DURHAM, N.C. -- The results of a Duke University Medical Center randomized controlled trial strongly suggests that not only can 30 minutes of daily walking prevent weight gain in most sedentary people, but that any further exercise can lead to additional loss of weight and fat.
The researchers said that their study provides the first scientific basis for a "more is better" approach to exercise. The 30 minutes of daily walking is the equivalent of 10 to 12 miles of walking each week, the researchers said, adding that for most sedentary people, this amount of exercise will offset the slow and incremental weight gain of inactivity.
The results of the Duke study were published Jan. 12, 2004, in the Archives of Internal Medicine.
The Duke team enrolled overweight and sedentary adults into a program of three escalating levels of exercise for more than eight months. Since the purpose of the trial was only to gauge the effects of different amounts and intensities of exercise, participants were encouraged not to change their normal diet. The main measurements were changes in weight, body composition and waist circumference.
"We found that the two low-exercise groups lost both weight and fat, while those in the more intensive group lost more of each in a 'dose-response' manner," said Cris Slentz, Ph.D. "Simply put, the more you exercise, the more you benefit. Just as importantly, the control group of participants who performed no exercise gained weight over the period of the trial."
"From the perspective of prevention, it appears that the 30 minutes per day will keep most people from gaining the additional weight associated with inactivity," Slentz said. "Given the increase in obesity in the U.S., it would seem likely that many in our society may have fallen below this minimal level of physical activity required to maintain body weight."
The Duke study was supported by a $4.3 million grant from the National Heart, Lung and Blood Institute. The trial, dubbed STRRIDE (Studies of Targeted Risk Reduction Interventions through Defined Exercise), was led by Duke cardiologist William Kraus, M.D.
Since the ability of exercise to lower weight and fat was demonstrated without any changes in diet, the researchers also believe a focus on dietary intake can only add to the benefits of exercise in reducing weight.
For the trial, researchers randomized 120 participants into one of four groups: no exercise, low dose/moderate intensity (equivalent of 12 miles of walking per week), low dose/vigorous intensity (equivalent of 12 miles of jogging per week) or high dose/vigorous intensity (equivalent of 20 miles of jogging per week). The exercise was carried out on treadmills, elliptical trainers or cycle ergometers in a supervised setting.
Participants in the high dose/vigorous intensity group saw reductions in all measures when compared to the two lower exercise and control groups.
Specifically, the high dose/vigorous intensity group experienced a 3.5 percent weight loss, while the two low-dose exercise groups experienced slightly greater than a 1 percent weight loss. During the same period, the inactive control group showed a 1.1 percent weight gain.
Interestingly, the two vigorous intensity groups saw similar increases in lean body mass, or muscle, which were twice as high as the 0.7 percent increase for the low-intensity group.
"The higher exercise intensity groups resulted in greater increases in lean body mass, which if confirmed by other studies, could have significant implications," Slentz said. "This finding suggests that while the amount of exercise determines total body weight change and fat mass loss, exercise intensity would appear to be the primary determinant of gain in lean body mass."
In terms of body fat mass, the inactive group experienced a 0.5 percent increase, while all the exercise groups saw important decreases: 2 percent decrease for low dose/moderate intensity; 2.6 percent decrease for low amount/vigorous intensity; and 4.9 percent decrease for high dose/vigorous intensity.
"This study revealed a clear dose-response effect between the amount of exercise and decreases in measurements of central obesity and total body fat mass, reversing the effects seen in the inactive group," Slentz said. "The close relationship between central body fat and cardiovascular disease, diabetes and hypertension lends further importance to this finding."
Participants who did not exercise experienced an average 0.8 percent increase in waist circumference, while all the exercise groups saw decreases: 1.6 percent decrease for low dose/moderate intensity; 1.4 percent decrease for low amount/vigorous intensity; and 3.4 percent decrease for high dose/vigorous intensity.
Joining Slentz were Duke colleagues Brian Duscha, Johanna Johnson, Kevin Ketchum, Lori Aiken, Gregory Samsa, Ph.D., and Connie Bales, Ph.D. Joseph Houmard, Ph.D., East Carolina University, was also a member of the team.