You’ve heard the recommendations. Exercise moderately for at least 30 minutes on most days of the week to improve your health. Or is it 60 minutes? It depends who you ask. The Centers for Disease Control and Prevention (CDC) and the Americal College of Sports Medicine (ACSM) recommend at least 30 minutes of moderate exercise (walking briskly, swimming, bicycling on level terrain) on most days of the week, while the Institute of Medicine (IOM) recently upped its recommendation to at least 60 minutes. Who’s right?

Regular exercise can help you lose weight and reduce your risk of heart disease , stroke , diabetes , and certain cancers, including breast cancer . But what is the minimum intensity and duration of exercise necessary to improve health? If you don’t exercise strenuously for an hour every day, is it even worth it?

A new study in the September 10, 2003 issue of the Journal of the American Medical Association found that exercising longer at a higher intensity did not significantly increase weight loss in previously sedentary women enrolled in a weight loss program. Another study in the same issue found that physical activity significantly reduced the risk of breast cancer in postmenopausal women, whether the activity was strenuous or not.

About the Studies

The first study included 201 sedentary women ages 21–45 with a body mass index (BMI, a measure of body fat based on height and weight) of 27–40. The women participated in a 12-month weight loss intervention that included the following:

  • Exercise, five days per week
  • Reduction of calorie intake to between 1,200 and 1,500 calories per day
  • Reduction of fat intake to between 20% and 30% of total calories
  • Weekly group meetings for the first six months, then biweekly meetings for the next six months
  • Biweekly supportive telephone calls during months 7–12

For the exercise component of the intervention, the women were given motorized treadmills and encouraged to walk in bouts of at least 10 minutes. They were randomly assigned to one of four groups:

  • Vigorous intensity/high duration
  • Moderate intensity/high duration
  • Vigorous intensity/moderate duration
  • Moderate intensity/moderate duration

Exercise intensity was measured using heart rate and perceived exertion scales. Duration of exercise ranged from 30–60 minutes, depending on exercise group.

Before the study began, and after six and 12 months, the women completed height and weight measurement, a cardiovascular fitness test, a diet questionnaire, and a leisure-time activity questionnaire. The women also logged the minutes, intensity, and type of exercise they did.

In the second study, researchers tracked the medical histories of 74,171 postmenopausal women ages 50–79 enrolled in the Women’s Health Initiative (WHI) study. The medical histories included information about whether the women had strenuously exercised (long enough to work up a sweat and make their heart beat fast) at least three times per week at ages 18, 35, and 50. The women also provided information about how often they currently exercised and whether that exercise was strenuous, moderate, or low-intensity.

The Findings

In the weight-loss study, all four groups lost a significant amount of weight over 12 months—an average of 20 pounds for the vigorous intensity/high duration group, 18 pounds for the moderate intensity/high duration group, 15 pounds for the vigorous intensity/moderate duration group, and 14 pounds for the moderate intensity/moderate duration group. Their cardiovascular fitness also increased significantly after the 12 months—by 22% in the vigorous intensity/high duration group, 15% in the moderate intensity/high duration group, 19% in the vigorous intensity/moderate duration group, and 14% in the moderate intensity/moderate duration group. Although these changes were significant in all groups, they were not statistically significant between the groups. In other words, the observed differences were most likely due to chance. There was no difference among the groups in calorie intake.

Although there was no difference in weight loss or improvement of cardiovascular fitness between exercise groups, the researchers found that the women who reported exercising for 200 minutes per week or more at months six and 12 lost significantly more weight and gained significantly more cardiovascular fitness than women who reported less exercise.

In the WHI study, the women who reported regular strenuous physical activity at age 35 had a 14% decreased risk of breast cancer. Also, increasing amounts of current exercise was associated with decreasing risks of breast cancer.

Compared with sedentary women, women who participated in the equivalent of 1.25-2.5 hours of brisk walking per week had an 18% reduction in the risk of breast cancer. This reduction was not significantly different from the 21% reduction seen in women who exercised for more than seven hours per week. Current physical activity had the strongest effect on reducing risk of breast cancer in women with the lowest BMIs and the smallest waist circumferences.

How Does This Affect You?

Both of these studies support the CDC and ACSM recommendations of 30 minutes of moderate exercise on most days of the week. In the weight-loss study, high intensity exercise was no more effective than moderate intensity exercise in aiding weight loss and improving cardiovascular fitness. The WHI study suggested that it isn’t necessary for women to exercise strenuously to enjoy the protective effects of exercise—a couple of hours of brisk walking per week can reduce the risk of breast cancer.

Following the CDC and ACSM recommendation would equate to exercising for at least 120 minutes per week (30 minutes, four days a week). But the first study also indicates that getting more exercise would result in additional benefits. While all levels of exercise in the weight-loss study aided weight loss and improved cardiovascular fitness, the women who exercised for more than 200 minutes per week saw more substantial improvements. The researchers in this study conclude that weight loss programs should have an initial goal of at least 150 minutes of moderate intensity exercise per week, and—where appropriate—progress to the IOM’s recommendation of 60 minutes per day.

So what should you do? An editorial related to these studies suggests that people should exercise moderately for at least 30 minutes on most days of the week, and, if they are willing to do more, they should, because they will see additional health benefits.