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In a study of 868 Japanese and US men in their 40s, researchers found that the Japanese men had a substantially lower rate of atherosclerosis - a buildup of "plaques" in the arteries that can lead to a heart attack.
This was despite the fact that Japanese men had a far higher rate of smoking, and were as likely or more likely to have a number of other risk factors for heart disease - including high blood pressure and diabetes.
The key difference, the researchers found, seemed to be the two-fold higher level of heart-healthy omega-3 fatty acids in the Japanese men's blood. Omega-3 fatty acids are found primarily in fattier fish, like salmon, mackerel and, to a lesser extent, tuna.
The findings suggest that Japan's high fish consumption may help explain its low rate of heart disease, the investigators report in the Journal of the American College of Cardiology.
Genetics don't seem to play role
"The death rate from coronary heart disease in Japan has always been puzzlingly low," lead researcher Dr Akira Sekikawa of the University of Pittsburgh said in a statement.
"Our study suggests that the very low rates of coronary heart disease among Japanese living in Japan may be due to their lifelong high consumption of fish."
On the other hand, genetics do not appear to be playing a role, the researchers say.
The US study group included both white and Japanese-American men, and both had higher atherosclerosis rates and lower levels of omega-3 fatty acids than men from Japan did.
In fact, Japanese-American men had the highest rate of coronary artery calcification - a buildup of calcium deposits in the arteries that signals atherosclerosis. CT scans showed that 31 percent of Japanese-American men had calcium deposits, compared with 26 percent of white US men and just nine percent of Japanese men.
The results raise the question of whether Japanese Americans will actually start to become more vulnerable to heart disease than white Americans, according to Sekikawa's team.
Not only fish diet – but one low in saturated fat
On the more positive side, the researchers say, the findings also suggest a way to ease some of the burden of heart disease in the US - get Americans to eat more fish.
It's likely, the researchers explain, that the low rate of heart disease among Japanese adults stems from a lifetime of high fish intake - a protective effect that a short time on fish oil pills is unlikely to match.
But eating fish is not the whole dietary story, according to a commentary published with the study. Research has shown that certain other groups with high fish consumption - namely, Norwegians and Inuit populations in the Arctic - have "Western" levels of atherosclerosis, notes Dr William Harris of the University of South Dakota in Sioux Falls.
The difference, he writes, may be that the average Japanese diet is not only rich in omega-3 fatty acids, but also relatively low in saturated fat.
"The Japanese experience," Harris writes, "when contrasted to that of the Inuit and the Norwegians, suggests that the cardioprotective punch of the long-chain omega-3 fatty acids may be no match for diets high in fat, particularly saturated fat." – (Reuters Health, July 2008)
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