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Quitting smoking can lead on average to four to five kilograms of weight gain within a year but Australian experts say the benefits far outweigh the few extra pounds.
A large international study published in the British Medical Journal on Tuesday analysed 62 clinical trials of people who sought medical help to stop smoking.
After 12 months of abstinence people gained on average four to five kilograms. However, 16 to 21 per cent of people actually lost weight.
A larger portion, 35 to 38 per cent, gained less than 5kg, 29 to 34 per cent gained 5kg to 10kg, and 13 to 14 per cent put on more than 10kg.
However, the wide variation in weight gain meant the results could not be applied to everyone who quits smoking, the study's authors noted.
Simon Chapman from the Sydney School of Public Health said the results were concerning because it could deter people indefinitely from quitting.
Professor Chapman said the data were unreliable because they were gleaned from clinical trials of people who sought help to stop smoking, not the general population.
Smokers who enrolled in trials differed from those who quit without professional help, he said.
"Those who decide they need help to stop smoking tend to lack self-efficacy," Prof Chapman said in an editorial in the same journal.
"They might have similar problems with the dietary and physical activity behaviours important in weight control.
"So these results may not be generalisable to all smokers who quit because two-thirds to three-quarters of ex-smokers stop smoking without professional help or interventions."
Prof Chapman, who co-authored the editorial with Esteve Fernandez from Barcelona University, said major studies showed that modest weight gain, unlike smoking, did not increase the risk of death.
Robert Grenfell from the Heart Foundation said he was concerned that some people, especially women, would avoid attempting to quit over concerns about weight gain.
Smokers were twice as likely to suffer a heart attack and three times as likely to have a stroke, he said.
"Giving up smoking is the single biggest thing you can do to improve your heart health and, while being overweight is also a risk factor for heart disease, the health benefits of quitting far outweigh a few extra kilograms," Dr Grenfell said.
Most of the people included in the analysed trials were from North America, Europe and Australia with a smaller proportion from east Asia.
The majority of the data came from trials of drug treatments to help smokers quit, while a smaller number tested the impact of exercise on quitters and specific interventions for people concerned about weight gain.