Taking exercise three times a week in later life could dramatically reduce the risk of Alzheimer's, according to research published today. People over 65 who exercise at this level are up to 40% less likely to develop dementia than those who are less active. The exercise need not be vigorous, and each session need last only 15 minutes.

Scientists in Seattle followed 1,740 adults, aged 65 or over, with no initial mental degeneration, for six years on average. By the end of this period, 158 had developed dementia, with 107 of these having the most common form - Alzheimer's disease.

Participants were questioned every two years on how frequently they exercised, with each 15-minute session counting as an episode. Activities included hiking, aerobics, calisthenics, swimming, water aerobics, weight training and stretching, with walking being by far the most common.

The researchers, led by Eric Larson, director of the Group Health Cooperative in Seattle, found that the risk of dementia and Alzheimer's was 32%-38% lower for those who exercised three or more times a week than for those who exercised less than this. But exercising four or more times, or exercising at increased intensity, did not have an additional benefit.

"We learned that a modest amount of exercise would reduce a person's risk of dementia by about 40%," said Professor Larson. "And you didn't have to get into a sweat. At this age, the key is not to be vigorous - it's to be not inactive."

Prof Larson, whose research is published today in the American journal the Annals of Internal Medicine, also found that those patients who were the most physically frail at the start of the study had the greatest risk reduction - of 42%. "This means that older people really should 'use it even after you start to lose it' because exercise may slow the progression of age-related problems in thinking," he added.

The study builds on Swedish research published in the Lancet Neurology last year which suggests that exercising at least twice a week during midlife could cut the risk of dementia by 50%. Both studies suggest exercise might protect against dementia by preventing reduced blood flow to part of the brain.

The new findings will give fresh hope to the 750,000 Britons suffering from dementia, 500,000 of whom have Alzheimer's. While some drugs may slow the progression of some symptoms, there is no cure for the disease. The government recommends that adults take moderate aerobic exercise for 20 to 30 minutes, three to five times a week, to protect their lungs and hearts, but the new research suggests it could help further.

Clive Ballard, director of research at the Alzheimer's Society, said: "This very substantial study of more than 1,500 people adds further weight to the evidence that physical exercise can significantly reduce your risk of developing dementia.

"The study's findings are consistent with the Alzheimer's Society's message that leading a healthy life may reduce your risk of developing dementia. As we do not yet know the cause of Alzheimer's disease and other forms of dementia, and there is no cure, it is vital that people are aware of anything they can do to reduce their risk."

But Professor Ballard added: "It is difficult to separate exercise from other elements of healthy living that may affect your risk of developing Alzheimer's disease, and taking regular exercise is just one essential part of a healthy lifestyle. People should also eat healthily, make sure they get their blood pressure checked and take part in regular social activities."