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One Major Side Effect of Eating Fish, Says Science

It's worth you being informed of this side effect, but it's something that can easily be avoided if you take the right precautions.
FACT CHECKED BY Olivia Tarantino

There aren't many foods that offer more health benefits than fish. From lowering heart disease risk to supporting brain health, eating fish is one of the best things that you can do for your overall health.

But while enjoying a baked salmon filet or a grilled piece of snapper is a yummy way to live a healthy lifestyle, there is one side effect of eating too much fish that can't be ignored: the trace levels of methylmercury.

The one downside of eating fish is consuming mercury

Years ago, eating a piece of fish didn't come with many concerns for whether there were any contaminants in the healthy protein. But unfortunately, thanks to pollution, the fish on your plate can have the same unsavory and potentially toxic items that we found in the ocean – think methylmercury, polychlorinated biphenyls (PCBs), and dioxins.

Methylmercury (a toxic mercury compound) in particular is accumulated in fish through its life cycle. It is first absorbed by phytoplankton, or algae, which is then consumed by smaller marine animals, smaller fish, and eventually larger fish. So over time, all animals of the sea can contain some methylmercury, although certain species contain more than others.

Like fish that accumulate mercury via their diet, humans, too, can store mercury in their body if they eat ample mercury-containing fish.

Why is this a problem? Methylmercury is a toxic metal. And while a small exposure has not shown to offer a huge health threat, too much exposure can result in mercury poisoning, with effects like hearing, vision, and coordination challenges. Some may experience muscle weakness as well.

Mercury exposure: one scary side effect of eating too much fish

Mercury is a metal that can potentially damage a person's central nervous system, or the brain and spinal cord. When the central nervous system is damaged, a slew of outcomes can occur, including things as mild as feeling a headache to experiencing memory loss or muscle weakness. So, if you are eating way too much fish, you are potentially taking in too much mercury and can unknowingly cause damage to yourself.

And when a person is pregnant, the concern about the effects of mercury exposure is even more elevated. Since the brain of a fetus is rapidly developing during pregnancy, throwing too much mercury into the mix can result in brain damage or vision challenges. This is why pregnant people are told to limit their fish intake to 2-3 servings per week, depending on the fish selected.

Should people avoid fish because of mercury?

Although fish can contain methylmercury, it should not be viewed as a poisonous metal bomb that should be avoided like the plague by any means. Yes, fish can contain mercury, but it also contains too many important nutrients like DHA omega-3 fatty acids and vitamin B12 to validate eliminating it from diets. In fact, fish is one of the best sources of biologically active omega-3s you can get in your diet.

In fact, the American Heart Association recommends eating 2 servings of fish (particularly fatty fish) per week, and suggests that the benefits of eating this food far outweigh the potential risks when the amount eaten is within the recommendations established by the Food and Drug Administration (FDA) and Environmental Protection Agency (EPA).

And the updated Dietary Guidelines for Americans, 2020-2025 includes recommendations on eating fish as well, generally suggesting that adults consume between 8-10 ounces of fish or seafood a week, depending on the person's calorie requirements, age, and stage of life. Included in this recommendation is a specific call-out for pregnant people, stating that eating this food is linked to favorable measures of cognitive development in young children. Pregnant or lactating people should eat at least 8 and up to 12 ounces of a variety of seafood per week, from choices lower in methylmercury, which can be found in this chart put together by the EPA.

Plus, the new guidelines go as far as to state that babies should be offered fish as soon as complementary foods are introduced.

So, like most things, fish in moderation can be an important part of a healthy diet. Eating fish morning, noon, and night? Not necessarily the best idea.

How can people eat fish without worrying about too much mercury?

Eating fish in moderation is something that is recommended by many experts to support overall health, and can be absolutely delish. What is better than a fresh mahi mahi sandwich on a summer day or a decadent sauteed salmon enjoyed at a fancy restaurant?

Thankfully, there are some ways that people can enjoy fish without worrying about eating too much mercury:

  • Limit intake to 2-3 servings of fish and shellfish a week.
  • Choose options that are considered to be lower in mercury – typically smaller fish like anchovy, black sea bass, catfish, flounder, haddock, mackerel, pollock, salmon, sardine, and freshwater trout.
  • Swap out high mercury-containing fish like mackerel, shark, swordfish, and orange roughy with lower mercury-containing fish like salmon, pollock, and freshwater trout.
  • If choosing fish on the higher side of mercury — think grouper, Chilean sea bass, and albacore tuna — simply limit your fish intake to one serving that week. In addition to the chart from the EPA cited before, you can also follow the guide by the National Resource Defense Council to be familiar with which fish choices are higher and lower in mercury.

If you stick with the recommended quantities, typically 8-10 ounces of lower-mercury fish or 2-3 servings a week, they will not only reduce the risk of mercury poisoning but also reap all of the health benefits that eating fish can offer.

And when deciding whether to choose farmed vs. wild fish, it turns out that both can be great choices. While it is true that higher levels of contaminants were found in farmed salmon in the past, follow-up longer-term studies have found the opposite, highlighting that farmed salmon is safe and healthy. In fact, according to a study published in Environmental Research, levels of mercury were higher in wild salmon than in farmed salmon.

So, no need to steer clear of your favorite fish dish out of mercury concerns. As long as you are sticking to some general guidelines – choose smaller fish, limit servings to 2-3 per week – then you should be a-ok! For more on the health benefits of these sea-dwelling creatures, don't miss these Surprising Side Effects of Eating Fish, According to Science.

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Lauren Manaker, MS, RDN, LD, CLEC, CPT
Lauren Manaker is an award-winning registered dietitian, book author, and recipe developer who has been in practice for almost 20 years. Read more about Lauren
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