Women who want to lose a lot of weight should walk briskly for an hour a day while cutting calories, says a new study in the Journal of the American Medical Association.
But there's hope for time-pressed dieters: Even walking for 30 to 40 minutes can help women shed quite a few pounds.
These findings confirm other research on the importance of exercise for weight control.
For the latest study, researchers at Brown University and the University of Pittsburgh followed 184 sedentary women who weighed an average of 200 pounds. The women, ages 21 to 45, were told to consume 1,200 to 1,500 calories a day and exercise five days a week for a year.
Participants could exercise continuously or in 10-minute bouts. The women were given treadmills to use in their homes and wore special pedometers to track their activity. They also kept exercise logs and completed questionnaires.
The women worked up to walking briskly -- moving as if they were late for the bus or a meeting.
They also attended weekly meetings on changing eating and exercise behaviors for the first six months and then every other week for the next six months. Many lost weight during the first half of the study and then kept it off for the second half.
The findings at the end of the year:
The women who exercised for 50 to 60 minutes a day and consumed about 1,500 calories a day lost and kept off 12% to 14% of their starting weight, or about 25 to 30 pounds. They were burning 2,000 calories or more a week with exercise.
Those who exercised 30 to 40 minutes a day and consumed about 1,500 calories lost and kept off 9% of their starting weight, or about 16 to 20 pounds. They were burning about 1,000 to 1,500 calories a week with exercise.
To lose a lot of weight, you should modify your diet and "do as much exercise as you possibly can within safe limits," say lead author John Jakicic, director of the Physical Activity and Weight Management Research Center at the University of Pittsburgh.
"You can walk at a little slower pace (as long as it's not just strolling) and go longer, or you can go at a faster pace and do it a shorter period, as long as you burn the same amount of calories in the end."
If you are on a treadmill, the speed probably should be at least 3 mph to 4 mph, he says.
Although the researchers did not study men, a similar recommendation might be equally effective for them, Jakicic says.
The latest research confirms previous studies on exercise. Data from the National Weight Control Registry, a group of more than 4,000 people who have maintained an average of a 67-pound weight loss over five years, shows that members exercise for about 60 to 90 minutes a day.
And last September, a report from the National Academies' Institute of Medicine recommended that Americans do at least an hour of some physical activity -- walking, playing tag, biking -- a day to control their weight.