Non-smokers are twice as likely as smokers to carry a mutation in a gene that helps to rid the body of nicotine. Those smokers with the mutation are still likely to smoke significantly fewer cigarettes, the scientists found.
The gene, called CYP2A6, helps to break down nicotine in the bloodstream and brain. The scientists believe that people with a defective version of the gene have a greater natural aversion to tobacco than those with the more efficient gene.
Scientists identified the gene last year, but have now discovered in a study of more than 400 people that it appears to be instrumental in determining a person's predisposition to take up smoking.
The new research suggests that millions of people have a natural protection against smoking.
The scientists discovered that 20 per cent of non-smokers carry the defective gene compared with 10 per cent of smokers.
Smokers who carry mutations in the CYP2A6 gene are likely to smoke less because nicotine - an addictive component of cigarettes - is not rapidly removed from the brain and bloodstream.
Smokers with the efficient version of the gene will tend to smoke more heavily to compensate for nicotine being removed more rapidly, the scientists suggest in the journal Nature.
The scientists said that people with defective versions of the gene are also less likely to develop smoking-related disorders, such as lung cancer and heart disease.
People with the defective gene might be less prone to converting chemicals in tobacco smoke into carcinogens, the scientists said.
Rachel Tyndale, assistant professor of pharmacology at the University of Toronto, said that smokers with the mutation consumed an average of 129 cigarettes a week compared with 159 cigarettes a week for those smokers with the normal gene.
``It's a gene that when inactive protects against smoking, it is also heavily involved in whether your are at risk of smoking. It's the first gene specifically related to smoking,'' Professor Tyndale said.
``People with a defective gene are twice as likely to avoid smoking altogether.
"In North America, where 30 per cent of the adult population smokes, this translates into about 7.5 million people who are protected against smoking by carrying a single copy of this gene defect," she said.
The results could have implications for the nicotine-replacement therapies, such as nicotine patches and chewing gum, because some smokers will breakdown the nicotine more quickly than others and so will be less susceptible to the benefits of the treatment.
``We believe we can use this inherited ability to remove nicotine from the body to prevent a relapse in those smokers who have quit," Professor Tyndale said.
Professor Edward Sellers, a member of the research team, said: ``With these findings comes the possibility of developing a method to chemically inhibit the function of the enzyme produced from the gene - a prevention and treatment for tobacco smoking in other words.
"That is the next step."Reuse content