Smokers who keep smoking, and those who quit, will all gain weight over time, but those who smoke the most may gain more, a new study suggests.
How much more might also be predicted by the smokers' starting weights, researchers say. Light smokers and those who didn't start out obese gained the least weight after quitting.
"Nicotine is a metabolic stimulant (speeds up metabolism) and an appetite suppressant (makes people feel less hungry)," said lead author Susan Veldheer of Penn State College of Medicine in Hershey, Pennsylvania, in an email.
"So, when people quit, they tend to eat more because their appetite increases and they lose the metabolic boost they were getting from smoking . . . two things that then lead to weight gain," she told Reuters Health.
This study clearly indicates that the more nicotine someone took in, the greater the effect of the drug on metabolic rate and appetite, Veldheer said. "This, in turn, led to more weight gain when they quit."
Researchers analyzed data from the annual National Health and Nutrition Examination Survey between 2003 and 2012, focusing on people age 36 and older who answered questions about their height, current weight, weight 10 years prior, and smoking status.
Of 12,204 adults included in the analysis, 7,914 had never smoked, 3,105 were still smoking and 1,185 had quit smoking between one and 10 years earlier.
Among former smokers, 44 percent had smoked one to 14 cigarettes per day before they quit, while 23 percent had smoked 25 or more cigarettes per day. Current smokers had similar frequencies.
Across the whole group, people had gained an average of almost 10 pounds (4.5 kilograms) over the 10-year period.
Current smokers had gained an average of almost eight lb (3.5 kg), compared to 18 lb (8.4 kg) for those who had quit smoking, as reported in the International Journal of Obesity.
Based on current smokers' average weight gains, researchers calculated how much of the quitters' weight gain was attributable to not smoking. The proportion was highest for those who had smoked the most, and for those who were heaviest before quitting.
For those who had smoked no more than 14 cigarettes per day before quitting, just 4.4 lb (2 kg) of weight gain over the years was directly attributable to not smoking. That compared to almost 23 lb (10.3 kg) among those who had smoked more than 25 cigarettes per day.
For smokers who were obese at the start, 15.6 lb (7.1 kg) of weight gain was related to quitting smoking. For those who were normal weight, it was 9.7 lb (4.4 kg).
About a third of smokers say fear of weight gain keeps them from quitting, Veldheer said.
"Eating more when you stop smoking is partly due to an increased appetite, but behavioral and psychological factors may play a part too, for example eating instead of smoking in a habitual way, or comfort eating," said Deborah Lycett, principal lecturer in nutrition and dietetics at Coventry University in the U.K., who was not part of the new study.
"Stopping smoking should continue to be the biggest priority for improving your health, and the recommendations for those trying to quit are to stop smoking first and then address any weight gained afterwards," Lycett told Reuters Health by email.
Smoking is not an appropriate way to control weight, Veldheer said. "What I think our study does is help identify specific groups of smokers who may gain more weight when they quit so that they can be prepared and have realistic expectations," she said.
Former smokers may want to find calorie-free ways to replace the habit of smoking, like chewing sugar-free gum, chewing a straw or a toothpick, which could help, Veldheer said.
"As a dietitian with expertise in weight management, the answer is surprisingly similar for smokers, non-smokers, anyone who wants to lose weight . . . eat plenty of fruits and vegetables, choose whole grains over highly processed foods and avoid liquid calories," she said.