Nicotine affects brain cells that normally tell people to stop eating when they're full.
Researchers stumbled onto the finding while studying a nicotine-related substance in mice, and the mice began eating less.
Nicotine activates receptors on the surface of cells, which is how it triggers addiction. It also activates a region in the brain that regulates appetite.
Mice without those cells did not lose weight like normal mice when given nicotine.
On average, smokers gain less than 10 pounds when they quit. Still, it's a worry that many cite when asked why they don't try to quit. This new discovery might lead to better treatments to help people quit without worrying about weight.
Smoking causes cancer, heart attacks and many other ailments, so concerns of modest weight gain shouldn't deter someone from quitting. But smokers who do have that concern should try nicotine-based smoking-cessation treatments, said study senior author Marina Picciotto, a Yale professor of psychiatry and neurobiology.
The findings were published in Friday's edition of the journal Science.