In the midst of our horror about the earthquake, tsunami and nuclear reactor disasters in Japan and concerns about the latter’s impact on the air, ocean and life forms, I thought that the timing of the article in the March issue of The New England Journal of Medicine was intriguing. It brought up another less immediate but valid concern, that of the effect of coal on our food and risk of disease. (I hope that those of you reading this article acknowledge coal’s impact on climate change and the ecologic repercussions that are, in themselves, a disaster). The NEJM article was titled “Mercury Exposure and Risk of Cardiovascular Disease in Two U.S. Cohorts.”
A quick review: as we use coal for power, we contaminate our atmosphere with mercury. This then returns to the oceans of the earth and is incorporated into plankton where it is converted into organic methylmercury. The latter is stored in the fat of fish. As larger fish eat smaller fish, their levels of methylmercury go up. We then eat the fish and the methylmercury gets into our bodies (and is also stored in our fat). Chronic, low-level methylmercury exposure can cause neurodevelopment delay in infants. It’s currently recommended that women of childbearing age, pregnant or nursing mothers, and infants and young children eat no more than 2 servings of fish per week and also limit their intake of certain species of fish that are especially high in mercury. The worst culprits are the ones on the top of the fish eating food chain: shark, swordfish, king mackerel, and tilefish. Then to complicate matters, fish from streams and rivers in areas that have high mercury pollution may also be less than safe, especially if consumed regularly. (For information on mercury pollution check http://www.epa.gov/mercury/advisories.htm.)
For adults, the main health concern regarding chronic low levels of methylmercury (not high, toxic ones) is the risk for cardiovascular damage and disease. Government agencies, the Institute of Medicine and (I assume) the rest of us want to know if mercury exposure is correlated with cardiovascular disease.
There has been robust research that shows that fish consumption is heart healthy; indeed, fish intake has been shown to be inversely associated with the risk of coronary heart disease, especially fatal heart attack and stroke. So what is a fish eating person (like myself, who does not eat meat) to do?
Researchers from Harvard, the University of Washington, and the University of Missouri studied mercury exposure in 2 large groups of individuals. The first was comprised of male physicians followed from 1986 in the Health Professionals Follow-up Study (don’t get me started on my gender protests regarding this study) and the second was through the Nurses Health Study (as you may guess, all female) in which the nurses were followed from 1976. The two studies totaled 51,529 men and 121,700 women.
And here is where it gets really interesting… Their toenail clippings were stored! (I read this, believe it or not, while getting a pedicure and wondered if I should save my own toe nail clippings for research.) Apparently concentrations of mercury and selenium in toenails have been found to be excellent biomarkers of usual methylmercury and selenium exposure. The researchers wanted to check selenium (which we get from consuming plants grown on selenium –rich soil) because this trace element provides protection against mercury toxicity in some experimental studies.
They identified 3427 participants with cardiovascular disease and matched them to controls who were the same age, sex, race and smoking status. They also had information on their fish consumption and lifestyle habits. Their toenail mercury and selenium concentrations were assessed by (and I’m sure you will get this) the use of neutron-activation analysis. The usual complicated statistical analysis (actually called a multivariate analysis) was done and demonstrated that participants with higher mercury exposures did not have a higher risk of cardiovascular disease nor did selenium concentrations make a difference in the results.
The authors concluded that their findings “provide no support for clinically relevant adverse affects of typical levels of dietary methylmercury exposure on cardiovascular disease in U.S. adults”. They went on to state that the absence of an association “should not alter ongoing public health and policy efforts to reduce mercury contamination in fish and the environment.”
Bottom line: Most of us can continue to eat the “right” fish for our heart’s sake, but women who are pregnant or may become pregnant, or who are breast feeding should limit their fish consumption. Now, we can start worrying about the impact of radiation on those fish…