Giving up smoking is associated with an average weight gain of 4-5 kg after 12 months, most of which occurs within the first three months of quitting, finds a study published on the British Medical Journal website.

Although this figure is higher than previously thought, an accompanying editorial argues that the health benefits of quitting far outweigh this modest gain in body weight and should not deter people from quitting.

It is well known that giving up smoking is often followed by an increase in body weight, but estimates vary. Concern about weight gain is also widespread among smokers and it may deter some -- particularly women -- from trying to quit.

So a team of researchers based in France and the UK analysed the results of 62 studies to assess weight change among successful quitters -- with and without the help of nicotine replacement therapy -- after 12 months.

In untreated quitters, the average weight gain was 1.1 kg at one month, 2.3 kg at two months, 2.9 kg at three months, 4.2 kg at six months, and 4.7 kg at 12 months.

This is higher than the typical 2.9 kg often quoted in smoking cessation advice leaflets and more than the 2.3 kg many female smokers report being willing to tolerate, on average, before attempting to quit, say the authors.

However, the changes in body weight varied widely, with around 16% of quitters losing weight and 13% gaining more than 10 kg after 12 months. This, say the authors, indicates that the average value does not reflect the actual weight change of many people who give up smoking.

Estimates of weight gain for people using nicotine replacement therapy were similar, as were estimates from people especially concerned about weight gain.

Previous reports have underestimated the average amount of weight gained when people stop smoking, they conclude. "These data suggest that doctors might usefully give patients a range of expected weight gain."

They suggest that further research is needed to identify the people most at risk of gaining weight and to clarify the best way to prevent continued weight gain after quitting.

In an accompanying editorial, experts from the Catalan Institute of Oncology/University of Barcelona and University of Sydney say that more data is needed to settle this question, and they point out that previous studies have shown that many smokers gain more weight than never smokers for a few years, but then the rate of weight gain falls to that seen in people who have never smoked.

"Although obesity is positively associated with an increased risk of all cause mortality, cohort studies indicate that modest weight gain does not increase the risk of death; smoking does," they conclude.