Research shows that smoking actually helps keep weight down, while quitting tends to cause moderate weight gain. According to Health.com, the average smoker gains 4 to 10 pounds after quitting.
WebMD cites research concluding that nicotine raises metabolism, causing smokers to burn more calories than non-smokers. It is this stimulant effect that contributes to heart problems in many smokers. When a person quits smoking the metabolic rate returns to a normal level. If people who quit smoking continue to consume the same number of calories they did when smoking, they are bound to gain weight.
Smoking also acts as an appetite suppressant, according to Health.com. It releases glycogen, which causes an increase in blood-sugar levels that decreases hunger.
However, doctors caution against fears of weight gain derailing smoking cessation plans. WebMD quotes Scott McIntosh, University of Rochester associate professor of community and preventative medicine as saying that "not every smoker gains weight, and those who do are still better off in terms of general health than they were when smoking."
Emily Rubin, RD at Thomas Jefferson University's Digestive Disease Institute in Philadelphia concurs, telling Health.com that a former smoker would have to gain 100 pounds to experience the same health risks as a pack-a-day smoker.