Health experts have long urged Americans to get up and get moving. But what kind of exercise should they do? And how lengthy and vigorous must it be for them to receive the greatest benefit?
Despite the apparent consensus of a number of health groups that have, in recent years, recommended that Americans engage in exercise of moderate intensity, scientists remain sharply divided over those questions.
Some researchers contend that short bouts of moderate exercise throughout the day -- walking, gardening, housework -- are sufficient for good health, while others maintain that only sustained, vigorous activities, like running, will do.
Their disagreement is much more than an academic dispute. According to the National Coalition for Promoting Physical Activity, a coalition of health and fitness groups that in August began a campaign to get sedentary and overweight Americans moving, more than 250,000 deaths per year in this country are directly attributable to prolonged inactivity.
The controversy over exercise intensity began in 1993, when the American College of Sports Medicine, a professional group, dropped its long-standing recommendation that Americans do a minimum of 30 minutes of sustained vigorous exercise three times a week.
Instead, the college, along with the Centers for Disease Control and Prevention, the National Institutes of Health, the U.S. Surgeon General, and the American Heart Association, settled on 30 minutes of moderate exercise, spread throughout the day, as a means to get Americans moving.
But many scientists say the research data do not support such a change. What's more, they add, the lower standard is misleading. "The perception is that you don't need to sweat to get the benefits of exercise," says Paul T. Williams, in an interview here at the University of California's Lawrence Berkeley Laboratory. That, he maintains, is just plain wrong.
For the past six years, Dr. Williams has sought to determine what kinds of health benefits result from differing amounts and intensities of exercise. A biostatistician specializing in the evaluation of health data, he is conducting the nation's largest continuing study of physical activity, examining the health, diet, and exercise habits of 55,000 people who run regularly for exercise. Much of what he has uncovered through the "National Runners' Health Study," as his survey is called, is at odds with the government's own exercise prescriptions.
Current government guidelines, for example, state that the health benefits of exercise depend largely on the total amount of physical activity, rather than on its intensity.
But Dr. Williams found in a study of a portion of the runners in his survey that prolonged, vigorous exercise -- enough to make a participant winded or break into a sweat -- produces health benefits beyond those provided by the recommended daily walk.
Medical data forwarded by the physicians of more than 10,000 of the runners, along with participants' weekly running mileage and their best times in 10-kilometer races, showed that the faster runners -- those who typically exercised the most vigorously -- had lower blood pressures and triglycerides and narrower waistlines, he says. All of those factors reduce the risk of coronary disease.
Running a longer distance, a less vigorous form of exercise, on the other hand, raised the level of high-density lipoproteins in the blood -- the so-called "good" cholesterol, another mitigating factor in the risk of coronary disease -- six times as high as did simply running fast.
Dr. Williams's studies also found that the benefits of vigorous activity continue beyond the point assumed by most exercise researchers. Many scientists, he says, have assumed that the health benefits of running level off after 30 miles per week. But he found that reductions in the risk of heart disease increase linearly at least through 40 miles per week, while reductions in body weight -- a risk factor for diabetes as well as heart disease -- continue through at least 50 miles per week.
Because only a small percentage of runners train more than 50 miles per week, he adds, statistically valid statements can't be made beyond that distance. "A lot of people like to point out that it must stop at some point, and they're probably right," he says. "But I think that for most people, the issue of whether or not they're doing more exercise than they're going to get benefit from is really a non-issue."
Peter Wood, an emeritus professor of medicine at the Stanford University medical school who spent his career studying the health effects of exercise, says those and other results show that the government's exercise guidelines fall far short of what people should do.
"You need a substantial amount of exercise in both length of time and intensity" to receive most of the health benefits, he says. "I think the guidelines understated that."
Paul Thompson is president-elect of the American College of Sports Medicine and director of preventive cardiology at Hartford Hospital, in Connecticut. "The guidelines imply that you don't need to do very much," he says. "But it really looks like the more you do, the more benefit you get. I would prefer my patients -- the ones who don't have heart problems -- to have one vigorous exercise session every day."
Russell R. Pate, chairman of the department of exercise science at the University of South Carolina and principal author of the government's exercise recommendations, concedes that intense exercise does provide some additional health benefits over moderate exercise. But he defends the current guidelines as the best scientific statement on exercise, from a public-health perspective, for a largely sedentary nation.
"If we're interested in public health," he says, "I think it would be questionable to base our recommendations on the data" from Dr. Williams's studies. "These are people who are at the high end of the exercise spectrum."
Harold W. Kohl III, director of research at the Baylor College of Medicine's Sports Medicine Clinic, concurs: "A lot of people will never be able to do intense exercise, but most people can benefit from getting off the couch."
Dr. Williams -- himself a recreational runner who logs about 50 miles a week -- notes that his studies also include people who run less than 10 miles a week, an amount of exercise, in terms of calories expended, that would just meet the current guidelines.
Even so, he says, the main value of his data base is that "it goes beyond where all of the other studies stop." Previous studies of the relationship between exercise and health weren't designed as exercise studies, he explains. "These were surveys of health, and exercise was one of the questions that just happened to get in there. But the real distinction between all of the other studies and ours is that we happen to focus on one activity: running. That allows us to come out with a very specific statement."
Dr. Williams obtained the subjects for his data base from among subscribers to the monthly magazine Runner's World and from participants in major races around the country involving recreational athletes. Because the data base includes a large number of physically active subjects, all participating in one activity, it can address many questions for the first time, he says. And because runners are typically diligent about tracking their weekly mileage, approximate training pace, and best performance times, "it makes them ideal for quantifying what they're doing."
Nevertheless, Dr. Pate says Dr. Williams's results have only limited value, because they measure coronary risk factors. Those factors, he argues, are not as reliable as data from clinical trials, where subjects are followed until they develop a disease or die.
But Stanford's Dr. Wood notes that a clinical trial to collect such information is impossible. "Of course it would be much more powerful to assign people to specific running mileages and follow them to their death," he says. "But that would take 50 years. It will never be done." He calls Dr. Williams' survey "ingenious" because it inexpensively gathers a large amount of data to evaluate the effects of differing intensities of exercise in a statistically reliable way. "He's done a study that would cost millions of dollars for less than $ 100,000."
The runners' health study, which includes thousands of women runners, is also unique in focusing on many questions about women and exercise, he says.
"A lot of scientists have said that women don't benefit the same way that men do with exercise," Dr. Williams says. "We found results that contradicted those notions. We saw the same improvements in H.D.L. in women as we saw in the men."
His most surprising finding came to light in May, when he reported in the American Journal of Clinical Nutrition that a study of 4,769 runners had revealed that weight gain occurred at the same rate, regardless of the number of miles run per week.
"What was most surprising was that the increase in waistline and the increase in body weight with age were identical if you were running less than 10 miles per week or 40 miles per week," Dr. Williams said. As a result, he concluded, if people want to compensate for their middle-age weight gain, they would have to exercise more. For runners, he calculated, that would require increasing their weekly running distance by 1.4 miles each year. "What this means is that runners who average 10 miles per week at age 30 should increase their weekly running distance to 24 miles by age 40 if they plan to still fit into the tuxedo they bought a decade earlier."
Some colleagues criticized Dr. Williams at last May's annual meeting of the American College of Sports Medicine in Denver. His conclusions, they argued, were obstacles in efforts to promote physical activity to a sedentary population.
"What would be achieved with guidelines if you recommended to individuals that unless you run 30 to 40 miles a week, you're not going to get any benefit?" asked William L. Haskell, a professor of medicine at Stanford. "And also, that every time you get older, you've got to do more exercise? I'm not sure that would contribute a lot."
Dr. Williams responded that scientists should be more worried about the accuracy of their statements than about how the public perceives them. "I believe that the current exercise guidelines basically come down to reducing the level of what we expect of people, so that more people can be winners," he said. "When you recommend more-vigorous exercise, I don't think you're discouraging people from attempting to increase their level of physical activity. The last thing we need is the government tossing in the towel for people."
Even Ralph S. Paffenbarger, Jr., an emeritus professor of epidemiology at Stanford who helped to formulate the government's guidelines, is concerned that they may be leading people away from exercising more vigorously.
But he also recommends caution in applying the results of Dr. Williams's studies to the public at large: "The studies are well done, the results are clear, but there's a question of generalizability. We're dealing with a population that may or may not be a random sample."
Dr. Williams hopes to clear up some of those concerns by extending his study to other groups of exercisers. He is gathering subjects from among the subscribers of Walking magazine, and he expects within the next month to have a site on the World-Wide Web where people who engage in other activities -- swimming, rowing, basketball -- can participate in a long-term study of their health and the health benefits of their particular activity.
"I don't think the results we've observed are restricted just to running," he says. "Sustained and vigorous are the key characteristics of running, so as long as other activities meet those criteria, we expect to get the same benefits."
While he compares the health benefits of other activities, Dr. Williams has no intention of dropping his study of runners. "The runners are only going to get more valuable. They're going to get older -- and to that extent, they're going to be just like the rest of the country's population."
No comments:
Post a Comment