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Selenium in tuna protects against mercury

By Staff | Dec 17, 2009

The evidence is overwhelming that eating ocean fish, including tuna, is part of a healthy diet and lifestyle despite the low levels of mercury present.

While it is agreed that mercury at high concentrations is toxic, the toxicity of trace levels of mercury found in open ocean fish like tuna remains controversial.

Yellowfin tuna was first shown in 1972 to protect against mercury toxicity, not cause it. The rich levels of selenium in tuna were responsible for the protective effect. Tuna has a relatively high content of selenium and tends to accumulate additional selenium when mercury is present.

Selenium, an essential trace element in our diet, is vital to the body’s antioxidant system and proper immune system function. It has anti-cancer effects and is known to detoxify metals, including mercury. It has been shown to protect against mercury in every animal model tested.

Selenium has an extremely high binding attraction and strength with mercury, forming a biologically inactive compound, mercury selenide. For this reason, it is important to have an excess of selenium over mercury in the diet, or run the risk of selenium deficiency and the toxic effects of mercury.

If the ratio of selenium to mercury determines if a food is safe, what are the ratios in Hawaii fish? In a Hawaii Seafood Project study supported by NOAA, Dr. John Kaneko of PacMar Inc. in Honolulu and Dr. Nick Ralston of the Energy and Environmental Research Center in North Dakota analyzed selenium and mercury in 15 pelagic fish caught near Hawaii.

They found that all tuna species sampled, including bigeye, yellowfin, albacore and skipjack, contained a healthy excess of selenium over the mercury content.    

For this reason, eating tuna, an excellent source of selenium, is more likely to protect against mercury toxicity than cause it.

Tuna is also a rich source of high-quality protein and omega-3 fatty acids, and should be considered part of a healthy diet. Tuna is health food – it’s good for your heart. It can result in a 17 percent lower risk of heart disease and a 12 percent lower risk of stroke.

The omega-3 fatty acids in tuna can also protect against Alzheimer’s disease, arthritis, breast and prostate cancer, and many other conditions.

Billfish species and most other pelagic fish species also contained an excess of health promoting selenium over mercury content. Mako shark was the only fish in the study that had more mercury than selenium.

For this reason, most Hawaii fish are not only a healthy source of high-quality protein and omega-3 fatty acids, they are also excellent sources of selenium.

Our favorite fish are more likely to protect against mercury toxicity than cause it. The good news for Hawaii seafood lovers? The selenium is in every bite.