The relationship to dementia with these activities remained strong no matter how much physical activity a person did, the authors wrote in Proceedings of the National Academy of Sciences.
Both watching TV and using a computer have been linked to increased risk of chronic disease and mortality, while exercise and physical activity (PA) has shown benefits in reducing cognitive decline, structural brain atrophy, and dementia risk in older adults, the authors wrote.
The authors said they wanted to try to understand the effects of watching TV and using computers on dementia risk because people in the United States and Europe have been engaging in both of these activities more often. They concluded that it’s not the sitting part of sedentary behavior (SB) that potentially has the effect on dementia but what people are doing while sitting.
Some of the results were surprising, lead author David Raichlen, PhD, professor of Human and Evolutionary Biology at University of Southern California, Los Angeles, said in an interview. Previous literature on sedentary behaviors has documented their negative effects on a wide range of health outcomes, rather than finding positive associations, he explained.
The researchers conducted their prospective cohort study using data from the United Kingdom Biobank. After excluding people younger than 60, those with prevalent dementia at the start of follow-up, and those without complete data, 146,651 participants were included. The participants were followed from their baseline visit until they received a dementia diagnosis, died, were lost to follow-up, or were last admitted to the hospital.
TV-watching time was linked with an increased risk of incident dementia (HR [95% confidence interval] = 1.31 [1.23-1.40]), and computer use was linked with a reduced risk of incident dementia HR [95% CI] = 0.80 [0.76-0.85]). TV’s link with higher dementia risk increased in those who had the highest use, compared with those who had the lowest use (HR [95% CI] = 1.28 [1.18-1.39]. Similarly, the link with risk reduction for dementia with computer use increased with more use. Both medium and high computer time were associated with reduced risk of incident dementia (HR [95% CI] = 0.70 [0.64-0.76] and HR [95% CI] = 0.76 [0.70-0.83] respectively).
Raichlen pointed out that the high use of TV in this study was 4 or more hours a day and computer use – which included leisure use, not work use – had benefits on dementia risk after just half an hour. These results remained significant after researchers adjusted for demographic, health, and lifestyle variables, including time spent on physical activity, sleeping, obesity, alcohol consumption, smoking status, diet scores, education level, body mass index, and employment type.
Parvathy Sukumaran 
