Quitting smoking makes you fat, and now scientists know why!

As often happens in science, it was an accidental discovery. It solves the mystery of why a lot of people who smoke look so skinny and, more importantly, why many people who quit the habit gain weight.

"Interestingly, initially we were not looking into feeding behavior but depression," says Yann Maneur, an associate research scientist at Yale. "We were trying to find new drugs to treat depression. And as I was testing these new drugs I realized the animals were not eating as much."

At the time, Mineur and his team were testing nicotine. Curious about this unexpected effect, they looked further. Maneur says, "We found that nicotine, when it enters the brain, activates specific nicotine receptors that are located on specific neurons known to decrease feeding and increase energy expenditure when activated."

So nicotine, it turns out, triggers a brain pathway that essentially tells you: you've had enough, put that cake down. And it signals your body to start using up some energy. But does that mean people should use it to try to lose weight? "That's the trick question," Mineur says with a chuckle, "because we do not want to advocate smoking, of course."

There may be alternatives that could do the same trick. Maneur claims that there is potential for "drugs to mimic this effect to help people maybe lose weight. Or even better, when people try to quit smoking they could use drugs that are already available and known to trigger this pathway in order to potentially limit their weight gain."

The smoking cessation drug cytisine, which is sold in Eastern Europe, also triggered this brain effect on eating in this study. Developing specific drugs to do the same thing could prove to be tricky, though. The same receptors that react this way to nicotine help control the way the body deals with stress.

This study is reported in the journal Science.