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People 60 and older who spent more time being sedentary -- passively watching TV, for example -- had a higher risk of incident all-cause dementia, prospective data suggested.

The study followed about 50,000 U.K. Biobank participants living in England, Scotland, or Wales for nearly 7 years. Participants wore a wrist accelerometer for a week to assess their activity patterns.

Dementia risk rose when sedentary behavior was 10 hours a day (HR 1.08), and climbed when it reached 12 hours a day (HR 1.63) and 15 hours a day (HR 3.21), reported David Raichlen, PhD, of the University of Southern California in Los Angeles and co-authors, in . Hazard ratios were compared with the study median of 9.27 sedentary hours a day (P<0.001).

The association was consistent whether sedentary time occurred in extended continuous periods or was spread intermittently throughout the day, Raichlen and colleagues said.

The relationship was nonlinear and there was no strong association with dementia risk before 10 daily hours of sedentary behavior, the researchers noted. "The mean time spent in sedentary behavior in the U.S. in 2019 was , which falls close to the level when the risk of dementia began to increase in this study," they wrote.

Sedentary behavior has been shown to increase all-cause mortality and risk for cardiovascular disease, diabetes, hypertension, obesity, and depression, Claire Sexton, DPhil, of the Alzheimer's Association in Chicago, told 番茄社区app.

Past research has shown mixed results about sedentary behavior and dementia risk, noted Sexton, who wasn't involved with the study. While the U.K. Biobank data do not establish causality, they emphasize the importance of doctor-patient conversations about movement and activity, she observed. "This is especially important given the prevalence of sedentary behavior," she said.

"Regular physical activity -- how much you move, how often, and how vigorously -- is central to your health, yet even this vital component cannot be viewed in isolation," Sexton pointed out.

"Multiple factors contribute to dementia risk, several of which are also associated with sedentary behaviors," she continued. "Trials are underway to further examine the potential of multimodal interventions targeting physical activity, cognitive activity, social activity, diet, and health status both in the U.S. -- the study -- and ."

Raichlen and co-authors applied an algorithm to raw accelerometry data to derive an objective measure of sedentary time in 30-second time windows. Sedentary behavior bouts were defined as more than two consecutive 30-second epochs classified as waking sedentary behaviors according to in other sources (sleep was not included).

The study evaluated accelerometer data from 1 week of activity in 49,841 participants in 2013-2015. Participants were free of dementia at the start of the study and had an average age of 67. About 55% were women.

Biobank participants were followed for a mean of 6.72 years. Incident dementia was determined by inpatient hospital records and death registry data; a total of 414 cases were reported.

The mean daily sedentary bout in the study was 0.48 hours and the mean maximum daily sedentary bout was 1.95 hours, with dementia risks rising as bout lengths rose. After adjusting for time spent in sedentary behavior, mean and maximum bout lengths were no longer significantly tied to incident dementia. The number of sedentary bouts per day was not correlated with incident dementia risk.

The study's observational study design meant that unmeasured confounding may have influenced results, Raichlen and colleagues acknowledged. Reverse causality also may have occurred, and cases of dementia may have been missed. In addition, the algorithm was validated in a sample that included participants of all ages, not just older adults.

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