The amount of weight gained on quitting smoking is higher than previous estimates (Reuters)

Smokers who quit can expect to put on more weight than previously thought, according to a new report.

Scientists in France and Birmingham have concluded that giving up the habit could lead to an average weight gain of 4-5kg over a year.

The majority of the weight is expected to be put on in the first three months after quitting, according to the report published in the British Medical Journal.

It is widely held that giving up smoking, which is an appetite suppressant, leads to an increase in weight, as ex-smokers often snack in order to stave off cravings.

A research team analysed the results of 62 studies to assess the weight gained by successful quitters over 12 months, both those who used nicotine replacement therapy and those who did not.

The average weight gain in those who did not use therapy was found to be 1.1kg in the first month, 2.3kg by the second month, 2.9kg by the third month, 4.2kg at six months and 4.7kg after a year.

Leaflets often state that smokers can expect to gain 2.9kg in the 12 months after they quit smoking, a figure significantly lower than that found in the latest study. According to its authors, many female smokers would be put off quitting if their weight gain was any higher than 2.3kg.

However, the report suggests that levels of weight gain vary significantly from one quitter to the next. While 16 percent of untreated quitters sampled by the study lost weight, 13 percent gained more than 10kg in 12 months. Drawing on these contrasting figures, the study's authors claim that the average values uncovered in their research do not necessarily signal the level of weight gain that can be expected by someone who quits smoking.

In an editorial accompanying the study, Esteve Fernández, associate producer of epidemiology at Barcelona University and Simon Chapman, professor of public health at Sydney University, write: "The data were extracted from clinical trials, not from 'real world' population-based studies of cessation. Those who enrol in trials are known to differ in important respects from non-participants.

"Smokers who take part in trials and attend cessation clinics are a self-selecting minority of smokers who may differ in important respects from those who quit without professional assistance.

"Those who decide they need help to stop smoking tend to lack self-efficacy. They may have similar problems with the dietary and physical activity behaviours important in weight control."

The authors suggest that further research is needed in order to identify the people most at risk of gaining weight, before work can be done to clarify the best way to prevent continued weight gain after kicking the habit.