Experts divided over ideal duration of daily exercise

By Ira Dreyfuss
Associated Press

The surgeon general says people should get at least 30 minutes of moderate daily physical activity. A report by the Institute of Medicine says it should be at least an hour.

People with the healthiest weights often do one hour of exercise daily. Advertiser library photo • 1999

The recommendations that the institute issued Sept. 5 have sparked a debate about how to encourage Americans to be healthier.

A key question is whether to focus on inactivity or obesity as the best target for reducing risks of conditions ranging from diabetes to heart attacks. An active life reduces the risk; so does a svelte body. Both together are best, but either exercise or weight control separately has some benefit.

The surgeon general's 1996 report leans toward encouraging physical activity, and says 30 minutes at a moderate intensity, such as brisk walking, is the minimum needed.

The institute thinks that's not enough — that people must control their weight as well, and burn the calories to do this even if it takes longer.

The institute is part of the National Academy of Sciences, an independent organization chartered by Congress to guide the government on scientific issues.

Its recommendations will form the basis for updates of healthy eating guidelines by government agencies in the United States and Canada. The report, written by a panel of scientists, offers more flexibility in the amount of fats and carbohydrates eaten, suggesting 45 percent to 65 percent of calories from carbohydrates and 20 percent to 35 percent from fats.

But the report is the first to include recommendations for physical activity, and they are based on the activity levels of people with the healthiest weights, said a panel member, Dr. Benjamin Caballero of Johns Hopkins University.

"One of the goals of our recommendations is to maintain a normal body weight," Caballero said. "You should do physical activity that is associated with that weight."

People with the healthiest weights do at least an hour of daily moderate physical activity such as walking, Caballero said. Therefore, the panel decided this would be a good goal for reducing obesity and its health risks.

Caballero said those who wish to spend less time will have to be more active — jogging, for instance, instead of walking. People also can be physically active through daily living — taking the stairs, for instance, instead of an elevator.

Although the institute study says there is some value in the surgeon general's recommendations, a researcher whose work was a keystone of the surgeon general's report wishes those guidelines had gotten more emphasis.

"The public needs to be reassured the three 10-minute walks are still important and will still provide a lot of health benefits, and they should do them," said Steven Blair of the Cooper Institute, a Dallas-based research organization that focuses on exercise.

Blair found that people who go from getting no exercise to getting the surgeon general's recommended 30-minute minimum can reduce their risk of death from all causes. The Institute of Medicine didn't spell out specific health benefits of its proposed additional 30 minutes of activity, he said.

"It seems the principal reason for the 60 minutes is they think that it is what people with ideal (weights) do," Blair said. "That seems a rather narrow interpretation to me."

Weight control and exercise are important, said I-Min Lee, an assistant professor at the Harvard School of Public Health and a leading researcher in the role of physical activity in promoting health and preventing chronic disease.

"In the research I've done, overweight and inactivity seem to be equivalently weighted risk factors," Lee said.

But federal statistics show about 60 percent of Americans get too little activity even by the surgeon general's standards, and Lee feared that doubling the requirement would raise the bar too high.

"I would like to see everyone exercising an hour a day in moderate intensity, but most people are not going to do it," Lee said. "People are really busy, and it requires will power and sweating and effort to do it."

People might also control their weight by eating less. "Part of it is cultural," Lee said. "American portion sizes are the largest I have seen in any country."

The institute's recommendations will be considered in the update of the federal dietary guidelines due out in 2005, said Agriculture Department spokesman John Webster. USDA and the Department of Health and Human Services issue the guidelines jointly.

HHS also includes the surgeon general's office, and HHS spokesman Bill Pierce said the institute's exercise recommendations will be something for surgeon general Richard H. Carmona "to visit in the future."