A new study suggests that genes have a big role in a person’s body weight by determining what types of bacteria are in the intestines.

Ruth Ley, senior study author at Cornell University, said that up until now scientists explained the abundance of gut microbes by diet, the environment, and lifestyle, but the latest study suggests that certain types of microbes come from your genes, according to WebMD.

Calling it the first study to firmly establish that gut microcobes are heritable, Ley said the study proves that environmental influences aren’t the only cause of genotype variation in humans.

Researchers looked at 416 pairs of twins and identified intestinal bacteria that skinny people had more often. The genes in this family of bacteria are genetic, which means parents can easily pass them to their offspring.

Researchers also tried to transplant the bacteria to mice, and the mice were able to avoid gaining weight as a result.

The findings could result in therapies for people suffering from obesity due to their genetic makeup. It could result in better disease risk prediction, allowing better weight loss techniques to be prescribed to those at highest risk.

The study was published Nov. 6 in the journal Cell.