DALLAS, Nov. 4 -- A new study of more than 20,000 children on three continents has found significant differences in blood cholesterol -- particularly in cholesterol's healthiest component, abbreviated HDL.
This study marks the first report of these striking differences in HDL (high-density lipoprotein) cholesterol among Japanese, Australian and U.S. children, says Terry Dwyer, M.D., in a study published this week in the American Heart Association journal Circulation. Dwyer is director of the Menzies Centre for Population Health Research at the University of Tasmania in Australia. "Coronary heart disease mortality for both men and women in Australia and the United States is more than five times higher than it is Japan," he says. This relatively low death rate from heart disease is puzzling since the rates of cigarette smoking are higher among Japanese men than among those in the other two countries.
Researchers also have wondered how the Japanese have been able to adopt an increasingly "westernized" lifestyle without experiencing a concurrent jump in heart disease, stroke and other cardiovascular diseases.
"The relatively high ratio of blood HDL to total cholesterol and the relatively small decline in HDL as boys age may help to explain why the coronary heart disease mortality rate in Japan is low compared with that in other developed countries," says Dwyer.
HDL, the so-called "good" cholesterol, helps protect against the plaque buildup that can obstruct blood flow in vessels by clearing fats from the bloodstream, The researchers studied data on more than 20,000 children ages 7 to 15 years in Japan, Australia and the United States. The data included cholesterol levels for more than 7,000 of the children.
Total cholesterol levels among children were very similar among the three countries. But when researchers looked at the various components of total cholesterol, they found two significant differences. First, the amount of HDL was higher in Japanese children than in those in Australia and the United States. Second, the amount of HDL declined much less as the Japanese boys aged than it did in boys in the other two countries. HDL normally declines as children age.
The principal influences on HDL in adults and children are levels of body fat, physical activity and hormones, the researchers say.
There were no major differences in body mass index, a measure of obesity, between Japanese and Australian children, although U.S. numbers were higher at each age. However, the Japanese children were more active than those in Australia were at all ages and in both sexes.
In addition, Japanese children ate less total fat -- 27 percent of their energy came from fat compared to 37 percent for the Australian youngsters and 36 percent in the youngest U.S. children in the study.
Several other dietary differences were reported. For instance, the major cereal is rice in Japan and wheat in Australia. In addition, Japanese children eat more soybean products, specifically tofu.
"Tofu has been shown to contain phytoestrogens, which in the amounts consumed by the Japanese children might be responsible for their higher HDL," the researchers say. Phytoestrogens are substances that seem to act like hormones in the body due to their similarity to human hormones.
The study raises many questions for future research.
"Foremost among the hypotheses to be tested is that childhood differences in food intake and physical activity may importantly affect later risk of coronary heart disease death via a pathway that involves adolescent HDL concentration," they write.
The researchers are based at the Menzies Centre, Tokyo Medical College and Tulane University in New Orleans. Their data came from several studies including the Australian Schools Health and Fitness Survey, 1985; the Australian Schools Dietary Survey, 1985; the Japanese Society of School Health Survey, 1993; the Japanese Physical Education and School Health Centre Dietary Survey, 1991 and 1992; and the Bogalusa Heart Study in Louisiana.
Co-authors are Malcolm Riley, Ph.D.; Hisao Iwane, M.D.; Kimberlie Dean, B.Med.Sci.; Yuko Odagiri, M.D.; Teruichi Shimomitsu, M.D., Ph.D.; Leigh Blizzard, M.Ec.; Sathanur Srinivasan, Ph.D.; Theresa Nicklas, Dr.P.H.; Wendy Wattigney, M.S.; and Gerald Berenson, M.D.