Exercise could be bad for few people and might trigger heart and blood pressure risk, reveals study.
By studying the data from six rigorous exercise studies involving 1,687 people, the group found that about 10 percent actually got worse on at least one measure related to heart disease: blood pressure and levels of insulin, HDL cholesterol or triglycerides and about 7 percent got worse on at least two measures.
But the researchers don't have answer to such peculiar finding.
"It is bizarre," the New York Times quoted Claude Bouchard, lead author of the paper and a professor of genetics and nutrition at the Pennington Biomedical Research Center, part of the Louisian State University system as saying.
While many researchers have applauded the study, there are many who are worried about the consequences.
"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 ten percent," said Haskell
But counterbalancing the 10 percent, who got worse were about the same proportion who had an exceedingly 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 got better anywhere from 20 to 50 percent
"That should make folks happy," said Dr. William E. Kraus, a co-author of the study who is professor of medicine and director of clinical research at Duke.
Kraus was a member of the committee providing the scientific overview for the Department of Health and Human Services' national exercise guidelines, which advise moderate exercise for at least 150 minutes a week.
Researchers point out that the problem with studies of exercise and health, is that, while they often measure things like blood pressure or insulin levels, they do not follow people long enough to see if improvements translate into fewer heart attacks or a longer life.
Instead scientists infer that such changes lead to better outcomes - something that may or may not be true.
Some critics have claimed that there is no indication that those who had what Dr. Bouchard is calling an adverse response to exercise actually had more heart attacks or other bad health outcomes.
But Dr. Bouchard insisted that if people wanted to use changes in risk factors to infer that those who exercise are healthier, they could not then turn around and say there is no evidence of harm when the risk factor changes go in the wrong direction.
"You can't have it both ways," Dr. Bouchard said.
The national guidelines for exercise are based on such deductions and also on studies that compared the health of people who exercised with that of people who did not, a weak form of evidence often said to be hypothesis-generating rather than proof.
"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 there is a lot of debate over how strong the guidelines should be in light of weak evidence," he said.
Researchers also claim that, people should continue to exercise as before, but, might also consider getting their heart disease risk factors checked on a regular basis.
Kraus said No intervention, including drugs, works for everyone. So it should not be surprising that exercise does not work for some.
"I am an exercise guy - I believe in exercise for health. I would rather have everyone exercise. But you can't ignore the data," Dr. Kraus said.
Yet, he added that even if someone does not get the expected advantage in some heart risk factors, there are other reasons to exercise - for mental health and to improve physical functioning.
"It is not possible yet to make more specific recommendations because we do not understand why this is happening," Dr. Bouchard said.
Dr. Bouchard stumbled upon the adverse exercise effects when he looked at data from his own study that analyzed genetics and responses to exercise.
He observed 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.
He then looked for other clinical experiments that also examined exercise under controlled conditions, making sure that participants actually exercised and did not change their diets, and carefully measuring heart risk factors and how they changed with an exercise program.
He discovered 5 studies in addition to his own and in all the studies, about 10 percent, had at least one measurement of heart disease risk that went in the wrong direction.
Then the scientists asked if there was some way of predicting who would have an adverse effect.
They found it was not connected to how fit the people were at the start of the study, nor to how much their fitness improved with exercise.
Age, race, gender had nothing to do with it. In some studies participants were allowed to take medications to control their blood pressure or cholesterol levels while in others they were not.
Drug use did not matter. Nothing predicted who would have an adverse response.
Some, like Dr. Benjamin Levine, a cardiologist and professor of exercise sciences at the University of Texas Southwestern Medical Center, asked whether the adverse responses signified just random fluctuations in these heart risk measures.
Would the same proportion of people, who did not exercise also get worse over the same span of time? Or what about seasonal differences in things like cholesterol? Maybe the bad effects just reflected the time of year when people entered the study.
But the researchers examined those hypotheses by looking in detail at some of the studies and found that they did not hold up.
Dr. Kraus revealed that researchers needed to figure out how to tailor exercise prescriptions to an individual's needs.
For example, people who have good cholesterol and insulin levels but bothersome blood pressure would want to know if exercise made their blood pressure rise.
An increase in blood pressure would not be compensated by improvements in already good cholesterol or insulin levels.
Dr. Lauer claimed that if nothing else, the study pointed out the need to know more about what exercise actually does.
"If we are going to think of exercise as a therapeutic intervention, like all interventions there will be adverse effects," he said.
"There is a price for everything," he added.
This study has been published in PLoS One.