Omega-3 fatty acids and mercury, both found in fish, appear to have opposite affects on heart health, according to a northern European study.
Researchers, whose conclusions were published in the American Journal of Clinical Nutrition, looked at data from more than 1,600 men from Sweden and Finland to find that men with high levels of mercury in their 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.
Researcher Maria Wennberg said that while the study can't clarify cause and effect, 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 and swordfish are at the top of the marine food chain and for that reason concentrate mercury from the environment in their tissues.
The heavy metal is known to be toxic to the nervous system, especially in foetuses and children, and the US Environmental Protection Agency warns women of childbearing age and children against eating predatory fish.
The men in the study 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, and 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 heart.
The results don't prove that the high mercury levels were responsible for the increased risk of heart attack, merely that the two are linked.
Dariush Mozaffarian, at the Harvard School of Public Health in Boston, said that other factors such as less education 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. - Reuters