For older adults, a lack of exercise may put their risk of developing dementia on par with that of adults who are genetically predisposed to the disease. This is the conclusion of a new study published in the Journal of Alzheimer's Disease.

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Researchers suggest that sedentary behavior puts older adults at just as much risk of developing dementia as those who are genetically predisposed to the disease.

Worldwide, there are around 47.5 million people living with dementia. By 2030, this number is expected to rise to approximately 75.6 million.

Alzheimer's disease is the most common form of dementia, accounting for about 60-80 percent of all cases. In the United States alone, an estimated 5.4 million people have Alzheimer's.

One of the biggest risk factors for Alzheimer's disease is the apolipoprotein E (APOE) e4 gene. According to the Alzheimer's Association, adults who possess one copy of the APOE e4 gene are three times more likely to develop the disease than those without the gene, while those with two copies are 8-12 times more likely to develop Alzheimer's.

However, the researchers of the new study - including Jennifer Heisz, an assistant professor in the Department of Kinesiology at McMaster University in Canada - suggest that the risk of dementia may be just as high for older adults exhibiting sedentary behavior.

Inactivity may 'negate the protective effects of healthy genes'

The Physical Activity Guidelines for Americans state that older adults should engage in around 150 minutes of moderate-intensity aerobic activity, or 75 minutes of vigorous-intensity activity, every week.

However, a 2015 review published in the Journal of Aging and Physical Activity found that adults aged 60 and older spend approximately 9.4 hours a day sedentary, which is equivalent to about 65-80 percent of their waking day.

For their study, Heisz and colleagues set out to investigate the association between physical activity and dementia risk among older adults with and without the APOE e4 gene.

The researchers came to their findings by analyzing the physical activity and dementia development of 1,646 older adults who were part of the Canadian Study of Health and Aging. All participants were free of dementia at study baseline and followed up for around 5 years.

Among adults who did not carry the APOE e4 gene, the researchers found that those who did not exercise were more likely to develop dementia than those who exercised.

For APOE e4 gene carriers, however, there was no significant difference in dementia risk between those who exercised and those who did not.

According to the researchers, these findings indicate that a lack of exercise may be just as risky for dementia development than carrying the APOE e4 gene.

"The important message here is that being inactive may completely negate the protective effects of a healthy set of genes."

Jennifer Heisz

Increasing exercise may protect against dementia

It is not all bad news; the study results also suggest that increasing physical activity may protect against the development of dementia in people without the APOE e4 gene.

"Although age is an important marker for dementia, there is more and more research showing the link between genetic and lifestyle factors," says study co-author Parminder Raina, a professor in the Department of Health Evidence and Impact at McMaster.

"This research shows that exercise can mitigate the risk of dementia for people without the variant of the apolipoprotein genotype," he adds. "However, more research is needed to determine the implications from a public health perspective."

Lead study author Barbara Fenesi, a postdoctoral fellow at McMaster, points out that further studies are needed in order to pinpoint the type of exercise that is most beneficial for brain health.

"A physically active lifestyle helps the brain operate more effectively. However, if a physician were to ask us today what type of exercise to prescribe for a patient to reduce the risk of dementia, the honest answer is 'we really don't know,'" she says.

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