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Australian researchers have discovered why cigarette smoking suppresses appetite, providing hope for better treatments to help people quit without piling on weight.
The Melbourne and Sydney scientists found a brain chemical known as neuropeptide Y (NPY), a key appetite regulator, is affected by smoking.
They studied three groups of mice over a month, comparing those exposed to moderate levels of cigarette smoke with two smoke-free groups.
Some of the smoke-free mice were fed the same amount of food as eaten by the smoke-exposed mice while others ate without restriction.
Hui Chen, a PhD student at the University of Melbourne, and colleagues found smoke exposure reduced NPY levels in the hypothalamus, a part of the brain involved in appetite.
"We know anecdotally from humans that appetite goes down in smokers. This research looks at changes in the chemistry in the brain behind that," explained lead researcher neuroscientist Margaret Morris.
"NPY normally stimulates feeding. It's a very potent stimulator of appetite. In the smoke-exposed mice, NPY has gone down. They don't feel like eating.
"In terms of appetite control, the human is quite similar to the mouse.
"All the chemical pathways are very similar so we think there is relevance to the human in terms of the appetite regulation."
The research, published in the American Journal of Respiratory Critical Care Medicine, sheds light on why smokers gain weight when they quit.
Once they give up, the scientists believe NPY levels probably surge, stimulating appetite.
Professor Morris, based at the University of NSW, said the battle of the bulge people faced once they quit smoking was a key reason some men and women went back to cigarettes.
Her Melbourne colleague, pharmacologist Gary Anderson, said the research may eventually lead to better drugs to help people stay off cigarettes for good, based on blocking levels of NPY.
But these may be some years away.
"The brain is a tricky organ to target drugs at," Prof Morris explained.
"You run the risk of adverse effects and you have to get the drug into the brain... which can be quite difficult."
Smokers wanting to quit should phone the National Quit-line on 13 7848 (13-QUIT).
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