In a study published in the journal PLOS One, the researchers recruited 35 people (aged between 45 and 75 years).
The researchers, including Prabha Siddarth from the University of California, Los Angeles in the US, asked the participants about their physical activity levels and the average number of hours per day they spent sitting over the previous week.
Each person had a high-resolution MRI scan, which provides a detailed look at the medial temporal lobe, or MTL, a brain region involved in the formation of new memories.
The researchers found that sedentary behaviour is a significant predictor of thinning of the MTL and that physical activity, even at high levels, is insufficient to offset the harmful effects of sitting for extended periods.
MTL thinning can be a precursor to cognitive decline and dementia in middle-aged and older adults.
Reducing sedentary behaviour may be a possible target for interventions designed to improve brain health in people at risk for Alzheimer's disease, said Siddarth.
This study does not prove that too much sitting causes thinner brain structures, but instead that more hours spent sitting are associated with thinner regions, he said.