Past researches proved that exercise is good for health. But now, a group of researchers, including one who helped write the scientific paper justifying national guidelines that promote exercise for all, is not agreeing now with the above statement. By analyzing data from six rigorous exercise studies involving 1,687 people, the group found that about 10 percent actually got worse on at least one of the measures related to heart disease: blood pressure and levels of insulin, HDL cholesterol or triglycerides. About 7 percent got worse on at least two measures. And the researchers say they do not know why.

“There are a lot of people out there looking for any excuse not to exercise,” said William Haskell, Emeritus Professor of Medicine at the Stanford Prevention Research Center. “This might be an excuse for them to say, ‘Oh, I must be one of those 10 percent.’ ” But counterbalancing the 10 percent who got worse were about the same proportion who had an exaggeratedly good response on at least one measure. Others had responses ranging from little or no change up to big changes, seen in about 10 percent, where risk factor measurements improved anywhere from 20 percent to 50 percent.

“We do not know whether implementing exercise programs for unfit people assures better outcomes,” said Dr. Lauer of the Heart Institute. “That has not been established.” And so, he said, “there is a lot of debate over how strong the guidelines should be in light of weak evidence.”

Dr. Bouchard stumbled upon the adverse exercise effects when he looked at data from his own study that examined genetics and responses to exercise. He noticed that about 8 percent seemed to be getting worse on at least one measure of heart disease risk. “I thought that was potentially explosive,” he said.