A daily stroll around the park switches on a brain process that can help protect the brain against Alzheimer's disease, according to a new study.

The study found that a stress hormone produced during moderate exercise may protect the brain from memory changes linked to the mind-robbing disease.

Scientists say that the latest findings, published in the Journal of Alzheimer's Disease, could also explain why people susceptible to stress are at more risk of developing dementia.

Researchers from the University of Nottingham say that people who are more vulnerable to the disease may have abnormal functioning of the protective hormone and its related brain receptor.

Previous studies have found that physical and mental activity can cut the changes of developing Alzheimer's or slowing its progression. However, researchers say that until now it has been unclear what mechanism is involved in protecting against the neurodegenerative disease.

Researchers decided to look at the stress hormone CRF (corticotrophin-releasing factor). The hormone is associated with producing stress and is found in high levels in people with anxiety and depressive diseases.

However, normal levels of the stress hormone benefit the brain by keeping the mental faculties sharp and aiding the survival of nerve cells. In the past researchers found that people with Alzheimer's disease have a significantly lower level of CRF.

Lead researcher Dr. Marie-Christine Pardon and her team wanted to see the role of CRF in the onset of the disease. They used an experimental drug to block CRF in the brains of mice with Alzheimer's disease. Researchers found that these mice had reduced anxiety but increased reaction when exposed to a stressful situation, like being placed in a new environment.

Researchers explained that the results were caused by the abnormal functioning of the brain receptor CRFR1 which is normally activated by CRF. The team said that the findings explain why people susceptible to stress are more likely to develop Alzheimer's.

They found that stopping the hormone from binding to the CRFR1 receptor blocked the improvement of memory normally promoted by exercise. However, researchers found that in mice with Alzheimer's, a repeated regime of moderate exercise restored the normal function of the CRF system, therefore bringing back its memory enhancing effects.

Researchers concluded that the turning on of this particular brain receptor during exercise increased the density of synapses. Researchers explained that these synapses make the connection between nerve cells, and the loss of them is thought to be responsible for the early memory loss seen in Alzheimer's patients.

"This is the first time that researchers have been able to identify a brain process directly responsible for the beneficial effects of exercise in slowing down the progression of the early memory decline characteristics of Alzheimer's disease," Pardon said in a statement.

"Overall, this research provides further evidence that a healthy lifestyle involving exercise slows down the risk of Alzheimer's disease and opens avenues for the new interventions targeting the altered CRFR1 function associated with the early stages of the disease," she concluded.