Regular exercise reduces the risk of memory disorders such as dementia and Alzheimer's disease, say scientists. Workouts may also help to delay progression of the condition in people who begin to develop these symptoms.

Researchers led by Eric Larson at the University of Washington studied the effect of exercise on the people suffering from such disorders and found it reduces risk by 40 percent, reported the online edition of BBC News.

The study followed 1,740 people aged 65 and older over a six-year period. At the start of the study none showed signs of dementia. After six years, 158 participants had developed dementia, of which 107 had been diagnosed with Alzheimer's disease.

People who exercised three or more times a week had a 30 percent to 40 percent lower risk of developing dementia compared with those who exercised fewer than three times per week, it found.

Larson believes exercise may improve brain function by boosting blood flow to areas of the brain used for memory. The frailer a person, the more likely that exercise would help.

A regular gentle workout was enough to produce a positive effect - even for people aged over 65. Even if you're 75 and have never exercised before, you can still benefit by starting to exercise now, the study said.

Larson said walking for 15 minutes three times a week was enough to cut the risk.