Heavy smokers and those who are obese gain more weight after quitting smoking, a new study finds.

People who quit smoking often put on extra pounds, but the amount of weight gain depends on the individual and can range from a few pounds to more than 25 pounds.

However, the factors that affect weight gain after quitting smoking haven't been well understood, according to the authors of the new study.

"Many smokers are concerned about gaining weight after quitting smoking and this can be a barrier for them when they are considering whether or not to make a quit attempt," Susan Veldheer, a registered dietitian in the department of public health sciences at Penn State College of Medicine, said in a university news release.

"Being able to easily identify smokers who may gain more weight when they quit is important so that we can work with patients to tailor their treatment plan," she added.

The researchers looked at information from more than 12,000 nonsmokers, smokers and former smokers in the United States who took part in a national health survey.

Quitting smoking led to significant average weight gains among those who had been heavy smokers (23 pounds) or were obese before quitting (16 pounds), the study found.

"Although this may seem like a lot of weight, it is important for all smokers to remember that quitting smoking is the single most important thing they can do for their health," Veldheer said.

"That being said, for heavy smokers and obese smokers, it may be a good idea to work on quitting smoking while also making other healthy lifestyle changes to control their weight," she added.

Among smokers who had fewer than 15 cigarettes a day, there was no significant difference in 10-year weight gain between those who quit smoking and those who kept smoking.

"This is good news for light to moderate smokers who are concerned about weight gain. It means that in the long term, quitting smoking will not make that big of an impact on their weight," Veldheer said.

The findings were published recently in the International Journal of Obesity.