In a new study of some 3,000 older adults, those with the highest levels of omega-3 fatty acids in their blood were 30 percent less likely to develop an irregular heartbeat over the next 14 years than peers with the lowest blood levels of omega-3s.
"A 30 percent lower risk of the most common chronic arrhythmia in the United States population is a pretty big effect," said Dr. Dariush Mozaffarian, senior author of the new report and a professor at the Harvard School of Public Health.
According to some estimates, up to 9 percent of Americans will develop atrial fibrillation, a heart-rhythm abnormality that can lead to stroke and heart failure, by the time they reach their 80s.
There are few treatments for the condition and they largely center on preventing strokes with blood-thinning drugs.
Some previous studies have suggested that people who eat a lot of fish have a lower risk of developing atrial fibrillation to begin with. But others haven't found the same link.
The omega-3 fatty acids measured in the new study — eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA) — are found in oily fish and some enriched foods, like eggs, as well as in fish oil supplements.
The earlier studies relied on questionnaires about how much fish people ate, which can only estimate the levels of omega-3s they ingested, Mozaffarian noted.
"Any given fish species can vary in its omega-3s by 10-fold," he told Reuters Health.
To get a more accurate measurement of how much fish oil the people in the study actually ingested, the researchers sampled blood from more than 3,300 adults over age 65.
Over 14 years, they tracked the seniors' health and found that 789 had developed atrial fibrillation.
Those with top 25 percent omega-3 levels in their bloodstreams at the beginning of the study were about 30 percent less likely to end up with the arrhythmia compared to those with bottom 25 percent blood levels of the fatty acids.
The difference in risk isn't huge, but "these are meaningful reductions in risk," said Dr. Alvaro Alonso, a professor at the University of Minnesota School of Public Health who was not involved in this study.
A 30 percent reduction in risk would mean that instead of 25 out of every 100 people developing a condition, only about 17 out of every 100 people would get it.
Another study from Finland used the same approach of measuring fatty acids in the blood and found a similar reduction in the risk of atrial fibrillation among those with the highest levels.
Mozaffarian's group tried to tease out which of the omega-3 fats might be responsible for the lower risk, and found that high DHA levels were linked to a 23 percent lower risk for atrial fibrillation while EPA and DPA were not tied to any reduced risk.
DHA is highly concentrated in heart muscle cell membranes, Mozaffarian and his colleagues point out in their report, published in the journal Circulation.
Alonso cautioned that this study doesn't prove eating fish is responsible for the lower rate of atrial fibrillation, but he said there is some idea that the fatty acids found in fish could work by stabilizing the excitability of heart muscle cells.
He said that the results seem promising enough to warrant further studies experimenting with using fish oil as a potential preventive measure against atrial fibrillation.
An earlier study of fish oil pills found that they didn't help the symptoms of atrial fibrillation in people who had already developed the arrhythmia.
The American Heart Association, the Department of Agriculture, and other groups recommend eating fish at least twice a week.
Mozaffarian said most Americans don't meet those goals.
He said his study "doesn't change current guidelines, but I think this should change people's motivation."
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