Offering clues to why smokers often gain weight after quitting, a new study suggests that smoking enhances the activity of a gene that helps break down body fat.

Researchers found that compared with non-smokers, a group of healthy smokers showed greater activity in a gene called AZGP1 in cell samples taken from their airways.

Because the gene is thought to be important in breaking down fat and controlling weight, the findings point to one possible reason that smokers tend to weigh less than non-smokers — and why people often put on pounds after quitting.

Celebrities Who Have Quit Smoking

Dr. Holly Vanni and colleagues at Weill Medical College of Cornell University in New York report the findings in the journal Chest.

Researchers have pointed to various potential reasons for the weight gain that often follows smoking cessation. People who quit may start to eat more, for example, while at the same time losing the metabolism boost that nicotine provides.

Meanwhile, studies on AZGP1 have shown that the gene helps regulate body fat in mice, and that patients with cancer-induced weight loss may have increased activity in the gene.

The current findings, according to Vanni's team, suggest that the smoking-weight connection has a molecular basis — namely, revved up AZGP1 activity.

However, that doesn't mean that weight gain after smoking cessation is simply a matter of genetics. The relationship between smoking and weight is "undoubtedly complex," the researchers write, and likely involves modifiable factors like diet and exercise.