Watching a lot of TV and having a low physical activity level as a young adult may result in worse brain function 25 years later in midlife, a new study suggests. Despite increasing concerns on the prevalence of sedentary or screen-based activities among young people, only a few studies have investigated the association between physical activity in early adulthood and cognitive function later in life.

A new study, published online by JAMA Psychiatry, sought to examine associations between 25-year patterns of television viewing and physical activity and midlife cognition. For the research, the team involved more than 3,000 adults aged between 18 and 30 years old.

The researchers defined high television viewing as watching TV for more than three hours per day for more than two-thirds of the visits, and they measured exercise as units based on time and intensity. After 25 years, the team evaluated cognitive function of the participants using three tests that assessed processing speed, executive function and verbal memory.

Based on their findings, adults with both high television and low physical activity had almost two times higher chances of poor cognitive performance. In particular, these behaviours were associated with slower processing speed and worse executive function but not with verbal memory, according to the researchers.

They noted that participants with the least active patterns of behaviour, or both low physical activity and high television viewing time, were the most likely to have poor cognitive function. Meanwhile, participants with either high television viewing or low physical activity within 25 years were associated with poor performance on some of the tests.

The study’s authors acknowledged a few limitations, including possible selection bias and physical activity and TV viewing being self-reported. Individuals with both low physical activity and high sedentary behaviour may represent a critical target group, the study concludes.

Watching too much TV is also linked with changes in the participants' growing brains, according to Tohoku University researchers in their 2013 study. For the study, the team studied kids aged between 5 and 18 who watched, on average, about two hours of television a day. Their brain scans revealed that more TV time resulted in enlargement in certain parts of the brain. In this case, higher brain volume in the participants was associated with a lower verbal intelligent quotient. This may be linked to previously known effects of TV viewing on verbal competence, aggression and physical activity, the researchers said.

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