People who are physically active may have a significantly reduced risk of developing Alzheimer's disease, a new study has shown.

US researchers looked at almost 900 people who were taking part in a 30-year heart health study. The average age of participants was 78 years. All had their physical activity habits assessed and all underwent MRI brain scans.

The study found that many different physical activities, including walking, dancing and gardening, can improve brain volume and reduce the risk of Alzheimer's by as much as 50%.

The study noted that increased physical activity is linked with larger brain volumes in the frontal, temporal and parietal lobes. Those participants who increased their physical activity levels had a 50% reduced risk of developing Alzheimer's.

Furthermore, while one in four participants already displayed signs of mild cognitive impairment associated with Alzheimer's, increasing physical activity also appeared to benefit their brain volumes.

"This is the first study in which we have been able to correlate the predictive benefit of different kinds of physical activity with the reduction of Alzheimer's risk through specific relationships with better brain volume in such a large sample," the researchers from UCLA noted.

They insisted that because there is no cure for the disease, ‘focus needs to be on prevention'.

Alzheimer's disease affects around 40,000 people in Ireland. While memory loss is probably the most well-known symptom, other symptoms can include changes in mood, changes in personality and difficulty performing familiar tasks.

Details of these findings are published in the Journal of Alzheimer's Disease. According to its editor in chief, Dr George Perry, ‘currently the greatest promise in Alzheimer's disease research is lifestyle intervention including increased exercise'.

"This is a landmark study that links exercise to increases in grey matter and opens the field of lifestyle intervention to objective biological measurement," he added.