New Research Reveals Why Avoiding Tuna Is Still a Good Idea

tuna mercury levels

Story at-a-glance -

  • Due to heavy contamination, the health risks of eating tuna outweigh the benefits of the healthy omega-3 fats it contains; some tuna may contain 36 times more toxins depending on its source
  • Tuna may have large amounts of persistent organic pollutants and mercury that increase your risk for certain cancers, neurobehavioral disorders, genotoxicity and mercury, which is linked with an increased risk of hypertension
  • Your best choices are small, cold-water, fatty fish, which are an ideal source of omega-3s with a low risk of contamination. Healthy fish to eat more of include anchovies, sardines, mackerel, herring and wild-caught Alaskan salmon

By Dr. Mercola

Fish has always been the best dietary source of the animal-based omega-3 fats EPA and DHA. While your body may be able to produce some of these fats, the process is relatively inefficient, so they are considered conditionally essential.1 This means to acquire sufficient amounts for optimal health, you need to consume the omega-3 fats EPA and DHA from your food.

Unfortunately, as the levels of pollution in the oceans have increased, so have the toxins you consume. If you aren't choosy about where your fish is harvested, the pollutants in the fish may outweigh the benefits of the healthy fats. At the same time, many fish species have been overfished and dangerously depleted. Orange roughy is one example. A slow growing, deep water fish, they don't usually reproduce until age 20 and may live up to 149 years.2

In the 1980s the popularity of orange roughy exploded and led to driving it almost to extinction. In some areas, fishing for roughy has been restricted and scientists expect it to take many years for the population to recover, if it ever does. Farm-raised fish are also not the answer as they present problems for both the environment and the consumer.3

Raising fish in a confined area results in the same kind of waste pollution problems as land-based factory animal farms struggle with. Fish farms also reduce biodiversity, spread disease and sea lice, and contaminate water with pesticides. It is important to consider not only your health but also the impact on the environment when choosing seafood for your family.

Why Tuna Is Best Avoided

Historically, tuna has been a very popular fish, and among the most widely consumed. They are known for a distinctive flavor and versatility in the kitchen. There are at least 15 different species,4 but only five are commonly fished and eaten. Depending upon the species, the adults can weigh from 10 pounds up to 500 pounds. The largest is the Atlantic bluefin tuna that is migratory and can travel at great speeds over long distances.

Yellowfin tuna are smaller, weighing approximately 125 pounds as adults, and tend to remain in their local waters. They are carnivorous from birth, eating a number of different fish, such as mackerel, herring, hake, squid and crustaceans that feed along the ocean floor.5 Their only predator, aside from humans, are orcas and sharks, placing them near the top of the food chain and increasing the amount of toxins stored in their flesh.

Clean tuna contains a number of valuable nutrients, including selenium, iron, magnesium and potassium.6 Tuna also has high levels of vitamins B12, B6, C, zinc, niacin and riboflavin, along with omega-3 fats. Unfortunately, tuna is also among the riskiest, thanks to their often-high levels of toxins. Recent research shows that, depending on where they're caught, some tuna can contain up to 36 times more toxic chemicals than others.7

Problems with persistent organic pollutants (POPs) occurs around the world. This short news video describes cleanup efforts the Australian government has put into place.

Toxicity Levels Vary Widely From Location to Location

Researchers from Scripps Institution of Oceanography at the University of California embarked on a one-of-a-kind study involving harvesting yellowfin tuna from around the world and analyzing the contaminant levels found in each.8 Yellowfin is the second most harvested tuna species, with more than 1.3 million tons caught every year.

Since yellowfin tend to stay in the same general region from birth to death, the researchers were able to use this fish to determine if geographic location made a difference in the accumulation of toxic chemicals in the upper levels of the ocean food chain. Yellowfin tuna are also found around the world, giving scientists a unique ability to compare toxicity levels in the same species of fish caught in different locations.

The researchers were interested in measuring levels of POPs, as these pollutants are known to be resistant to degradation and thus bioaccumulate through the food chain from the tiniest plankton to the tuna harvested for your dinner plate. They analyzed 117 fish from 12 locations around the world and found every fish caught had some level of pollutants in its body.

Fish caught close to industrialized areas, such as off the coasts of North America and Europe, had an average of 36 times more toxic chemicals than the same species caught in more remote locations. However, those were the averages. The differences between individual fish were much greater as toxic levels between the greatest and least contaminated samples varied by 180 times.

The highest contamination levels were found in tuna collected from the Northern Hemisphere, including the Gulf of Mexico. Those fish from off the coast of Asia and around the Pacific Islands were relatively clean. However, it's important to remember that every fish was found to have toxic chemicals — which should be a highly unnatural state for an animal. Only those around Asia and the Pacific Islands were relatively clean.

Toxins found included pesticides, flame retardants and polychlorinated biphenyls (PCBs). The researchers also found 90 percent of the fish caught in the northeast Atlantic Ocean, off the coasts of Maine north to Nova Scotia, and 60 percent caught in the Gulf of Mexico, had levels of toxic chemicals that would trigger health advisories in segments of the population at greater risk, such as pregnant and nursing women.

POPs Are a Serious Health Threat

Lead author Sascha Nicklisch, Ph.D., biochemist and postdoctoral researcher at Scripps, commented on the results, saying, "The pollutant levels in seafood and tuna in our case can be heavily determined by the location where it was caught. It is important to know the origin of catch of the fish, to know the amount of pollutants in your fish."

POPs came into wide use after World War II as thousands of chemicals were introduced into manufacturing and commercial use. The most well-known of this class of chemicals are PCBs, DDT and dioxins. Their potential for long-range transport and their persistence in the environment make these chemicals of concern across the globe. Even in countries where they are not manufactured or used commercially, people test positive for toxic chemicals in their body.9

Exposure to these pollutants, even at low levels, may increase your risk of certain cancers, neurobehavioral impairment, genotoxicity, immune problems and endocrine disruption.10

Tuna Is Also High in Mercury

The study originally began by measuring levels of POPs, but a secondary report was also issued, discussing the levels of mercury found in the tuna researchers harvested.11 As with the POPs, researchers found the levels of mercury in the fish were dependent on the site. The levels were only weakly associated with the size of the fish or the amount of fat found in the individual fish. In other words, it appeared that even the youngest and smallest had the same level of mercury as older and larger fish.

In one study from the U.S. Geological Survey, all tuna tested had fairly high amounts of mercury,12 and levels in yellowfin tuna are rising by 3.8 percent each year.13 The fish sold in restaurants actually contain higher amounts than the store-bought varieties.14 Restaurants tend to favor specific species of tuna that often test higher in mercury, such as the bluefin akami and bigeye tuna. As with POPs, mercury bioaccumulates in smaller fish and becomes increasingly elevated higher in the food chain.

Mercury also accumulates in greater amounts in lean muscle rather than fat, making the lean tuna species more susceptible to higher amounts of contamination. Canned albacore tuna has been found to contain up to 0.32 parts per million (ppm) of mercury.15 Approximately 33 percent of exposure to mercury in the U.S. comes from eating contaminated tuna16,17 and nearly 75 percent of all mercury exposure in humans comes from eating fish.

Other Fish to Avoid Due to High Mercury Contamination

Even in small amounts, exposure to mercury may cause serious health problems and is of significant danger to a developing child.18 Toxic effects occur in the nervous system, digestive tract, immune system and in the lungs, kidneys and eyes. Although mercury is naturally occurring, the levels in the ocean are boosted by industrial activity and dentist offices that use mercury amalgam.19

Once consumed by smaller fish, the mercury bioaccumulates up the food chain, as the animals do not metabolize or excrete it from their body. Up to 10 percent of American women carry enough mercury in their body to put their developing child at risk of systemic disorders. The Environmental Defense Fund runs a handy Seafood Selector20 to help you evaluate the amount of mercury you may be consuming. They recommend you steer clear of the fish that contains the most mercury, namely:

Bluefin tuna

Walleye

Shark

King mackerel

Marlin

Bluefish

Swordfish

Wild sturgeon

Opah and bigeye tuna

Mercury-Contaminated Seafood May Contribute to Hypertension

Although including fish in your diet is recommended to increase your omega-3 level, mercury-contaminated fish may be a hidden contributor to hypertension. Other sources of mercury exposure include dental amalgam (so-called "silver" fillings, which are actually 50 percent mercury), thimerosal (a vaccine preservative), coal mines and fluorescent light industries.21 Researchers from the National Institutes of Health collaborated on a review article, in which they state:22

"In the cardiovascular system, mercury induces hypertension in humans and animals that has wide-ranging consequences, including alterations in endothelial [lining of the interior surface of blood vessels and lymphatic vessels] function.

The mechanism by which mercury produces toxic effects on the cardiovascular system is not fully elucidated, but this mechanism is believed to involve an increase in oxidative stress. Exposure to mercury increases the production of free radicals potentially because of the role of mercury in the Fenton reaction and a reduction in the activity of antioxidant enzymes, such as glutathione peroxidase."

According the American Society of Hypertension:23

"Mercury toxicity should be evaluated in any patient with hypertension, coronary heart disease, cerebral vascular disease, cerebrovascular accident, or other vascular disease. Specific testing for acute and chronic toxicity and total body burden using hair, toenail, urine and serum should be performed."

To read more about detoxifying from your mercury exposure and reducing your potential risk for illness, read my previous article, "Revised Protocol for Detoxifying Your Body From Mercury Exposure."

Fish You Might Consider Eating More Often

While it is essential to get optimal amounts of omega-3 fats in your diet, it's important to avoid toxic sources that raise your risk of disease. As a general rule, your best choice are small, cold-water, fatty fish, which are an ideal source of omega-3s with a low risk of contamination. The healthiest fish I suggest you consider eating more often include anchovies, sardines, mackerel, herring and wild-caught Alaskan salmon.

To encourage you to try these fish, I recommend you experiment by adding anchovies or sardines to your next salad. You may also want to check out my personal lunch recipe and healthy baked salmon recipe.

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