Fish, which is rich in omega-3 fatty acids, is considered an important part of a heart-healthy diet. But fish can sometimes be contaminated with mercury, which some studies have found to be associated with increased risk for cardiovascular disease, among other medical problems. Because fish intake is a primary source of exposure to mercury, researchers studied whether mercury exposure from fish might counteract the cardioprotective effects of fish. The results are published in the November 27, 2002 issue of the
New England Journal of Medicine
About the study
Researchers from eight European countries and Israel studied 684 men (aged 70 or younger) who had just had their first heart attack and 724 men of similar age and characteristics who had no history of heart disease (control group).
They then compared levels of mercury and docosahexanoic acid (or DHA, a particular type of fish oil) between the groups, to see if there existed any associations between the two substances and heart attack risk.
To measure the subjects’ mercury exposure, the researchers analyzed their toenail clippings for the presence of the metal, a common research technique. To assess their DHA levels, the researchers obtained a small fat tissue sample (taken with a fine needle) from each of the subjects’ buttocks.
The findings were significant. The mercury levels in the men who had heart attacks were 15% higher than the men without heart attacks. Men with the highest mercury levels had more than twice the risk of heart attack than did men with the lowest levels of mercury. After adjusting for mercury levels, the analyses showed that DHA was associated with lower risk for heart attack: men with the highest levels of DHA had a 41% lower risk for heart attack compared with men with the lowest levels of DHA. The results led the researchers to conclude, “high mercury content may diminish the cardioprotective effect of fish intake.”
In doing the analyses, the researchers controlled for factors including age and geographic location of the subjects. They also did separate analyses controlling for DHA and mercury levels, to see if either of these factors influenced the effects of the other.
While the results are interesting, there are some limitations of this study:
The analyses were based on one-time measurements of mercury and DHA levels, which were subject to random errors in measurement (there were no other measurements to confirm or contradict them).
There was a greater participation rate among men with heart disease than among control subjects, which may have skewed the results.
Because the study was conducted only in men, the results cannot necessarily be generalized to women. Also, because the study only included men with heart attacks who survived until hospitalization, the results cannot necessarily be generalized to men who have fatal heart attacks.
How does this affect you?
While these results pose an interesting question of whether the mercury in fish may counteract the heart-health benefits of the omega-3 fatty acids in fish, they do not prove that this is the case. In fact, another study in the same issue of the
, which studied mercury levels from toenail clippings of a much larger group (more than 33,000 US men),
find an association between mercury exposure and coronary heart disease. The results of both of these studies point to the need for further research on this topic.
In the meantime, experts are not recommending that people stop eating fish altogether. However, it may be wise to avoid the species of fish known to have the highest levels of mercury contamination, include tilefish, swordfish, king mackerel, and shark (see FDA link below for a table showing the mercury content in several species of fish and seafood). (Note: It is already recommended that pregnant and breastfeeding women, women who may become pregnant, and young children avoid these species).
And it’s important to remember that a heart-healthy lifestyle includes not only a diet containing omega-3 fatty acids, but also plenty of fruits, vegetables, and whole grains, as well as regular exercise, stress management, and no smoking.
Please be aware that this information is provided to supplement the care
provided by your physician. It is neither intended nor implied to be a
substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER
IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the
advice of your physician or other qualified health provider prior to
starting any new treatment or with any questions you may have regarding a