SPENDING more than 10 hours a day sitting down "rapidly" increases your risk of dementia, research suggests.
Scientists found older adults who were largely inactive were more likely to suffer from the brain-robbing disease than those who moved more.
And it didn't matter whether the sedentary periods were in one long chunk or dotted throughout the day. Both had a similar effect.
Reassuringly though, the dementia findings, published in the journal Jama, revealed that being idle for less than 10 hours was not associated with an increased risk of developing the condition.
The researchers said it provided "some reassurance" for people with office jobs that involve "prolonged periods of sitting".
Study author Gene Alexander, professor of psychology and psychiatry at the Evelyn F McKnight Brain Institute at the University of Arizona and Arizona Alzheimer's Disease Research Centre in the US, said: "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.
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"This suggests that it is the total time spent sedentary that drove the relationship between sedentary behaviour and dementia risk, but importantly lower levels of sedentary behaviour, up to around 10 hours, were not associated with increased risk."
In England, adults of working age average about 9.5 hours per day of sedentary time.
This includes watching TV, using a computer, reading, doing homework and travelling by car, bus or train – but does not include sleeping.
Between the ages of 65 and 74, average sedentary time in men and women increases to 10 hours per day or more, according to figures from the British Heart Foundation.
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For the study, the researchers looked at data from more than 49,000 people aged 60 and above from the UK Biobank – an online database of medical and lifestyle records of half a million Brits.
These people did not have dementia at the start of the study and were followed for more than six years.
Participants were given devices to wear on their wrists to track movement for 24 hours a day for one week.
Using a type of artificial intelligence known as machine-learning algorithms, the researchers classified types of movement, including differentiating between sleeping and sitting still.
Over the course of the study, 414 people developed dementia.
When adjusting for lifestyle factors that could affect brain health (such as diet, smoking, alcohol use and self-reported mental health) and demographics (such as age, sex, education level, ethnicity, chronic conditions and genetics), the team found that prolonged lack of movement was linked with increased risk of dementia.
Compared to adults who spent around nine hours a day sitting down, those who were sedentary for 10 hours were eight per cent more likely to develop dementia.
And people who were sedentary for 12 hours were 63 per cent more likely to develop the condition.
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Study author David Raichlen, professor of biological sciences and anthropology at the University of California's Dornsife College of Letters, Arts and Sciences, said: "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.
"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."
Prof Raichlen said more research is needed to fully answer the question of whether physical activity can attenuate the risk after being sedentary for 10 or more hours.
He added: "As more cases of dementia occur, we will be able to better address that question."
Meanwhile, another study, published in the journal Jama Network Open, has found untreated high blood pressure to be associated with greater dementia risk.
The research, which was an analysis of data from 14 different studies and involved more than 34,000 people aged 60 and above, showed that individuals with untreated high blood pressure had a 42 per cent increased risk of dementia compared with healthy people and 26 per cent increased risk compared with those who received treatment for the condition.
Dr Richard Oakley, associate director of research and innovation at Alzheimer's Society, said: "Dementia affects 900,000 people in the UK and is the country's biggest killer.
"We know some people are at greater risk than others due to factors such as their age, genes and lifestyle.
"This study showed that being inactive for more than 10 hours a day increases the risk of going on to develop dementia.
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"As a rule of thumb, what's good for the heart is good for the head, and taking care of your cardiovascular health could help reduce your risk of getting dementia in the future.
"Eating a balanced diet, avoiding smoking and heavy drinking, and exercising regularly can all help too."
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