Menacing Mercury Levels Found in Some Seafood

FDA cautions some against certain seafood consumption

Salmon and tuna sushi
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In all fairness, many types of seafood like salmon, shrimp, and cod contain little metabolized mercury.  Moreover, trepidation about metabolized mercury found in seafood is nothing new, and, overall, fish is very nutritious. However, the FDA is currently in the process of updating its advice concerning the consumption of seafood by pregnant women, nursing mothers, women anticipating pregnancy and young children.

 Specifically, the feds and many others are concerned about the profound neurological damage that such mercury can wreak on our physiology--especially its effects on fetuses and very young children who are still growing and developing.

How does mercury make its way into seafood?

Mercury makes its way into seafood, fish, and shellfish, via circuitous means.

First, vaporized metallic mercury (Hg0) produced by both natural processes (think forest fires or volcanic activity) and pollution (burning of fossil fuels) rises into the atmosphere. Once in the atmosphere, this mercury is oxidized to form divalent inorganic mercury (Hg2+) which then falls back to the Earth's surface as rain. Aquatic bacteria like plankton methylates this mercury (forming MeHg+ or methylmercury), and this metabolized methylmercury deposits in the lipids or fat of animals, making its way all the way up the food chain.

Longer-lived predator fish at the top of the food chain like shark, swordfish, and king mackerel end up accumulating relative high levels of mercury, making their consumption by pregnant women and young children with developing nervous systems particularly concerning.

Why is methylmercury dangerous?

Both ionized mercury and methylmercury bond with sulfur found in proteins thus disrupting the biology of our bodies in numerous and disquieting ways. More specifically, mercury generates oxidative stress on cells (think formation of free radicals), messes with microtubules (think messed up cell division) and may even trigger dangerous autoimmunity.

Verily, we don't exactly understand how mercury messes us up, but what we do know about mercury poisoning is scary.

Here are some bad ways in which methylmercury can mess with our bodies:

  • blindness
  • cerebral palsy
  • growth problems
  • intellectual disability
  • fatigue
  • hearing loss
  • ataxia (loss of voluntary coordination of muscle movements)
  • muscle tremor
  • movements
  • paralysis (with severe poisoning)
  • death (with severe poisoning)
  • heart disease (some very new research suggests that methylmercury may contribute to atherosclerosis)

Of note, some of these neurological adverse effects like cerebral palsy and growth problems apply to fetuses and young children. Thus, the FDA and various health care organizations have issued warnings specific to these populations.

Although mercury is excreted by the liver and kidneys, it has a particularly long half-life in the body. In other words, it takes a long time for us to rid ourselves of ingested mercury. Furthermore, methylmercury is almost completely absorbed from the gastrointestinal tract and is very good at crossing the blood-brain barrier, making the brain and spinal cord particularly sensitive to its effects.

Poisoning with methylmercury is insidious, and unlike other more acute forms of mercury poisoning, chelation and emesis (vomiting) fail to rid your body of this heavy metal. In fact, prevention is the only defense that we have against methylmercury. In other words, the best way to deal with methylmercury poisoning is to avoid its ingestion in the first place.

What does methylmercury in seafood mean to you?

As pointed out by the FDA and countless others, we must remember that seafood is a particularly healthy source of proteins, minerals, and vitamins. Moreover, seafood is low in saturated fat and high in omega-3 fatty acids. ​We all, even pregnant woman and young children, should include healthy, recommended and nutritious amounts of seafood in our diet.

In fact, with its recently drafted advice, the FDA actually encourages pregnant women, nursing mothers and mothers anticipating pregnancy to eat 8 to 12 ounces of seafood a week, and children aged 2 to 8 years should eat 3 to 6 ounces. In other words, such people should eat about 2 to 3 servings of seafood a week.

However, the FDA is advising pregnant women, nursing mothers, women anticipating pregnancy and young children to eat fish typically lower in levels of methylmercury like tilapia, catfish, and cod. Moreover, the feds recommend against the consumption of shark, swordfish, king mackerel and tilefish caught off the Gulf Coast. Additionally, people who consume fish from freshwater reservoirs should heed local advisories about mercury levels and be wary of fish harvested from areas without advisories.

Of note, although the FDA does list light canned tuna as low in mercury, this assessment is disputed by many experts who warn its best for certain mothers and young children to stay away from tuna altogether. (On a personal note, I empathize that we all live on a budget and canned tuna is cheap. However, personally, I agree with the assessment that in these certain populations, it's best to steer clear of tuna altogether.  Better safe than sorry.)

Please keep in mind that despite what I've shared with you so far, the FDA's advice is just advice. It's very rare that limited exposure to even problem seafood will result in methylmercury poisoning. For instance, if you're pregnant or nursing and you slip and eat the occasional swordfish steak during Valentine's brunch with your spouse, no need to freak out. Just try to stay away from such fish most of the time and decrease the consumption of fish for the rest of the week--a point the FDA makes, too.

Despite concerns about methylmercury in seafood, the vast majority of Americans harbor low levels of mercury in their bodies--even among those who eat freshwater fish, more fish than average, or both. For example, one recent study pegged the number of adults with total EPA blood-mercury concentrations considered potentially harmful (greater or equal to 5.8 micrograms per liter) at 4.6 percent. Moreover, another large study estimated that a mere 0.5 percent of youths aged 1 to 19 years registered concerning blood mercury levels.

While researching this article, I stumbled on some wonderful resources that will help you better calculate and appreciate methylmercury exposure attributable to seafood including:

Selected Sources

Byrns MC, Penning TM. Chapter 67. Environmental Toxicology: Carcinogens and Heavy Metals. In: Brunton LL, Chabner BA, Knollmann BC. eds. Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12e. New York, NY: McGraw-Hill; 2011.

Article titled "A longitudinal study of mercury exposure associated with consumption of freshwater fish from a reservoir in rural south central USA" by Z Dong and co-authors published in Environmental Research in 2015. 

Article titled "To Eat Fish or Not to Eat Fish" by R Nelson from AJN published in 2015. 

Article titled "More Than Half of US Youth Consume Seafood and Most Have Blood Mercury Concentrations below the EPA Reference Level, 2009-2012" by SJ Nielsen and co-authors published in The Journal of Nutrition Nutritional Epidemiology

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