Why do people lose weight from smoking? And what causes them to pack on pounds after quitting?
Scientists think they're finally able to answer both questions. The findings could lead to more effective smoking cessation treatments that reduce weight gain, according to research in the journal Science.
In the United States, 46.6 million people smoke, and previous work shows that fears of gaining weight discourage some people from quitting. Though other factors such as diet, genetics and behavior influence weight gain, smokers have lower body mass index measures on average, with one analysis suggesting the tobacco industry may use chemicals as a strategy to keep consumers skinny.
But on a basic level, nicotine -- the addictive chemical found in most tobacco products such as cigarettes and cigars -- affects neurons in the brain that transmit important messages, including when to eat, to the rest of the body.
By conducting experiments with mice, scientists now know that nicotine binds to neurons in thehypothalamus, an area of the brain that regulates hunger, sleep and other moods. Specifically, researchers discovered that a drug called cytisine used in smoking cessation actually activates a nicotine receptor that turns on other neurons in the hypothalamus. These neurons, also called pro-opiomelanocortin cells, serve to suppress the appetite when activated.
In previous studies, animals receiving the drug ate less, but it wasn't clear why. It turns out that both nicotine and the experimental drug affect these cells and whether they send messages urging people to eat more or less.
To pinpoint clear relationships among the drug, pathway and nicotine, scientists exposed mice -- both with and without pro-opiomelanocortin cells -- to cytisine and nicotine in different experiments. Mice without the cells didn't lose weight, while those with the cells intact did lose weight when each group received nicotine doses.