Sitting too much is linked to thinning in a brain region critical to memory among middle-aged and older adults‚ a new study shows.

The researchers also found that high levels of activity do not mitigate the “harmful effects of sitting for extended periods”.

Being very sedentary is bad for your metabolism and heart‚ raising the risk of heart disease‚ diabetes and early death‚ much like smoking‚ previous research has proved.

Researchers from the University of California in Los Angeles enrolled 35 people aged 45 to 75 in the study and asked them on average how many hours they spent sitting in the previous week and how active they were. They did not ask them if they took breaks during their sitting.

They did high-resolution MRI scans on each volunteer to look at the medial temporal lobe (MTL)‚ a brain region involved in making new memories. Thinning in this region precedes cognitive decline and dementia.

“Sedentary behaviour is a significant predictor of thinning of the MTL and physical activity‚ even at high levels‚ is insufficient to offset the harmful effects of sitting for extended periods‚” said biostatistician Prabha Siddarth‚ from the Semel Institute for Neuroscience and Human Behavior at UCLA.

A recent big study of dementia — a decline in memory that hampers your ability to perform everyday tasks — found that older adults in the UK with “slower walking speeds” and who had a bigger decline in walking speeds over time appeared to have a greater risk of developing dementia than those who walked faster.