The decades-old belief that smoking cigarettes keeps you slim is being challenged by new research that has found exposure to secondhand smoke can cause weight gain.

“For people who are in a home with a smoker, particularly children, the increased risk of cardiovascular or metabolic problems is massive,” study author Benjamin Bikman, a professor of physiology and developmental biology at Brigham Young University, said in a news release.

In an animal study, researchers exposed lab mice to secondhand smoke and followed their metabolic progression. Those who were exposed to smoke gained weight. Researchers found that smoke triggered an alteration in cell mitochondria, disrupting normal function and inhibiting cells’ ability to respond to insulin.

“The lungs provide a vast interface with our environment and this research shows that a response to involuntary smoking includes altering systemic sensitivity to insulin,” study author Paul Reynolds said in a news release. “Once someone becomes insulin resistant, their body needs more insulin. And any time you have insulin go up, you have fat being made in the body.”

Researchers discovered the key to reversing the effects of cigarette smoke is to inhibit ceramide; now they’re searching for a ceramide inhibitor that is safe for humans.

“The idea that there might be some therapy we could give to innocent bystanders to help protect them from the consequences of being raised in a home with a smoker is quite gratifying,” Reynolds said.

As for smokers, researchers noted that their findings could help motivate them to quit, as their habits affect loved ones.

According to the news release, half of the U.S. population is exposed at least once daily to secondhand cigarette smoke and approximately 20 percent of young children live with someone who smokes in the home.