While the study didn’t tease out cause and effect, lead researcher Maria Wennberg of Umeå University in Sweden told Reuters there are ways to get fish oil naturally without getting a lot of mercury. "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," she said.
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 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 three micrograms of mercury per gram of hair had a somewhat increased risk of heart attacks compared with men with one 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 didn’t prove that the high mercury levels were responsible for the increased risk of heart attack, but merely that the two are linked.
Dr. Wennberg said her findings highlight the need to consider omega-3s when studying the link between mercury and disease.