Omega 3s vs. Mercury—Is Seafood Good for You?

Michael Conathan breaks down the health benefits and detriments of fish in your diet.

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The preponderance of evidence shows that in virtually all circumstances, the health benefits of fish outweigh the detriments. (AP/ Elaine Thompson)
The preponderance of evidence shows that in virtually all circumstances, the health benefits of fish outweigh the detriments. (AP/ Elaine Thompson)

It seems there’s a never-ending see-saw battle in scientific research about certain consumables. Red wine will decrease incidence of cardiovascular disease! No it won’t. Dark chocolate will lower your body mass index! Or not.

Seafood is no different. For every report that Omega 3 fatty acids are the fountain of youth, there’s another study warning seafood lovers about looming poison from excessive quantities of heavy metals, especially mercury. But are Omega 3s really that beneficial? And what to make of reports that selenium in fish can counterbalance the negative effects of mercury? And just what the hell is selenium, anyway? What’s the truth about fish?

Of course, there’s no black-and-white answer, but I’ll try to sort through a few of the bigger issues and provide a bit of guidance about what to look for at the fish counter to maximize the benefits and reduce your risk.

First of all, a disclaimer: I’m an ocean policy wonk, not a doctor, so take all this info with a grain of salt (figuratively, people, watch that blood pressure!) and ask your doctor if you have deeper questions—particularly if you’re pregnant.

Omega 3s

Here’s what the Mayo Clinic has to say about Omega 3’s. (I’m using the Mayo primarily because my mother-in-law treats it like an oracle. And if there’s one lesson from this column that has nothing to do with fish it’s that you should seize every chance you get to make nice with your mother-in-law.)

There is supportive evidence from multiple studies that suggests the intake of recommended amounts of [fatty acids] DHA and EPA in the form of dietary fish or fish oil supplements lowers triglycerides; reduces the risk of death, heart attack, dangerous abnormal heart rhythms, and strokes in people with known cardiovascular disease; slows the buildup of atherosclerotic plaques (“hardening of the arteries”), and lowers blood pressure slightly.

Omega 3s from fish are considered superior to Omega 3s from other sources such as eggs or flax seeds because they contain the fatty acids DHA and EPA (let’s go ahead and skip the multisyllabic mouthfuls that make up these guys’ actual names), which are the gold standards when it comes to proteins.

Or are they? Earlier this week, The Wall Street Journal wrote about a study that found no evidence of benefits from use of Omega 3 supplements. Yet the article goes on to describe various ways in which this study was potentially skewed, and it notes that:

Thousands of studies since the 1970s have shown that people with high levels of omega-3s have lower triglycerides, lower blood pressure, lower LDL cholesterol, less inflammation and a lower risk of heart disease. Those with low levels of omega-3s are more likely to be depressed, to commit suicide and have memory loss and brain shrinkage as they age.

So let’s see, one study saying there’s no discernible positive impact, against thousands of studies over four decades showing Omega 3s can make your heart work better for longer and prevent your brain from shrinking. I know whose side I’m on. Score one big thumbs up for fish.

Mercury and selenium

One thing that’s crystal clear in all this haziness is that mercury is dangerous. I still remember dropping a mercury thermometer as a kid and being intrigued by the quicksilver beads that oozed out of the shards—until my mother, not a yeller, shrieked at me not to touch the stuff. Less understood is the role played by selenium, a beneficial mineral found in tiny amounts in the human body, in counteracting the negative effects of mercury.

Mercury accumulates in fish as a byproduct, primarily of burning fossil fuels. Mercury finds its way into the ocean from precipitation and settling of emissions from coal plants; it is then consumed by microscopic organisms that are in turn eaten by larger organisms and so on up the food chain. (For more on the relationship between coal and mercury, particularly the battle over an EPA regulation to control it, see the work of my colleagues Daniel J. Weiss and Jackie Weidman.)

The result of this so-called bioaccumulation is that larger, carnivorous fish tend to have higher concentrations of mercury in their flesh because they have spent their lives eating and processing lots of smaller fish, and while the nutritious elements of the prey are turned into energy, the mercury is stored rather than cleansed out of the larger fish’s system.

On the other side of the ledger, large ocean fish also tend to have higher concentrations of selenium, a trace mineral essential to human health that bonds to the mercury and prevents it from being absorbed in your brain. Which, simply put, is a good thing.

Many larger fish are widely cited as having the highest concentrations of mercury, particularly king mackerel, swordfish, shark, tilefish, and some tunas. After growing up on the coast, and working on fisheries issues for over a decade, I’ve never been served either a tilefish or a king mackerel, so those shouldn’t be tough to give up. And there are plenty of reasons beyond mercury not to eat shark—many species are overfished primarily as a result of the inflated value of their fins for shark fin soup, but also because sharks are very slow to reproduce.

Swordfish and most tunas, on the other hand, are a popular menu items and, at least in the United States, sustainably harvested. In addition, according to a study sponsored by the National Oceanic and Atmospheric Administration, the Energy and Environmental Research Center’s Center for Air Toxic Metals, and other government and nongovernment entities, these species also contain more selenium than mercury. This study concludes that other than for young children and pregnant mothers, “there are no recommendations to avoid any ocean fish,” including swordfish and tunas.

This finding was backed by a new study published this month in the American Journal of Clinical Nutrition, stating that most seafood’s benefits outweighed its detriments.


The preponderance of evidence shows that in virtually all circumstances, the health benefits of fish outweigh the detriments. Omega 3 fatty acids are widely thought to be a major health boon and while consumers should be cognizant of mercury in their fish, particularly when pregnant or feeding young children, for the majority of adults, even this environmental toxin shouldn’t scare you off your swordfish steak.

As always, the best ways to pick your seafood are to seek fresh, local (when possible), sustainably harvested products—remember, American fisheries are among the most sustainably managed in the world and the number one rule of sustainability is buy American—and when in doubt, follow the age-old rule of thumb, “All things in moderation. Even moderation.”

Particularly when it comes to red wine and chocolate.

Michael Conathan is the Director of Ocean Policy at the Center for American Progress.

The positions of American Progress, and our policy experts, are independent, and the findings and conclusions presented are those of American Progress alone. A full list of supporters is available here. American Progress would like to acknowledge the many generous supporters who make our work possible.


Michael Conathan

Director, Ocean Policy

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