Too much time in front of the TV during middle age may be harmful to brain health, with new research linking heavy viewing habits to changes associated with Alzheimer's disease.

Published in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association, the study followed more than 1,700 adults for nearly 24 years to examine whether midlife TV habits were linked to brain health later in life. Participants reported their viewing habits in their early 50s, and decades later, researchers used MRI scans in their mid-70s to identify brain changes associated with aging and Alzheimer’s disease.

Researchers found that men who reported watching TV frequently had more brain changes linked to cognitive decline, including greater white matter damage, which has been associated with dementia, and smaller frontal and occipital lobes, regions responsible for decision-making, planning and visual processing. However, the same association was not found among women.

"This could be because men and women accumulate sedentary time differently – for example, women may interrupt their sitting more often – or because of biological differences in how the brain responds to prolonged sedentary behavior,” biological scientist Natan Feter from the University of Southern California, and lead author of the study, told ScienceAlert.

The findings also suggest that not all sitting is equally harmful. People whose jobs required long periods of sitting at a desk did not show the same brain changes, leading researchers to believe that mentally engaging activities while sitting may be less damaging than passive behaviors like watching TV.

"For years we've focused on how much people sit. Our findings suggest we should also pay attention to what they're doing while they're sitting," David Raichlen, professor of biological sciences and anthropology at USC and a senior author of the study, said.

“Sedentary behavior has become a major part of modern life, but not all sedentary activities are the same,” Feter added. “Spending an hour reading a book is very different from spending an hour passively watching television.”

Researchers found that the link between heavy TV watching and brain changes remained even after accounting for physical activity, suggesting that regular exercise alone may not completely offset the potential effects of prolonged passive viewing.

Feter told ScienceAlert that the stronger association seen among men may be linked to differences in TV habits, as men in the study were more likely to watch TV for longer, uninterrupted stretches.

Researchers also noted that biological and hormonal differences between men and women could play a role, though more research is needed to understand why the association appeared stronger in men.

The study did not examine the types of programs participants watched, so researchers cannot determine whether certain content affects the brain differently.

However, Dr. Soma Sengupta, chair of neurology and neurologist-in-chief at Tufts Medical Center, told Prevention that mentally engaging programs, such as documentaries, educational shows or complex dramas, may provide more cognitive stimulation than passive entertainment.

“If TV is watched in ways that force you to think, predict, remember key plot elements, or discuss ideas, it likely behaves more like cognitively active ‘sitting’ than passive viewing,” Kang Hsu, M.D., an internal medicine physician in Washington, told Prevention.