Non-smokers who had a unique gene variation usually associated with smoking weighed more than people without the smoking gene. (Photo: Colourbox)

Smoking gene could be causing weight increase in non-smokers

New study suggests relationship between smoking genes and increased weight.

Could a gene associated with triggering heavy smoking also be causing weight gains in non-smokers?

It seems so, according to a new study which has surprised scientists.

The study aimed to examine how smoking affected the weight of smokers who had a specific genetic variant linked to a tendency to smoke heavily but when the scientists looked at the data, they found that the genetic variant was also associated with increased weight among non-smokers.

The smokers with the genetic variant had a smaller BMI than smokers who did not have the gene. However, non-smokers had a tendency to weigh more if they had the genetic variant.

The study tells the scientists that the 'smoker gene' has a weight-boosting effect in non-smokers. But what is even more interesting is that it shows that environmental factors, such as smoking, may have a veiled effect on large-scale obesity studies.

"To date, some 100 different genes have been found which influence body weight -- all of which have an extremely modest effect. The new study shows that we may have to go down other routes to find the rest and take into consideration other conditions to which the body is exposed," says study co-author, Professor Thorkild I. A. Sørensen of the Department of Public Health at the University of Copenhagen.

These other conditions, he says, could be other lifestyle factors such as what we eat how much and how much we exercise, and chemical or psycho-social environmental influences.

150,000 test subjects reveaed small effect

The surprising conclusion is based on an international study of genetic data from 150,000 test participants selected from 29 cohort studies around the world.

The participants were divided into smokers, former smokers, and people who had never smoked.

"We used a special design known as Mendelian Randomisation analysis. This involves the use of genetic variants as a proxy for a variable associated with disease. This variable could, for example, be smoking," says co-author Christine Dalgård, an associate professor at the University of Southern Denmark.

"The genetic variant we used in our study was one which causes smokers to smoke more and we were able to establish that smoking is causally related to a lower BMI. But to our great surprise we also found that this genetic variant increased the BMI of people who didn't smoke," says Dalgård

Gene had very limited effect

The groups of genes, known as CHRNA5-A3-B4 proved to have a very slight effect in people who had never smoked, says Dalgård:

"The effect of the genetic variant is so small that it’s the equivalent of 0.09 of a unit of BMI. That is to say that if you have two people of the same height who have never smoked, the person with the genetic variant will weight a few hundred grams more than the one without the genetic variant in question.

She explains that the genetic variant belongs to a group of genes which go together in what’s known as a gene cluster.

This cluster could have something to do with the brain's reward system -- the system that -- among other things -- is responsible for making us feel good after we’ve consumed food.

Dalgård emphasises that the reason for the effect is still extremely speculative and that the results should be verified by other studies.


Read the original story in Danish on

Translated by: Hugh Matthews

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