If you want to use exercise to help control your weight, don't stop. Paul Williams of the University of California at Berkeley showed that interrupting an exercise program can cause you to gain weight that won't come off easily even after you resume training (Medicine & Science in Sports & Exercise, February 2008).
Williams compared 17,280 men and 5,970 women who decreased their running distances with 4,632 men and 1,953 women who increased their running distances over an eight year period. He found that runners who decreased their distance from five to zero miles per week gained four times as much weight as those who decreased their distance from 25 to 20 miles per week. He also found that people who started running after an exercise layoff didn't lose weight until their mileage exceeded 20 miles per week in men, and 10 miles per week in women.
The people in the lower mileage groups were not able to lose the weight gained during time off if they just resumed the same exercise regimen. This study explains why exercise programs designed to prevent obesity may fall short if the exercise is irregular, seasonal, or often interrupted. "Many scientists attribute the obesity epidemic to excess calories rather than [lack of] exercise, because dieting has been shown to produce more weight loss than exercise," says Williams. "My findings suggest that calorie intake and body weight may be self regulating in active individuals." More on exercise and weight control