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Smokers wanting to kick the habit need not worry about gaining a lot of weight after quitting, research based on the longitudinal ''Dunedin Study'' shows.
The University of Otago findings - which go against previous studies - came from the world-renowned Dunedin Multidisciplinary Health and Development Study, which closely follows the progress of about 1000 people born in Dunedin in 1972-73.
Researchers measured participants' smoking habits and weight at regular intervals from ages 15 to 38.
About one-third of the group were smokers at age 21, and by age 38 about 40% of them had quit.
Researchers found, on average, the weight of those who kicked the habit returned to the same level as people of similar age who had never smoked.
Furthermore, those who quit gained, on average, only about 5kg more than those who carried on smoking.
Assistant research fellow Lindsay Robertson, who led the research, said she hoped the research would help ''debunk the myth'' that quitting smoking resulted in large weight gains, and encourage more people to quit.
''They should not be put off by the fear of putting on large amounts of weight. It is important to be aware that a small weight gain is unlikely to offset the health benefits of quitting,'' she said.
The researchers also found that being a smoker did not prevent long-term weight gain and all groups tended to put on weight over time, regardless of their smoking status.
The findings went against previous studies which showed people who quit might gain large amounts of weight, including a recent study suggesting people who quit gained about 5kg within a year.
Many of the other studies were not reliable, she said.
''The Dunedin Study ... has such a wealth of data, so we were able to put things in our statistical model that other studies weren't able to account for.''
The study was published online in the journal Nicotine & Tobacco Research.