Of all the lifestyle changes that have been studied, taking regular physical exercise appears to be one of the best things that you can do to reduce your risk of getting dementia.
Several studies looking at the effect of aerobic exercise (exercise that increases your heart rate) in middle-aged or older adults have reported improvements in thinking and memory, and reduced rates of dementia.
Exercising in mid-life
Prospective studies follow the health and behaviour of a group of people over time. Several prospective studies have looked at middle-aged people and the effects of physical exercise on their thinking and memory in later life. Combining the results of 11 studies shows that regular exercise can significantly reduce the risk of developing dementia by about 30 per cent. For Alzheimer's disease specifically, the risk was reduced by 45 per cent.
One particular study looked at health behaviours of over 2,000 men in Wales, and followed them for 35 years. Of the five behaviours that were assessed (regular exercise, not smoking, moderate alcohol intake, healthy body weight and healthy diet), exercise had the greatest effect in terms of reducing dementia risk. Overall, people who followed four or five of the above behaviours were up to 60 per cent less likely to develop dementia.
In the short term, aerobic exercise can also improve the performance of healthy adults on thinking tests. Pulling together the results of 29 clinical trials, a month or more of regular aerobic exercise resulted in improvements in memory, attention and processing speed when compared with regular non-aerobic exercise such as stretching and toning.
Exercising in later life
Although less research has been done with healthy older people, there is some evidence to show older people can also reduce their risk of dementia with regular exercise. In a study of 716 people with an average age of 82 years, people who were in the bottom 10 per cent in terms of amount of daily physical activity were more than twice as likely to develop Alzheimer's disease as those in the top 10 per cent.
A literature review found 27 studies looking at the effect of physical activity on brain function in people over 60 years of age. In 26 of the studies there was a clear link between physical activity levels and cognitive performance, suggesting that exercise might be an effective way to reduce cognitive decline in later life.
Aerobic exercise has also been shown to affect the brains of healthy older people. In a modest-sized controlled trial, one year of aerobic exercise resulted in a small increase in the size of the hippocampus (the key brain area involved in memory), which was the equivalent of reversing one to two years of age-related shrinkage. A study of 638 people in Scotland that asked people about their activity levels found those who were physically active at age 70 experienced less brain shrinkage over three years than those who were not.
What does 'physical activity' mean?
The research studies in this area do not all use the same definition of 'physical activity' or exercise. In general they are referring to aerobic exercise performed for a sustained period of time, perhaps 20–30 minutes. Most of the studies report on the effects of aerobic exercise done several times a week and maintained for at least a year.
However, physical exercise does not just mean playing a sport or running. It can also mean a daily activity such as brisk walking, cleaning or gardening. One study found that the risk of Alzheimer's disease can be reduced by daily physical tasks such as cooking and washing up.
Areas of uncertainty
Usually, we would like there to be randomised controlled trials (RCTs) that show how one factor can influence another. To date, no RCT has shown an exercise regime can prevent dementia, although there are several in progress across Europe.
We need more research to understand the level and intensity of exercise that is most effective; this is also likely to vary throughout someone's life. We also need more research to understand the role that physical activity plays in reducing the risk of different types of dementia.