It's positively crazy to think – given everything we know about the dangers of smoking – that lots of people actually put off quitting the deadly habit because they're concerned about the potential negative impact on their waistline. Weight gain isn't good, sure, but smoking is just about the worst thing you can do to your body. And to others.

Anyway, here we are. And despite what you may have heard, this weight gain smokers face after quitting cigarettes is very real and not just an urban myth. Researchers at Penn State University in the US have found that the amount of weight quitters stand to gain is related to the volume of cigarettes they smoked during their habit. In other words, heavy smokers make for heavier quitters.

To figure out how personal factors might contribute to smokers' weight gain after quitting, the researchers looked at the data of more than 12,000 people who participated in the US National Health and Nutrition Examination Survey. Their research, published in the International Journal of Obesity, looked at weight gain in both continuing smokers, those who quit, and non-smokers over a 10-year period. After 10 years, the average weight for all groups had increased.

"People tend to put on some weight over time and everyone in the study gained weight. The non-smokers gained about a pound [450 grams] a year for 10 years," said Susan Veldheer, a registered dietician and lead author of the study, in a press release.

For smokers of fewer than 15 cigarettes a day, there was no difference in weight gain between those who continued smoking and those who quit. "This is good news for light to moderate smokers who are concerned about weight gain," said Veldheer. "It means that in the long term, quitting smoking will not make that big of an impact on their weight."

The findings were not so good for heavy smokers quitting, or for smokers who were already obese prior to quitting. For people who smoked more than 25 cigarettes per day – or who had a body mass index of 30 or more before giving up – the weight gains over 10 years were markedly increased. Heavy smokers who quit gained 10.4 kg on average over the 10-year period, while obese quitters gained 7.2 kg.

"Although this may seem like a lot of weight, it is important for all smokers to remember that quitting smoking is the single most important thing they can do for their health," said Veldheer.

"That being said, for heavy smokers and obese smokers, it may be a good idea to work on quitting smoking while also making other healthy lifestyle changes to control their weight."