Why is it that women find it harder to quit smoking than men? A US study suggests it could be environmental and social factors, rather than a physical dependence on the nicotine itself.
Scientists at the Yale University School of Medicine scanned the brains of 58 women and 52 men, using radioactive markers that bind to a specific group of receptors responsible for nicotine addiction.
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It is thought that these nicotine receptors increase in number as people continue to smoke cigarettes, binding to the nicotine and reinforcing the habit.
In men's brains, researchers found this theory to be true. Smokers had a greater number of nicotine receptors than their non-smoking counterparts.
But the way a woman's brain responds to nicotine is vastly different. In fact, researchers found that female smokers and non-smokers had roughly the same number of nicotine receptors.
"When you look at it by gender, you see this big difference," said study researcher Kelly Cosgrove, an assistant professor of psychiatry at the School of Medicine.
Breakdown of receptors in the brain
In the study, around half of the participants were smokers who had abstained from the habit for a week, so that their nicotine receptors were free to bind to the radioactive markers.
Researchers were then able to break down the specific areas where the nicotine receptors were located in smokers' brains, and discover exactly how many more receptors there were in those regions than in the brains of non-smokers.
They found that male smokers had 16 per cent more receptors located in the striatum, 17 per cent more in the cerebellum, and 13 per cent more in the cortical regions of their brains than male non-smokers.
On the other hand, female smokers and female non-smokers had similar numbers of nicotine receptors in each of these areas of the brain.
Related: Diseases you didn't know smoking caused.
What does this mean?
The findings of the study suggest that women do not form a physical dependence on nicotine like men do, but rather that they are more susceptible to social and environmental triggers such as the act of holding a cigarette, or even the smell of one.
The findings indicate that women may benefit from different forms of treatment other than mainstream nicotine-replacement therapies such as nicotine gum or patches.
Cosgrove suggests that behavioral therapies may also be the way to go. Relaxation techniques such as breathing exercises, and non-nicotine medications including natural herbal remedies, could all be explored.
Related: Is social smoking really that bad?.
The researchers have not worked out the exact reasons for the differences between the sexes in the study, but suggested it could have something to do with hormone fluctuations that occur in women each month.
Interestingly, the study also found a link between higher levels of the hormone progesterone with a lower number of available nicotine receptors to bind to the nicotine.
It could be that during different stages of the menstrual cycle, especially after ovulation, increased progesterone production in women's bodies may be blocking the receptors.
The study is published in this month's issue of Archives of General Psychiatry.
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