However, a new study suggests that moderate exercise might not slow down the disease.
Although the difference between the two groups was small, the researchers say exercise should not be recommended for people with dementia and called for future trials to 'consider the possibility that some types of exercise intervention might worsen cognitive impairment'.
Regarding the question of whether exercise might worsen dementia, they noted that those who did the most exercise had worse outcomes, saying it was "possible" the programme may have worsened mental abilities.
It affects 47.5 million people worldwide and manifests in a variety of different ways. Alzheimer's disease is the most common form of dementia, accounting for 60 to 80 percent of all cases in the United States, according to the Alzheimer's Association. Few studies have involved humans.
In the study, a team led by Sarah Lamb, of Oxford's Center for Rehabilitation Research and Center for Statistics in Medicine, tracked outcomes for almost 500 people averaging 77 years of age.
The researchers were surprised to find that individuals assigned the exercise program had higher ADAS-cog scores compared with those who didn't. Each session involved aerobic exercise (30 minutes of moderate- to hard-intensity cycling on a stationary bike) and strength training (various exercises using hand-held weights). They said they can not exclude the possibility that exercise may have made dementia worse, although the differences in decline were small. Those taking part in the exercise programme had their physical fitness measured at the start of the programme and again after 6 weeks.
Those in the "usual care" group (165 people) received care in accordance with British clinical guidelines, which included counseling, medications for any physical symptoms and brief advice about physical activity.
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Participants were assessed at six and 12 months after starting the programme.
The study found that cognition declined in both groups.
"These benefits do not, however, translate into improvements in cognitive impairment, activities in daily living, behaviour, or health-related quality of life".
Although the exercise programme improved physical fitness, it can not be recommended as a treatment option for cognitive impairment in dementia, say the researchers. But recent reviews of trials of exercise training in people with dementia have shown conflicting results. People who exercise more are less likely to get dementia, possibly because it maintains blood flow to the brain. Lamb commented that dementia is a complicated issue to tackle.
"We don't want to alarm members of the public with dementia and their families".
It's important to note this does not change what we know about exercise's ability to protect against dementia.