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July 28, 2008 — It might be called the Japanese paradox.
So why is it that the rate of heart disease among men living in Japan is less than half that of men living in the U.S. and that Japanese men tend to have less artherosclerosis — the artery-clogging plaque that leads to heart attacks and strokes?
A new study suggests that the answer may be found in the sea.
High Omega-3 Levels
Because they ate more fish, men living in Japan who participated in the study had twice the levels of omega-3 fatty acids in their blood as white men and Japanese men living in the U.S. They also had less severe degrees of atherosclerosis.
The finding lends support to the hypothesis that omega-3, which is found primarily in fatty fish like tuna, mackerel, and salmon, protects against plaque buildup in the arteries.
Sources of omega-3 fatty acids are fish oils, which contain docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), and plant sources. Alpha-linolenic acid (ALA), which is converted into omega-3 fatty acids in the body, is a plant-source omega-3 fatty acid.
Studies have generally used fish oils. While plant sources with ALA may have the same benefits, less is known about them.
"The extremely high intake of fish in Japan may explain the much lower rate of atherosclerosis and subsequent coronary heart disease," researcher Akira Sekikawa, MD, PhD, tells WebMD. "This study does not prove that omega-3 is protecting these men, but we showed that artery thickness decreased as omega-3 levels went up."
The Japanese diet has become increasingly westernized since the end of World War II, but fish consumption in Japan is still among the highest in the world.
People in Japan eat an average of 3 ounces of fish every day, while the average American finds it difficult to manage the two servings of fish a week recommended for heart health by the American Heart Association, omega-3 researcher William Harris, PhD, tells WebMD.
He adds that the average omega-3 intake in Japan of 1 gram a day is about eight times higher than the amount the typical American gets.
"We are not a nation that loves fish, and that isn't likely to change," he says. "But it is increasingly clear that we need to get more omega-3 into our diets."
Fish oil supplements are one way of doing this. Studies in individuals with heart disease have shown a benefit of supplemental omega-3 fatty acids. Based on these studies, the American Heart Association recommends that people with heart disease take 1 gram of EPA plus DHA daily.
Other good sources of omega-3 include foods such as flaxseed and canola oils, soybean, tofu, and walnuts. Omega-3 fatty acids are often sold as capsules but can upset the stomach and should be taken with food.
Harris is working to develop a soybean-based omega-3-enriched oil through a grant from the company Monsanto.
"Either we get people in the U.S. to start liking oily fish, which probably isn't going to happen, or we find another way of getting it into our food," he says.
Fish Eaters Had Less Plaque
The study by Sekikawa and colleagues included 281 Japanese men living in Japan, an equal number of Japanese men living in the U.S., and 306 white men who lived in the U.S.
All the men were in their 40s, and all underwent blood testing to determine serum levels of fatty acids, including omega-3. The men also had two tests for atherosclerosis — one measuring the thickness of the artery wall in a major neck artery that sends blood to the brain and the other measuring plaque in arteries leading to the heart.
While total fatty acid levels were similar in the three groups, blood omega-3 levels in the Japanese men living in Japan were 45% higher and 80% higher, respectively, than in Japanese men and white men living in the U.S.
And both measures of artherosclerosis showed less plaque buildup in the arteries of the Japanese men living in Japan. Atherosclerosis levels were similar in both Japanese-Americans and in white Americans.
The study appears in the Aug. 5 issue of the Journal of theAmerican College of Cardiology.
"This indicates that much lower death rates from coronary heart disease in the Japanese in Japan is very unlikely due to genetic factors," Sekikawa says.
The 'Omega-3 Hypothesis'
In an editorial accompanying the study, Harris writes that what he calls the "omega-3 hypothesis" grew from research on the Inuit Eskimos of Greenland conducted almost four decades ago.
Despite eating a diet low in fruits, vegetables, and complex carbohydrates and high in fat and cholesterol, the Eskimos had very low rates of heart disease. Researchers concluded that the reason was the incredibly high levels of omega-3 in their diets from the consumption of large amounts of fish, whale, and seal.
But recent studies suggest that heart disease rates among Alaskan Eskimos are now higher than among whites in the U.S., even though fish consumption in the population remains high.
"At least part of the problem in Alaska appears to be not a lack of omega-3 but the introduction of massive amounts of shortenings and other saturated fats into their Westernizing diet," Harris writes.
He concludes this and other research suggest that the "cardioprotective punch of the long-chain omega-3 fatty acids may be no match for diets high in fat, particularly saturated fat."
Cardiologist Robert Eckel, MD, who is a past president of the American Heart Association, says studies in heart attack patients treated with very high doses of omega-3 have generally proven disappointing.
Eckel is a professor of medicine at the University of Colorado Denver School of Medicine.
"We have known for some time that people who eat more fish seem to have less heart disease," Eckel tells WebMD. "This study supports that, but more research is needed."
SOURCES: Sekikawa, A. Journal of the American College of Cardiology, Aug. 5, 2008; vol 52: pp 417-424. Akira Sekikawa, MD, PhD, assistant professor of epidemiology, University of Pittsburgh. William Harris, PhD, director, Metabolism and Nutrition Research Center, Sanford Research, University of South Dakota. Robert Eckel, MD, professor of medicine, University of Colorado-Denver School of Medicine; past-president, American Heart Association; professor of medicine, University of Colorado Denver School of Medicine.
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