Heavy smokers may be successful in staying slim – but not in improving their health, according to a new genetic analysis.
The "benefit" of smoking is enjoyed by people with a genetic susceptibility to tobacco addiction, Danish researchers found.
The finding may help explain why some people take up smoking in the belief it will help them stay slim.
Overall the Danish study found heavy smokers weighed about 0.5kg (1 lb) more than average.
But those who had two copies of the "smoking gene" weighed 1.2kg less than average.
The study, in the International Journal of Epidemiology, involved more than 80,000 people, including 15,000 smokers.
The researchers found that the smoking gene made no difference to weight among non-smokers – suggesting it needs to be combined with tobacco to help shed a few pounds.
Researcher Professor Børge Nordestgaard, of Herlev Hospital, Copenhagen University Hospital, said there was a "clear disagreement" between the findings of genetic analysis and the figures from simple observation of smokers and non-smokers.
He said: "Reverse causality may also be a factor, where having a higher body weight in itself could lead to someone taking up smoking, or increasing their tobacco consumption.
“Smoking and obesity remain two leading modifiable risk factors for death. Which of them is associated with the greater health risk has not been subject to much investigation, and despite our findings, we do not recommend the decision to start or continue smoking in order to control body weight.”
High tobacco consumption lowers body weight: A Mendelian randomisation study of the Copenhagen General Population Study. International Journal of Epidemiology 17 March 2015; doi: 10.1093/ije/dyu276
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