Adults age 60 and older who spend more time engaging in sedentary behaviors, such as sitting while watching TV or driving, may be at increased risk for developing dementia, finds a new study by University of Arizona and University of Southern California researchers.

The study shows the risk of dementia significantly increases among adults who spend over 10 hours a day engaging in sedentary behaviors like sitting — a notable finding considering the average American is sedentary for about 9.5 hours each day.

The study, published Tuesday in the Journal of the American Medical Association, also revealed that the way sedentary behavior is accumulated over the course of the day doesn't matter as much as the total time spent sedentary each day.

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"We were surprised to find that the risk of dementia begins to rapidly increase after 10 hours spent sedentary each day, regardless of how the sedentary time was accumulated," said study co-author Gene Alexander, a UArizona professor of psychology and psychiatry and a professor in the university's Evelyn F. McKnight Brain Institute and BIO5 Institute. "This suggests that it is the total time spent sedentary that drove the relationship between sedentary behavior and dementia risk. But, importantly, lower levels of sedentary behavior, up to around 10 hours, were not associated with increased risk."

That should provide some reassurance to those with office jobs involving prolonged periods of sitting, said lead study author David Raichlen, professor of biological sciences and anthropology at USC.

"Many of us are familiar with the common advice to break up long periods of sitting by getting up every 30 minutes or so to stand or walk around," Raichlen said. "We wanted to see if those types of patterns are associated with dementia risk. We found that once you take into account the total time spent sedentary, the length of individual sedentary periods didn't really matter."

The researchers used data from the U.K. Biobank, a large-scale biomedical database of participants across the United Kingdom, to investigate possible links between sedentary behavior and dementia risk.

As part of a U.K. Biobank sub-study, over 100,000 adults agreed to wear accelerometers, wrist-worn devices for measuring movement, for 24 hours a day for one week. From this sub-study, the researchers focused on a sample of approximately 50,000 adults over the age of 60 who did not have a diagnosis of dementia at the start of the study.

The researchers then applied a machine learning algorithm to analyze the large dataset of accelerometer readings and classify behaviors based on different intensities of physical activity. The algorithm was able to discern between different types of activity, such as sedentary behavior versus sleeping. The accelerometer data, combined with advanced computing techniques, provided researchers with an objective measure of the time spent engaging in different types of sedentary behaviors.

After an average of six years of follow-up, the researchers used inpatient hospital records and death registry data to determine dementia diagnoses. They found 414 cases of dementia.

Then, the researchers adjusted their statistical analysis for certain demographics – including  age, sex, education level, race/ethnicity, chronic conditions and genetics – as well as lifestyle characteristics – such as physical activity, diet, smoking, alcohol use and self-reported mental health – that could affect brain health.

The study builds on their previous research, which used self-reported health data to investigate how certain types of sedentary behavior, like sitting and watching TV, impact dementia risk more than others.

"Our latest study is part of our larger effort to understand how sedentary behavior affects brain health from multiple perspectives," Raichlen said. "In this case, wearable accelerometers provide an objective view of how much time people dedicate to sedentary behavior that complements our past analyses."

More research is needed to establish causality and whether physical activity can mitigate the risk of developing dementia, the authors said.

In addition to Raichlen and Alexander, other authors of the study include Daniel H. Aslan, M. Katherine Sayre, Mark H.C. Lai and Rand R. Wilcox of USC; Pradyumna K. Bharadwaj, Madeline Ally and Yann C. Klimentidis of UArizona; and Silvio Maltagliati of the Universite of Grenoble Alpes, France.

The study was supported by grants P30AG072980, P30AG019610, R56AG067200, R01AG064587, and R01AG072445 from the National Institutes of Health and funding from the state of Arizona, the Arizona Department of Health Services and the McKnight Brain Research Foundation. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.