If you know you should stop smoking but fear you'll gain weight, you may want to listen up: Heavy smokers are more likely to develop pot bellies.

While smoking may help keep extra pounds at bay, scientists at the University of Glasgow found that smoking concentrates fat into the central area of the body, causing a bigger stomach that's in the shape of an apple.

Apple-shaped bodies, as opposed to pear-shaped bodies, are known to increase the risk of many diseases, including heart disease and diabetes.

Researchers analyzed 29 studies involving almost 150,000 people containing data on their smoking habits, weight, and waist circumference.

They found that a genetic variation in some smokers was associated with heavier smoking and a lower body mass index (BMI), confirming the theory that heavy smokers weigh less. But they also found that while their BMI was lower, their waist circumference was higher than non-smokers.

"Whilst smoking lessens weight overall, it tends to push fat more into the central area so waist circumference is preferentially higher," said Professor Naveed Sattar, of the Institute of Cardiovascular and Medical Sciences.

"So, when smokers put on weight, they will show bigger tummies for the same weight gain than non-smokers, and this may also be linked to their greater risk for diabetes.

"On the whole, weight goes down in smokers," Sattar said. "but it means smoking is lessening the chance of putting fat on in the 'safe bits.'

"If confirmed, a tendency for smokers to acquire an 'apple shape' due to increasing central adiposity might provide a novel health promotion message to encourage smoking cessation."

The analysis was published in the journal BMJ Open.