Quitting smoking leads to an average weight gain of up to five kilograms in the first year, significantly more than previously thought, according to a new study.

Most of the kilos are piled on in the first three months, a team of medical researchers write in the online journal British Medical Journal, as another group stresses that the health benefits of quitting far outweigh the risks of putting on weight.

Changes in body weight among successful quitters varied greatly. (iStock)

For quitters who did not use nicotine replacement therapy, the average weight gain was 1.1 kilograms at one month, 2.3 at two, 2.9 at three, 4.2 at six months and 4.7 after a year, the Australian Broadcasting Corporation said.

This was "substantially higher than the 2.9 kilograms often quoted in smoking cessation advice leaflets," write the team from France and Britain.

"Moreover, this mean weight gain is greater than the 2.3 kilograms gain that female smokers report being willing to tolerate, on average, before embarking on a quit attempt."

Earlier research showed that nicotine is an appetite suppressant and may increase the metabolic rate.

For the latest paper, the researchers collated data from earlier studies conducted between 1989 and 2011 in the United States, Europe, Australia and east Asia to assess weight changes among successful quitters.

The researchers stressed that changes in body weight varied greatly, with about 16 per cent of quitters losing weight and 13 per cent gaining more than 10 kilograms in the first year.

Smoking riskier than weight gain

In an editorial accompanying the paper, Associate Professor Esteve Fernández of the Universitat de Barcelona and Professor Simon Chapman of the University of Sydney say modest weight gain is far less life-threatening than smoking.

"Tobacco is the main cause of premature death worldwide, being responsible for 5.1 million deaths each year. Obesity, together with overweight, causes 2.8 million deaths," they write.

"Cohort studies indicate that modest weight gain does not increase the risk of death; smoking does."

They stress that fears of getting fat could deter some people from quitting and called for further research to identify those most at risk of gaining weight to be targeted for counselling.