Scientists have discovered how heart disease or a stroke may trigger Alzheimer's disease.
Amyloid protein damages brain cells
Both conditions lead to a reduction of oxygen flow to the brain.
A University of British Columbia team, studying mice, found this stimulates increased development of the protein clumps thought to cause Alzheimer's.
The lack of oxygen increases activity in a gene controlling production of the key protein, found the Proceedings of the National Academy of Sciences study.
This protein, beta amyloid, collects in knots in the brains of people with Alzheimer's disease, and is thought to cause damage to the brain cells.
The gene identified in the latest study, BACE1, is responsible for regulating the maturation of beta amyloid.
The researchers, who carried out their work on mice, say their findings suggest that boosting oxygen delivery to the brain may be one way to treat Alzheimer's disease.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said the charity was currently funding research to investigate the role of oxygen levels in amyloid plaque build-up.
She said: "Scientists have known for some time that people with certain diseases that restrict their supply of oxygen, such as stroke or lung disorders, are more prone to Alzheimer's and also that people who live at high altitudes are also at greater risk of developing the disease.
"This research provides a molecular explanation for that anecdotal finding and shows that cues from cells, genes and a person's environment all play a role in causing this dreadful disease.
"We hope that research in this area could suggest new methods to prevent and treat dementia in the future - particularly for those individuals who have suffered head trauma or a stroke."
Professor Clive Ballard, director of research at the Alzheimer's Society, said there was growing evidence that a good supply of blood and oxygen to the brain was vital to minimise the risk of developing dementia.
He said: "The Alzheimer's Society strongly believes that what is good for the heart is also good for the brain.
"You can reduce your risk of developing dementia by taking regular exercise, eating healthily and getting your blood pressure checked."