Eating fatty acids like those found in fish and certain plants may not prevent heart disease as well as previously believed, a new study concludes.
Danish researchers who analyzed the diets of nearly 3,300 people found that, in general, people who reported eating more fatty acids were no less likely over time to develop ischemic heart disease -- meaning reduced supply of blood to the heart -- than people who consumed less of the nutrients.
However, women who ate the most omega-3 fatty acids, common in fish and in fish oil supplements, did appear to benefit. They had nearly 40 percent lower risk of developing heart disease than women who reported eating the least fatty acids, according to findings that appear in the American Journal of Clinical Nutrition.
The study "raises important questions" about the interactions of these nutrients with cardiac health, said Alice Lichtenstein, director of the Cardiovascular Nutrition Laboratory at Tufts University. But, she cautioned, studies that focus on the dietary habits of a single country are tricky to extrapolate.
"Because food is so environmentally specific, you can't just assume what has been observed in one population is necessarily true for another," Lichtenstein told Reuters Health.
In 2004 the U.S. Food and Drug Administration said there is "supportive but not conclusive" evidence that omega-3 fatty acids can reduce a person's risk of heart disease. The agency recommended that people consume no more than three grams a day of the substance, with no more than two grams of that coming from dietary supplements.
In addition to omega-3 fatty acids, the current Danish study looked at the effects of linoleic acid, and a related substance, alpha-linolenic acid -- both of which come from plants -- on the risk of heart disease in 3,277 men and women living in the Copenhagen metropolitan area.
None of the subjects had been diagnosed with heart problems when they entered the study, which gathered information about their eating habits, exercise and other lifestyle patterns.
After an average of 23.3 years of follow-up, 471 participants had developed ischemic heart disease. But only women who reported eating the most omega-3 fatty acids -- some as much as 11.2 grams a day -- seemed to benefit from the nutrient. Their risk of heart disease was 38 percent lower than that of women who consumed less than 0.2 grams a day, the researchers found.
"High intake of omega-3 fatty acids had a significant cardioprotective effect among women," said Mia Sadowa Vedtofte, of the National Institute of Public Health at the University of Southern Denmark, in Copenhagen, and a co-author of the study.
Vedtofte said that men who ate more omega-3 fatty acid-rich foods also seemed to gain protection from heart disease, but that the statistical differences were small so the effect could be due to chance. To answer the question more conclusively, she added, the study would have required more people.
The estimated average daily intake of fatty acids by study participants was 1.2 grams for women and 1.6 grams for men, according to the researchers. That's somewhat higher than in other studies that have looked at fatty acid effects, but, at least for women, equal to the minimum recommended amount.
Vedtofte said she and her colleagues were "a little surprised" that the plant-based fatty acids they studied did not appear to prevent heart disease.
The results of this study do not prove that the omega-3s prevent heart disease either, even in the women who appeared to benefit significantly. Some other factor that goes along with a fish-rich diet could also be at work, for instance.
Lichtenstein said the basic advice for Americans shouldn't change: Eat a diet that's high in fruits and vegetables and low in red meat. Even if fish oil proves not to protect against heart disease, she added, "there's no... reason for not including fish in the diet. It may be displacing a food that's high in saturated fat."
Lichtenstein noted that the Danish study looked only at whether fatty acids could protect against heart disease, not whether they were helpful in preventing cardiovascular events, such as heart attacks and strokes, in people with established heart trouble.
"There are data to show that for people with established disease, they will benefit" from supplements, she said.
A major U.S. study involving 20,000 volunteers, the VITAL trial, is now underway investigating whether vitamin D and omega-3 fatty acid supplements can protect against heart disease, cancer and strokes in people without a history of such problems.