Want a brake from your training program? Think twice! It could cost you more than you thought, as found by a new research from the U.S. Department of Energy's Lawrence Berkeley National Laboratory and published in the journal "Medicine & Science in Sports and Exercise."

Weight gained during a rest period can be harder to get rid of when exercise is resumed later. The key of staying trim is to remain active year-round, year after year, avoiding seasonal and irregular exercise patterns, despite the pressures of family and work obligations, or various other reasons.

"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," said lead researcher Paul Williams.

Information coming from the National Runners' Health Study has revealed that increasing/decreasing exercise intensity impacts runners differently. At distances of over 20 miles (32 km) weekly in men and 10 miles (16 km) in women, the extra weight achieved by running less was similar to that lost by increasing the run distance. At these levels, exercising and quitting exercising are comparable, and the weight gains and losses connected to them could be reversed.

But for people running less, a gap in training is more disastrous, causing weight gain not simply lost by resuming the same exercises.

"At lower mileages, there is asymmetric weight gain and loss from increasing and decreasing exercise, leading to an expected weight gain from an exercise hiatus. In other words, if you stop exercising, you don't get to resume where you left off if you want to lose weight," said Williams.

Williams focused on 17,280 men and 5,970 women, who dwindled their running distance, with 4,632 men and 1,953 women who increased their running distance in an interval of 7.7 years. Those going from 5 miles per week to quitting gained four times more pounds than those running from 25 to 20 mi (40 to 32 km) per week. Those resuming running lost weight only when their weekly running distances were over 20 mi (32 km) in men, and 10 mi (16 km) in women.

The research shows that exercise aimed to impede obesity is ineffective if irregular, seasonal, or often interrupted.

"We are getting fat because we don't exercise sufficiently and consistently. The real solution to the obesity epidemic is getting people to exercise before they think they need it, and to stick with it. The ounce of prevention is indeed worth a pound of cure," said Williams.