What you do while sitting may matter as much as how long you sit.

Reading, doing a crossword or doing office work while seated may lower the risk of dementia, while spending long periods watching television may raise it, according to new research from Sweden.

The study, published in the American Journal of Preventive Medicine, found that different kinds of sedentary behaviour are linked to dementia risk in very different ways.

Researchers said mentally active sitting appeared to help protect against dementia, while mentally passive sitting, described in the study as “couch potato” behaviour, such as watching television, was linked to a higher risk.

It challenges the older view that all forms of sedentary behaviour carry the same dementia risk.

The research team said previous studies had already suggested that mentally passive sedentary behaviours increase dementia risk, while mentally active sedentary behaviour, such as reading or office work, seemed protective.

Most adults spend between nine and 10 hours a day sitting, the researchers said.

Earlier research has shown that extended, uninterrupted sitting is a risk factor for heart disease, type 2 diabetes and depression. It has also been associated with dementia.

The new study is the first to separate passive sitting from mentally active sitting when looking at dementia risk, according to the researchers.

They found that adults who spent long periods in mentally passive sedentary behaviour had a higher risk of dementia. Replacing passive sitting with mentally active sedentary behaviour was associated with a lower risk of dementia later in life.

Dementia is the third highest cause of death and the seventh largest cause of disability among older adults worldwide, the researchers said.

Lead investigator Professor Mats Hallgren, of the Karolinska Institute in Stockholm, said: “While all sitting involves minimal energy expenditure, it may be differentiated by the level of brain activity.

“How we use our brains while we are sitting appears to be a crucial determinant of future cognitive functioning and, as we have shown, may predict dementia onset.”

Researchers analysed data from a study involving more than 20,000 Swedish adults aged 35 to 64 who were followed over 19 years.

The initial survey included questions about sedentary behaviour, physical activity and other behaviours associated with dementia.

To identify dementia cases, the team linked the 1997 survey data with the Swedish National Patient Register and the Swedish Cause of Death Register.

Using statistical models, the researchers examined the dementia associations that came from substituting mentally passive sedentary behaviour with mentally active sedentary behaviour.

They found that mentally active sedentary behaviour was associated with a reduced risk of dementia among middle-aged and older adults.

Increasing time spent in mentally active sedentary behaviour was also associated with a “significant” reduction in dementia risk while maintaining levels of passive sedentary behaviour, light physical activity and moderate-to-vigorous physical activity.

The same pattern appeared when researchers looked at replacing time spent in mentally passive sedentary behaviour with an equivalent amount of mentally active sedentary behaviour. That substitution was also associated with a lower risk of developing dementia.

Because the survey collected data from 3,600 cities and villages across Sweden, the research team said the findings are likely generalisable to a wider global population.

“The prospective study design allowed us to establish the direction of these relationships and infers but does not establish causality,” said Hallgren. “Controlled trials are needed to confirm these important observational study findings.”

Hallgren, who also holds a post at Deakin University in Australia, said sedentary behaviour is common and can be changed, making it a target for public health efforts.

He said, “Sedentary behavior is a ubiquitous but modifiable risk factor for many health conditions, including dementia.

“Our study adds the observation that not all sedentary behaviors are equivalent; some may increase the risk of dementia, while others may be protective.

“It is important to remain physically active as we age, but also mentally active, especially when we are sitting.”