"The price to pay for quitting exercise is higher than expected, and this price may be an important factor in the obesity epidemic affecting Americans," Paul Williams of the U.S. Department of Energy's Lawrence Berkeley National Laboratory says in a statement.

The study, published in Medicine & Science in Sports and Exercise, shows the key to keeping weight off is exercising "sufficiently and consistently."

Williams compared 17,280 men and 5,970 women who decreased their running distance with 4,632 men and 1,953 women who increased their running distance over a 7.7-year period.

Runners who decreased their distance from 5 to zero miles per week gained four times as much weight as those who decreased their distance from 25 to 20 miles per week. Williams also found that people who started running after an exercise layoff didn't't lose weight until their mileage exceeded 20 miles per week in men, and 10 miles per week in women.