Exercise could protect against Alzheimer's by triggering a hormone which helps regrow brain cells, a study has found.

Scientists have known for some time that exercise reduces the risk of developing dementia, but did not know if there was a direct link or whether ill people were simply unable to work out.

Now a new series of studies had shown that a hormone released during exercise, called irisin, is depleted in the brains of people with Alzheimer’s, and can protect animals against memory loss and brain damage.

Mice who swam nearly every day for five weeks did not develop memory impairment despite getting infusions of beta amyloid - the brain-cell clogging protein implicated in Alzheimer's.

It suggests not only that exercise really does protect against dementia, but also that a drug which mimics irisin could be the answer to preventing the disease.

"I would certainly encourage everyone to exercise, to promote brain function and overall health," said Dr Ottavio Arancio, professor of pathology and cell biology and of medicine at Columbia University.

"But that's not possible for many people, especially those with age-related conditions like heart disease, arthritis, or dementia.

“For those individuals, there's a particular need for drugs that can mimic the effects of irisin and protect synapses and prevent cognitive decline."