Over half of Americans are overweight or obese . In fact, the prevalence of obesity has increased by almost 50% since 1991. This is no small matter, since overweight and obesity have been linked with increased risks for a range of medical problems including coronary heart disease , diabetes , and high blood pressure .

Diet and exercise are known contributors to weight change. The reason is simple: if you consume more calories than you expend, you gain weight; if you expend more calories than you consume, you lose weight. So, the key to weight maintenance is to find a balance between food intake and exercise expenditure. For most Americans, that means cutting calories and increasing exercise.

But how much exercise do we really need? In 1995, the Centers for Disease control and Prevention, along with the American College of Sports Medicine, recommended that all American adults get at least 30 minutes of moderate-intensity exercise on most—preferably all—days of the week. But the Institute of Medicine recently upped this recommendation to 60 minutes per day for the prevention of weight gain.

So, are they right? A new study in the January 12, 2004 issue of the Archives of Internal Medicine compared the effects of different amounts of exercise on the body weight and body composition of overweight, sedentary adults. The researchers found that the equivalent of 30 minutes of walking per day was enough to prevent weight gain and promote modest weight loss.

About the Study

This study included 120 men and women who were 40–65 years old, sedentary (exercising less than once per week), and overweight or mildly obese. The participants could not be currently dieting or have an intention to diet.

The participants were randomly assigned to one of the following exercise training programs, which lasted for eight months:

  • High amount/vigorous intensity group – the equivalent of jogging 20 miles per week
  • Low amount/vigorous intensity group – the equivalent of jogging 12 miles per week
  • Low amount/moderate intensity group – the equivalent of walking 12 miles per week
  • Control group – no exercise

The participants were instructed not to change their diet and encouraged to maintain their baseline body weight. To record their nutrient intake, the participants completed three-day food records and a 24-hour dietary recalls.

The researchers measured the participants’ height, weight, percentage body fat (using skinfold), and fat distribution (using waist circumference) both before and after the exercise program.

The Findings

As the amount and intensity of exercise increased, so did improvements in body weight, body composition, and fat distribution. Compared with the control group, all of the exercise groups had significant improvements in these measures.

The following table summarizes each group’s results:

MeasureHigh amount/ vigorous intensityLow amount/ vigorous intensityLow amount/ moderate intensityControl
Body weightLost 7.7 poundsLost 2.4 pounds*Lost 2.9 poundsGained 2.2 pounds
Body fatLost 10.8 poundsLost 5.7 poundsLost 4.4 poundsGained 1.1 pounds*
Lean tissueGained 3.1 poundsGained 3.3 poundsGained 1.5 poundsGained 1.3 pounds*
Waist circumferenceLost 1.3 inchesLost 0.6 inchesLost 0.6 inchesGained 0.3 inches*

*Change from baseline was not statistically significant

The participants in the high amount group had significantly better improvements in weight and body composition than the participants in the low amount groups. The low amount/vigorous intensity group saw no significant added benefit from the extra intensity, when compared to the low amount/moderate intensity group. Most of the participants in the low amount/moderate intensity group were able to complete the exercise requirements by exercising approximately 30 minutes per day.

From their data, the researchers estimated that the equivalent of 6–7 miles of walking or jogging per week was enough to prevent weight gain.

While these findings are intriguing, they are limited because one-third of the participants dropped out of the study. This is a substantial drop-out rate, which may compromise the reliability of the results.

How Does This Affect You?

These findings suggest that the amount of weekly exercise is directly associated with weight change in overweight, sedentary adults, even with no change in diet. Compared with no exercise, all amounts and intensities of exercise resulted in improvements, with greater amounts yielding greater improvements.

These results question whether the Institute of Medicine’s minimum recommendation of 60 minutes of exercise a day to prevent weight gain is really necessary. About 40% of American adults report no exercise at all, and a recommendation of an hour of exercise every day just to prevent weight gain may be discouraging. If you are unable—or unwilling—to devote seven hours of your week to exercise, is it worth exercising at all? According to this study, as little as 6–7 miles per week, which could be accomplished by walking 15 minutes per day, may be enough to prevent weight gain. Although this is encouraging, and is certainly worth a try, other studies still need to be done to confirm this conclusion.

Once again, these findings support the adage: “forget pills, forget fad diets—burn enough calories and you’ll lose the weight.” And remember—any amount of exercise is better than no exercise at all.