selenium to protect against its effects. Safe seafood varieties include but are not limited to: albacore tuna, anchovies, Atlantic mackerel, bigeye tuna, canned tuna, flounder, haddock, herring, lake whitefish, orange roughy, pollock (Boston bluefish), salmon, sardines, skipjack tuna, sole, trout, and shellfish (including oysters, clams, crab, scallops, lobster, mussels, and shrimp).

Basically, the selenium in seafood protects us from potential mercury contamination.  Not only is it perfectly safe to consume a lot of fish, including for breastfeeding and pregnant women, and young children, but there are many more benefits of eating fish! For instance, the extra selenium you get from eating fish can help protect you against mercury exposure from other sources!  Not to mention all of the other important nutrients typically found in seafood such as omega-3 fatty acids (DHA&EPA), carotenoids, and micronutrients, and the many health benefits associated with consuming seafood including but not limited to improvements in cardiovascular disease, depression and anxiety, cognitive impairment, inflammatory bowel disease, cancer, hearing loss, age-related macular degeneration, and total mortality.

How cool is that?  You could say it’s very bene-fish-ial!

To hear all about my thoughts on mercury and fish, check out my video.  If you are interested in learning more about selenium, you can find it here.

@drsarahballantyne

Replying to @asphesterosfelley For the vast majority of fish, mercury is not a concern. The selenium in seafood protects us from potential mercury contamination. In fact, the selenium in seafood can help protect us from mercury exposure from other sources, too. Selenium is a mineral that is required for activity of twenty-five to thirty different enzymes (selenoenzymes), whose job is to protect the brain from oxidative damage. Methylmercury irreversibly binds to selenium. This is bad if we’re exposed to methylmercury because it renders selenoenzymes inactive. In fact, this is the mechanism through which methylmercury is believed to damage the brain and nervous system: by inhibiting the ability of selenoenzymes to protect these tissues from oxidants. Very importantly, most typically consumed varieties of ocean fish contain much more selenium than methylmercury. This is good for the fish (they don’t die from mercury exposure) and even better for us. Selenium-bound methylmercury is not efficiently absorbed by our bodies. The methylmercury that is absorbed is already bound to selenium, so it can’t interfere with our selenoenzymes. I’ll cover filter feeders in another video! #nutrivore #mercuryinfish #selenium #seafoodsafety

♬ original sound – Sarah Ballantyne, PhD

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