Special gene is causing some smokers to stay slim

March 29, 2015 - 06:25

Heavy smokers are slimmer than smokers who lack a specific gene, shows new study with 80,000 participants.

Do you smoke to keep your weight down? It's a poor excuse -- even though the connection between lower body weight and smoking has once again been confirmed. (Photo: Shutterstock)

Smoking can actually make you lose weight. In a study involving more than 80,000 participants, scientists found a connection between smoking and fewer kilos on the body.

The link was discovered by looking at genes -- or rather one particular gene which has previously been proven to be closely related to how many cigarettes a person smokes.

"If you smoke and have that gene, then you weigh less than you would without it. The same doesn't apply, however, if you're a non-smoker. This suggests that it's the act of smoking that results in weight loss," says study co-author Børge Nordestgaard, MD, from the department of clinical biochemistry at Herlev Hospital. He is also a professor at the University of Copenhagen.

"These are not results that I'm happy about, but I am a scientist and I don't believe you should hold back information just because it doesn't suit you," he says.

Smokers with the special gene weighed on average 1.2 kg less than smokers without the gene.

Colleague: study's conclusions are solid

The study's conclusions are solid, according to obesity researcher Professor Thorkild I. A. Sørensen from the department of public health at the University of Copenhagen. He says that previous studies have confirmed that there is a connection between smoking and weighing less.

Sørensen does, however, have one reservation -- namely the gene the scientists have selected as their point of departure:

"A previous study shows that it's not so unambiguous to use this genetic variant for this kind of study as we had thought, since it is related to a slightly higher BMI in non-smokers," he says, although he adds that the new study appears to point out that the weight reducing effect of smoking is greater than previously thought.

"The study doesn't give us any reason to change our general recommendation that people should stop smoking, but one can perhaps say that smokers who want to keep their weight down need extra help," says Sørensen.

May remove prejudices that make it more difficult to quit smoking

Nordestgaard also emphasises that the study's conclusions are in no way an excuse for starting to smoke or continuing to do so.

On the other hand, the study can be used to remove some of the prejudices that surround smokers and makes it more difficult for them to kick the habit.

"This is absolutely no excuse for smoking. But when smokers say this is what stops them from quitting smoking we have to take them seriously -- because it's right enough! A lot of people probably think that the explanation lies in the fact that you have a special lifestyle when you smoke and that you change your lifestyle when you quit smoking."

Precisely because the scientists have examined the genes of smokers and non-smokers, we cannot preclude other sources of error, such as alcohol, exercising habits and diet, says Nordestgaard.

"We are exploiting a genetic variant which we inherit from our parents because genetics are entirely independent of, for instance, alcohol habits. You might say that it is nature's own randomised experiment -- the genes you get are entirely randomly distributed whatever you may do later in life. It's like drawing lots," he says.

Lower weight not necessarily the same as a healthy body

In the study the scientists reviewed a huge amount of data from the so-called Herlev-Østerbro study.  They have examined and compared the genes and bodyweight of 80,342 subjects, of whom 15,220 were smokers.

Here they discovered that smokers carrying two genetic defects in the gene CHRNA3 weighed, on average, 1.2 kg less than smokers without the same genetic defect. The scientists did not, however, only examine the subjects' weight; they also examined the distribution of fat in their bodies and their waistline-hit ratio -- and here they found that it was only weight which was positively affected by cigarette smoking.

So a lower weight was not necessarily the same as a healthier distribution of fat in smokers -- a factor which in previous studies has shown to be of even greater significance to health than weight.

The scientists do not have an explanation for the apparent connection between lower body weight and cigarette smoking. In the study they go into whether it might be nicotine which increases the smokers' metabolic rate, that is to say how quickly their bodies break down food and nutrients or reduce their appetites.

"But this is pure speculation. Cigarettes contain thousands of other substances; it's just that we are most familiar with nicotine, which means that it's what we think of first," says Nordestgaard.

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Read the original story in Danish on Videnskab.dk

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Translated by
Hugh Matthews