BOSTON -- Researchers using a new method for measuring cholesterol are offering a familiar message: Get some exercise.
A new study suggests that even modest exercise can benefit the heart by making cholesterol less damaging to the arteries, even if levels of the substance are unchanged.
Staying active has many health benefits, but improving cholesterol is not usually considered one of them. Workouts fail to lower LDL, the dangerous form of cholesterol, and only rigorous exercise can nudge up HDL, the good form that protects against heart attacks.
But the study, by Dr. William E. Kraus of Duke University, found a new way that exercise can affect cholesterol -- by altering the number and size of the particles that carry cholesterol through the bloodstream.
''People in the exercise field have always wondered why it doesn't affect total cholesterol and LDL,'' Kraus said. ''We always knew low levels of exercise are helpful. This helps solve that paradox.''
His work, published in November's issue of the New England Journal of Medicine, is the latest chapter in an evolving view of cholesterol's effects. A generation ago, doctors worried only about the total amount of cholesterol. Later, the importance of the main subtypes, especially HDL, became apparent. Now experts are turning their attention to the physical structure of cholesterol in the bloodstream.
Cholesterol is an essential fat, or lipid. It circulates through the body by attaching to protein particles. Cholesterol appears more likely to clog the arteries when it is carried by small, dense protein particles than when it is moved by relatively large, fluffy ones.
The latest study finds that people who exercise develop these bigger particles, even if their total amount of cholesterol stays the same.
''Using this analysis shows clearly that exercise has beneficial effects that are not revealed by standard tests,'' said Dr. Ronald M. Krauss of the Lawrence Berkeley National Laboratory, who studies the protein particles.
The study, conducted at Duke and East Carolina University, involved 111 sedentary, overweight men and women. They were randomly assigned to three exercise groups: the equivalent of walking 12 miles a week, jogging 12 miles a week or jogging 20 miles a week. All were instructed to eat enough to keep their weight constant.
They found that the cholesterol effects of walking and jogging 12 miles were the same, while jogging 20 miles resulted in more pronounced changes.
Measuring protein particle size is sometimes done in large medical centers, but it is not part of standard physicals. Kraus said he expects the tests, which cost two or three times more than standard cholesterol tests, to become more widely used.
Dr. Joann Manson, head of preventive medicine at Harvard's Brigham and Women's Hospital, noted that exercise has already been found to have many other benefits for the heart, including improvements in blood pressure, blood sugar, clotting and inflammation.
Studies show that briskly walking 30 minutes a day can lower the risk of heart disease by 30 percent to 40 percent.
''Lipids are not the full story,'' Manson said.
EDITOR'S NOTE: Medical Editor Daniel Q. Haney is a special correspondent for The Associated Press.
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