But the research has so far been limited to mice, perhaps offering addicts one last justification for hanging onto the habit.

In the research, published in the American Journal of Physiology, mice were exposed to cigarette smoke and fed either a low fat diet or a feast.

The results show smoking mice might have eaten less but they still kept the kilograms.

Professor of pharmacology at the University of New South Wales Margaret Morris says if it is true, humans who smoke are probably losing muscle and other precious organs, not fat.

Increased appetite

She says people who quit smoking and then complain that they have actually put on weight probably have because of increased appetite.

"In many people and in our animals, cigarette smoke suppresses signals that increase appetite," she said.

"So the powerful chemical message induced by smoking to suppress feeding and withdrawal of nicotine is unhelpful in that regard.

"So I think the message from this study is that using smoking to suppress body weight gain, if you're overweight, is not going to be helpful."

While the number of people smoking continues to fall, it has been tougher getting the message through to young women.

Dr Morris says although young women might think they look slimmer, they are now being told it is actually muscle not fat they are losing.

"Our work actually points out that this is a very powerful health message - if it's true in humans - that the combination of these two behaviours is very, very unhealthy," she said.

Dr Morris says it is important for more research to be done.

"It's very hard to do these studies in humans, clearly," she said.

"This is a long-term study, we've fed the animals for seven weeks, the smoking was equivalent to moderate cigarette smoking in humans, we know that they had carboxyhaemoglobin levels in that range.

"So, if we can make the leap into humans, yes, I think it is quite important." © 2007 Australian Broadcasting Corporation