Mercury, oils from fish at odds in heart health
By Kerry Grens
NEW YORK | Thu Aug 30, 2012 4:52pm EDT
(Reuters Health) - Mercury and omega-3 fatty acids - both found in fish - appear to have opposite links to heart health, scientists have found.
In an analysis of more than 1,600 men from Sweden and Finland, researchers found that men with high levels of mercury in the body had an increased risk of heart attacks, while those with a high concentration of omega-3s had a lower risk.
Fish are considered part of a healthy diet, but the balance between potential risks and benefits from the two compounds is not clear.
While the study can't tease out cause and effect, researcher Maria Wennberg said there are ways to get fish oil naturally without getting a lot of mercury, too.
"Fish consumption two to three times per week, with at least one meal of fatty, non-predatory fish (such as salmon) and an intake of predatory fish not exceeding once a week can be recommended," Wennberg, of Umea University in Sweden, told Reuters Health by email.
Predatory fish such as shark, swordfish, king mackerel and tilefish are at the top of the marine food chain and so concentrate mercury from the environment in their tissues.
The heavy metal is known to be toxic to the nervous system, especially in fetuses and children, and the U.S. Environmental Protection Agency warns women of childbearing age and children against eating predatory fish.
The men in the new study, published in the American Journal of Clinical Nutrition, submitted hair and blood samples to measure their mercury and omega-3 levels, as well as information on their health and lifestyle.
The average mercury level among the Swedish men was 0.57 micrograms per gram of hair, whereas it was more than twice as high in their Finnish peers. Swedes, however, had higher levels of omega-3s than did Finns.
The researchers found that men with at least 3 micrograms of mercury per gram of hair had a somewhat increased risk of heart attacks compared with men with 1 microgram per gram, although they didn't calculate the exact risk.
But this only held true if the men also had low levels of omega-3 fats. For men with more of the fats, it took higher levels of mercury to see an increased heart attack risk - suggesting the two compounds might have opposite effects on the ticker.
The results don't prove that the high mercury levels were responsible for the increased risk of heart attack, but merely that the two are linked.
Dr. Dariush Mozaffarian of the Harvard School of Public Health in Boston said other factors, such as low education levels among those with high mercury levels, could also be at work.
Previous studies by Mozaffarian, who was not involved in the new work, did not show a link between mercury and heart attacks. But that research involved mercury levels much lower than in the current study.
He said the new results probably don't apply to most Americans, who have lower mercury levels than the men studied by Wennberg and her colleagues. And few people have high mercury levels and low omega-3s, because mercury from fish often comes with the healthy fats.
According to Mozaffarian, it's likely that the Finnish men with high mercury levels and low omega-3 levels ate contaminated whitefish from lakes in northern parts of the country.
Wennberg said her findings highlight the need to consider omega-3s when studying the link between mercury and disease.
SOURCE: bit.ly/PQ4CVi American Journal of Clinical Nutrition, online August 15, 2012.
Myocardial infarction in relation to mercury and fatty acids from fish: a risk-benefit analysis based on pooled Finnish and Swedish data in men
Maria Wennberg, Ulf Stromberg, Ingvar A Bergdahl, Jan-Hakan Jansson, Jussi Kauhanen, Margareta Norberg, Jukka T Salonen, Staffan Skerfving, Tomi-Pekka Tuomainen, Bengt Vessby, and Jyrki K Virtanen
Am J Clin Nutr October 2012 ajcn.033795; First published online August 15, 2012. doi:10.3945/ajcn.111.033795
Background: Exposure to methylmercury from fish has been associated with increased risk of myocardial infarction (MI) in some studies. At the same time, marine n?3 (omega-3) PUFAs are an inherent constituent of fish and are regarded as beneficial. To our knowledge, no risk-benefit model on the basis of data on methylmercury, PUFA, and MI risk has yet been presented.
Objective: The objective of this study was to describe how exposure to both marine n?3 PUFAs and methylmercury relates to MI risk by using data from Finland and Sweden.
Design: We used matched case-control sets from Sweden and Finland that were nested in population-based, prospective cohort studies. We included 361 men with MI from Sweden and 211 men with MI from Finland. MI risk was estimated in a logistic regression model with the amount of mercury in hair (hair-Hg) and concentrations of n?3 PUFAs (EPA and DHA) in serum (S-PUFA) as independent variables.
Results: The median hair-Hg was 0.57 μg/g in Swedish and 1.32 μg/g in Finnish control subjects, whereas the percentage of S-PUFA was 4.21% and 3.83%, respectively. In combined analysis, hair-Hg was associated with higher (P = 0.005) and S-PUFA with lower (P = 0.011) MI risk. Our model indicated that even a small change in fish consumption (ie, by increasing S-PUFA by 1%) would prevent 7% of MIs, despite a small increase in mercury exposure. However, at a high hair-Hg, the modeled beneficial effect of PUFA on MI risk was counteracted by methylmercury.
Conclusions: Exposure to methylmercury was associated with increased risk of MI, and higher S-PUFA concentrations were associated with decreased risk of MI. Thus, MI risk may be reduced by the consumption of fish high in PUFAs and low in methylmercury.
Received December 29, 2011.
Accepted May 9, 2012.
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