Why does obesity raise the risk of developing cancer? A new study suggests that the wrong mix of gut bacteria could be to blame. Researchers report that obese mice carry altered communities of intestinal bugs, which produce DNA-damaging acid that leave the mice more susceptible to liver cancer. The findings hint that bacteria help drive cancer development and may eventually help scientists better predict and prevent the disease.

Obesity increases the odds of falling victim to certain types of cancer, including colorectal and liver tumors, but scientists haven’t been able to identify the mechanism behind this link. They’ve suspected that our gutmicrobiota—the complex community of trillions of microbes living in our intestines—play a role. After all, gut bugs have been linked to other diseases, including inflammatory bowel disease, allergies, and even heart disease, and are known to differ between lean and obese individuals.

Molecular biologist Eiji Hara of the Cancer Institute at the Japanese Foundation for Cancer Research in Tokyo and colleagues set out to connect the dots between cancer and gut microbes by studying the development of cancer in obese and lean mice. They began with two groups of mice: lean mice that grew up on a normal diet and mice that ate a fat-laden diet until they were obese. To induce the rodents to develop cancer—”Mice don’t smoke or drink alcohol, so they’re fairly cancer-free,” Hara says—they exposed the animals to a cancer-causing chemical shortly after birth.