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.
A researcher 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," Maria Wennberg, of Umea University in Sweden, said by email.
Predatory fish such as shark, swordfish, mackerel and tilefish 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 fetuses and children.