Couch potatoes have a higher risk of developing dementia in old age, a new study reports.
Moderate physical activity
Seniors who get little to no exercise have a 50 percent greater risk of dementia compared with those who regularly take part in moderate or heavy amounts of physical activity, the researchers found.
Moderate physical activity can include walking briskly, bicycling slower than 10 miles an hour, ballroom dancing or gardening, according to the US Centres for Disease Control and Prevention.
"It doesn't require intensive physical activity to decrease risk of dementia," said senior researcher Dr Zaldy Tan. He is medical director of the Alzheimer's and Dementia Care Programme at University of California, Los Angeles. "Even moderate amounts are fine."
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Study participants aged 75 or older gained the most protective benefit from exercise against the onset of dementia, the findings showed.
"The message here is that you're never too old to exercise and gain benefit from it," Tan said. "These patients derive the most benefit from exercise because they are the ones who are at the age of greatest risk for dementia."
Brain tends to shrink
Brain scans of participants showed that those who exercise are better able to withstand the effects of ageing on the brain, the study authors said.
With age, the brain tends to shrink. But people who regularly exercised tended to have larger brain volumes than those who were sedentary, Tan and his colleagues found.
The new study involved about 3,700 participants in the Framingham Heart Study, a federally funded health research project begun in 1948. All were 60 and older.
Researchers measured how often the participants exercised, and tracked them over a decade. During the study, 236 people developed dementia.
Read: Exercise gives dementia sufferers a boost
To see how physical activity might have affected dementia risk, the researchers broke the study population down into fifths that ranged from sedentary to highly active.
The one-fifth containing the most sedentary people were 50 percent more likely to develop dementia than the other four-fifths, the investigators found. In other words, even a little exercise helped.
Better connections in brain
The research team also compared physical activity to brain scans taken of about 2,000 study participants, and found a direct connection between exercise and brain size as people aged. Those who worked out had more total brain volume.
There are several theories why exercise might help brain health. Increased blood flow caused by physical activity might "beef up" the brain, increasing its volume and promoting the growth of additional neurons, said Dr Malaz Boustani. He is research director of the Healthy Ageing Brain Centre at the Indiana University Centre for Ageing Research and a spokesman for the American Federation for Ageing Research.
"Physical exercise might end up leading to increased density of the connections between the neurons and create alternative pathways for signals" that might otherwise be impeded due to age-related brain shrinkage, he added.
Boustani likened this process to a street system in a city. The more alternative routes are available to drivers, the less likely it is that a blockage on one street will lead to a city-wide traffic jam.
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Exercise also promotes secretion of helpful brain chemicals such as brain-derived neurotrophic factor (BDNF). Tan explained that "BDNF actually encourages the growth of new neurons, and the preservation of those we already have."
Heather Snyder, senior director of medical and scientific operations for the Alzheimer's Association, said that the true answer is likely a combination of factors related to exercise.
Benefit not fully explained
"It's likely there are multiple benefits, and they all funnel together," Snyder said.
According to Boustani, these results support other studies that have shown an association between exercise and protection against dementia, but clinical trials aimed at proving a definite link have so far been disappointing.
"When we take it to the next step and start doing experiments, randomising patients to physical exercise versus no physical exercise and see if that will protect their brain, the story becomes a little bit muddy and unclear," he said.
Regardless, Boustani said he prescribes moderate intensity physical exercise to his patients as one way to preserve their brain health – 5,000 steps a day for about a month, increasing to 10,000 steps over time.
"Given that there's no harm, and there's a possible benefit to the brain that hasn't been fully explained, I work with my patients and their families to help improve their physical activity," he said.
The findings were published online recently in Journals of Gerontology: Medical Sciences.
What is dementia?
Causes of dementia
Symptoms of dementia