Mercury & Heart Disease
Chester J. Zelasko, Ph.D. | December 3, 2002

Scientific research can often seem confusing--one study says one thing and another seems to say the complete opposite. Just such confusion was illustrated in a series of articles published in The New England Journal of Medicine (November 28, 2002), including two articles that examined the association between fish consumption, mercury, and heart disease. This Newsletter will try to make sense of the research.

The background on mercury
Mercury is a heavy metal used in a variety of manufacturing processes, and it's toxic to animals, including humans--it has no normal biochemical function in human physiology.

The two forms humans are exposed to most often are metallic mercury and methylmercury. Metallic mercury is the type found in dental fillings while the later is the form most often found in animal flesh, especially fish. Excess mercury exposure can be toxic to humans, especially to developing fetuses, and the nervous system of all humans. The U.S. Environmental Protection Agency (EPA) has set a limit of 2 parts of mercury per billion parts of drinking water (2 ppb); the U.S. Food and Drug Administration (FDA) has set a maximum permissible level of 1 part of methylmercury in a million parts of seafood (1 ppm).

Mercury has been implicated in a variety of diseases, including heart disease. It's important to keep in mind that being exposed to more than the recommended mercury limits does not necessarily mean you'll experience adverse health effects, according to the Centers for Disease Control. A significant safety margin is incorporated into all acceptable mercury exposure limits; they should not be viewed as absolute levels above which harm can be expected to occur. While most of us think that zero parts per anything should be the standard, that's not reasonable given the potential environmental exposure.

Study 1
In the first study, scientists examined the relationship between fish consumption, mercury content (measured from toenail samples), and heart attack in the Health Professionals Follow-up Study (1). The mercury level was significantly correlated with fish consumption. That is, the more fish they ate, the more mercury was found in their bodies. Levels were also higher in dentists than in other healthcare professionals.

However, they couldn't find a strong association with mercury intake from fish and heart attack. That doesn't mean that it doesn't exist--just that they could find only a weak association and therefore, couldn't confirm prior research showing a definite association.

Study 2
The second study examined the relationship between fish oil, mercury, and the risk of heart attack. Using European subjects with high fish consumption, mercury and omega-3 fatty acids levels were assessed from toenails and fat tissue, respectively.

In this study, mercury levels were directly related to the risk of having a heart attack and the omega-3 fatty acid levels were inversely related to heart attack. Stated simply, the higher the body's omega-3 fatty acid level, the lower the risk of heart attack. The researchers concluded that the mercury content of the fish eaten may reduce the protective effect of the omega-3 fatty acids from the fish.

So where does that leave us? Two studies--seemingly opposite results. Do we eat fish or not? Does the mercury we may get from fish outweigh the benefits of the omega-3 fatty acids found in the fish? It seems to be a Catch-22.

Our advice: Reserve judgment
That's a term statisticians use when the results are not conclusive. We could avoid eating fish while we wait to see how the research develops, or we could be prudent and choose wisely which fish we eat. That's probably the best course.

Based on an FDA report on the levels of mercury found in species of fish, avoid swordfish, mackerel, and shark (3). However, tuna, halibut, and salmon should be fine if you eat 12 ounces per week or less--even for pregnant women. For more information on mercury levels, check the link on Reference 3.

Another alternative for making sure you get the benefits of the omega-3 fatty acids is to take a high-quality fish oil supplement every day. Recommended amounts are typically 1-3 grams of purified fish oil per day. It's important that you thoroughly check out manufacturers to insure they use a multi-step purification process to eliminate contaminants.

Eventually more studies will be done and they'll reveal more definitive rules for eating fish. In the meantime, you work on getting your omega-3s and we'll keep watching for better answers.

References:

  1. Yoshizawa, K., et al. Mercury and the Risk of Coronary Heart Disease in Men. New England Journal of Medicine 2002. 347:1755-1760.
  2. Guallar, E., et al. Mercury, Fish Oils, and the Risk of Myocardial Infarction. New England Journal of Medicine 2002. 347:1747-1754.
  3. www.cfsan.fda.gov/~frf/sea-mehg.html
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