Replacing passive screen time with mentally engaging activities may help protect the brain for decades and provides new insights into how daily habits shape dementia risk.
Study: Mentally active versus passive sedentary behavior and risk of dementia: a 19-year cohort study. Image credit: Pressmaster/Shutterstock.com
Adults who are sedentary have an increased risk of depression, but the association with dementia is less clear. Recent research published in American Journal of Preventive Medicine reported that even among sedentary adults, mentally active sedentary behavior was associated with a lower risk of developing dementia during follow-up.
Sedentary lifestyles are increasingly linked to cognitive decline
Approximately 57 million people are currently living with dementia, which poses significant physical, psychological, and economic costs to patients, caregivers, and society. Previous research has linked sedentary behavior to a higher risk of developing dementia. Increasing physical activity in sedentary people may improve cognition.
mentally passive or mentally active
Sedentary activities that are mentally passive, such as watching television, are distinguished from those that are mentally active, such as office work. Prolonged periods of mentally passive, sedentary behavior can lead to a decline in cognitive function.
Previous observational studies suggest that adults who sit and watch television have a higher risk of dementia than adults who are more mentally active. The latter includes computer use, playing cards, hobbies, and reading. However, these were small studies with relatively short follow-up periods.
Large population study links habits and brain health
The current study investigated the association between mentally passive and mentally active sitting styles and new-onset dementia. Additionally, the researchers modeled the effects of replacing mentally passive sedentary behavior with mental activity or any level of physical activity.
The study included 20,811 adults (mostly women) whose data were obtained from the Swedish National March Cohort. All were between 35 and 64 years old at baseline in 1997. Using national registry data, we identified the onset of dementia in this cohort over a median follow-up of 19 years.
A questionnaire was used to classify sedentary adults into mentally passive and mentally active. Reported sedentary behavior, both mentally passive and mentally active, accounted for an average of 116 and 240 minutes per day, respectively. Physical activity was similarly rated as light or moderate to vigorous.
There were 569 new cases of dementia for a total of 393,104 person-years.
Mental activity is associated with lower dementia risk
After adjusting for known risk factors such as body mass index (BMI), age, gender, education, smoking, alcohol consumption, diet, and chronic disease, mentally passive sedentary behavior was not significantly associated with increased adjusted risk of dementia compared to mentally active people. However, the association remained directionally positive.
For every additional hour of mentally active sedentary behavior, sedentary behavior was associated with a 4% lower risk of developing dementia, especially among people aged 50 to 64.
Hypothesized mechanism
This is biologically plausible, as mental and/or physical activity in midlife is associated with increased gray matter volume and cerebral blood flow. However, not all mental activity is beneficial. For example, working in an office is different from cognitively challenging leisure activities such as reading or solving puzzles, which can cause stress in some situations.
It may also be that sedentary adults with good baseline cognitive function are more likely to choose mentally active behaviors, highlighting the possibility of reverse causation. That is, the observed association may not be completely causal. Mental passivity can also impair sleep quality, thereby increasing the risk of cognitive impairment. Future studies are needed to directly examine these mechanisms.
In a theoretical model, holding other behaviors constant, an additional hour of mental activity reduced the risk of new-onset dementia by 11 percent, even if there was no change in mentally passive sedentary behavior or physical activity. Furthermore, replacing the same period of mentally passive behavior with one hour of mental activity reduced the risk of developing dementia by 7%. However, in real time, mental or physical activity may replace, rather than simply increase, the time spent in mentally passive behavior.
These findings are consistent with the results of several recent studies, although the effects are small. This may be due to the lower baseline risk of dementia in the middle-aged study population.
No statistically significant association was observed between dementia risk and physical activity. The researchers suggest that this may reflect factors such as relatively low activity levels, lack of ascertainment of mild dementia cases, and specific characteristics of the cohort, rather than a true lack of effect.
strengths and limitations
Strengths of this study include the large cohort size, low baseline risk (reducing but not eliminating the risk of reverse causation), long follow-up period, and use of multiple markers to distinguish between mental activity and mental passivity.
However, this study did not view smartphone, video, and social media use as mentally passive, sedentary behaviors, as it had been set out to do in previous eras. This relationship should be captured in future studies, as such behaviors are likely to have significant effects on brain function.
Other limitations include the possibility that dementia cases are under-represented in registry data despite increasing over time, misclassification of dementia, and single-point assessment of sedentary behavior.
Click here to download your PDF copy.
