A new UCLA study has found a connection between how quickly the brain ages and the bacteria and chemical byproducts found in the gut, offering a clue to what drives brain aging long before symptoms appear.

Researchers have long used brain scans to estimate a person’s “brain age,” a measure that can differ from someone’s actual age in years. When a brain looks older than expected on a scan, prior research has linked it to poorer memory, thinking skills and mood. However, most of these studies involved older adults or people with existing brain diseases. Whether this gap between brain age and chronological age means anything in younger, generally healthy people has been unclear.

In a new study published in the journal eBioMedicine, UCLA Health researchers analyzed brain scans from nearly 1,500 adults across three separate groups using a method that measures how different regions of the brain communicate with one another while at rest.