Swapping passive screen time with mental activity may cut dementia risk

by · News-Medical

Swapping passive screen time for mentally engaging activities may help protect the brain over decades, offering new insight into how everyday habits shape dementia risk.

Sedentary lifestyles are increasingly linked to cognitive decline 

About 57 million people today live with dementia, which carries a huge physical, emotional, and economic cost to the patient and caregivers, as well as to society. Sedentary behavior has been associated in prior studies with a higher risk of developing dementia. Increasing physical activity among sedentary individuals may improve cognition.

Mentally passive versus mentally active

Sedentary behavior that is mentally passive, such as watching television, has been differentiated from that which is mentally active, such as office work. Cognitive function might decline with longer periods of mentally passive sedentary behavior.

Earlier observational studies have suggested a higher dementia risk among sedentary adults who watch television compared to those who are mentally active. The latter included computer use, playing cards, hobbies, and reading. However, these were small studies with relatively short follow-up periods.

Large population study links habits to brain health

The current study explored the associations of mentally passive and mentally active sedentary behavior with new-onset dementia. In addition, the researchers modeled the impact of substituting mentally passive sedentary behavior with either mental activity or any level of physical activity.

The study included 20,811 adults (mostly women) whose data were retrieved from the Swedish National March Cohort. All were 35-64 years old at baseline, in 1997. National registry data were used to identify dementia development in this cohort over a median follow-up period of 19 years.

Questionnaires were used to classify sedentary adults as mentally passive or mentally active. Reported sedentary behavior, both mentally passive and mentally active, occupied a mean of 116 and 240 minutes a day, respectively. Physical activity was also similarly assessed as light or moderate-to-active.

There were 569 new cases of dementia in a total of 393,104 person-years.

Mental activity linked to reduced dementia risk

After adjusting for known risk factors like body mass index (BMI), age, sex, education, smoking, drinking, diet, and chronic disease, mentally passive sedentary behavior was not significantly associated with higher dementia risk after adjustment compared to those who were mentally active, although the association remained directionally positive.

With each additional hour of mentally active sedentary behavior, sedentary behavior was linked to a 4 % lower risk of developing dementia, especially among those aged 50-64 years.

Hypothetical mechanisms

This is biologically plausible, as mid-life mental and/or physical activity is associated with increased grey matter volume and cerebral blood flow. However, not all mental activity is beneficial. For instance, office work may, in some contexts, induce stress, unlike cognitively challenging leisure activities like reading or solving puzzles.

It could also be that sedentary adults with better baseline cognitive function were more likely to choose mentally active behaviors, highlighting the possibility of reverse causality, meaning the observed association may not be fully causal. Mental passivity could also impair sleep quality, thereby increasing the risk of cognitive impairment. Future research is necessary to directly examine these mechanisms.

In theoretical models, when holding other behaviors constant, adding 1 hour of mental activity was associated with a 11 % decreased risk of new-onset dementia, even without any change in mentally passive sedentary behavior or physical activity. In addition, substituting one hour of mental activity for mentally passive behavior over the same period reduced the risk of developing dementia by 7 %. However, in real time, mental or physical activity is likely to replace rather than simply add to the time spent in mentally passive behavior.

These findings are consistent with those of some recent research, though with smaller effects. This could be due to the middle-aged study population's lower baseline risk of dementia.

No statistically significant association was observed between dementia risk and physical activity. The researchers suggest this may reflect factors such as relatively low activity levels, under-ascertainment of milder dementia cases, or the specific characteristics of the cohort, rather than a true absence of effect.

Strengths and limitations

The strengths of this study include the large cohort size, the lower baseline risk, which reduces but does not eliminate the risk of reverse causality, the long follow-up period, and the use of multiple markers to distinguish mental activity from mental passivity.

However, the study did not capture the use of smartphones, videos, and social media as mentally passive, sedentary behavior, as it was set in an earlier era. Since such behavior is likely to profoundly impact brain function, this relationship should be captured in future research.

Other limitations include the potential under-ascertainment of dementia cases from registry data, the misclassification of dementia, and the single-point assessment of sedentary behavior, even though this increases over time.

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