By Peter Galvin, MD

Half of U.S. adults spend more than 9.5 hours of their day sitting, including more than 80% of their leisure time. This is known as sedentary time. A sedentary, or inactive, lifestyle has been associated with a host of diseases and conditions including cardiovascular disease, diabetes and cognitive and structural brain aging. Studies based on self-reported activity or inactivity have shown a relationship between cognitive aging, or dementia, and inactivity. Last September, a large study was published that looked closer at the possible association between a sedentary lifestyle and the risk of early dementia. They used some newer technology that actually measured the test subjects’ activity.

The study defined sedentary behavior as “any waking behavior characterized by an energy expenditure of < 1.5 METs (metabolic equivalent units) while in a sitting or reclining posture.” In general, this included behaviors like sitting while using a computer, watching TV, and driving. Previous studies found that self-reported leisure-time sedentary behaviors were associated with a risk of developing all-cause dementia, but the risk depended on the activity done while sedentary (i.e., cognitively passive TV watching vs cognitively active computer use or reading). This new study measured activity using an accelerometer, which is a device similar to a pedometer, which measures the number of steps taken and distance walked over a period of time. The device they used was an Axivity AX3 3-axis logging accelerometer worn as a wristwatch. This measures and records activity in three dimensions, namely forward and backwards, up and down, and sideways. The study group was a large cohort of older adults in the UK (almost 50,000 adults, mean age 67.19, and 54.7% female).

The study again found an association between sedentary behavior time and dementia, especially if the daily sedentary time was 10 hours or longer. In contrast, they found that the risk of dementia was reduced in those who were more physically active, especially those who engaged in moderate to vigorous intensity physical activity. They also confirmed the results of previous studies that found that the risk of dementia is compounded in those whose leisure time was mostly cognitively passive (i.e., watching TV). I suppose this confirms that the moniker “the boob tube” is fairly accurate. Finally, they also confirmed data from previous studies that showed high rates of mortality in those with inactive lifestyles.

One of the study’s drawbacks was that they found that wrist-mounted accelerometers tended to be less accurate at recording overall activity than thigh-mounted ones, and they recommended that subsequent studies use thigh-mounted devices. At any rate, we now have further proof that, among older adults, an inactive lifestyle increases both the risk of dementia and early death, and that cognitively passive activity further adds to that risk. Whether cognitively passive inactivity actually causes dementia remains to be seen, but the association between the two is real. So, if you can’t be physically active because of disease or disability, at least occupy your time with active things like reading, doing puzzles, or playing video games, rather than just sitting there watching the boob tube.

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