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Professor Ramon Trullas, who led the research at the Institute of Biomedical Research of Barcelona, said: "If our initial findings can be replicated by other laboratories, the results will change the way we currently think about the causes of Alzheimer's.

"This discovery may enable us to search for more effective treatments that can be administered during the preclinical stage."

The research, which is published in the Annals of Neurology, found that a decrease in the amount of mitochondrial DNA in the cerebral spinal fluid of Alzheimer's patients.

Mitochondrial DNA is typically found inside tiny structures that act like the power stations of cells and are inherited through the maternal line only.

Professor Trullas and his colleagues believe that decreased levels of this mitochondrial DNA may reflect the diminished ability of these power stations to provide energy for brain cells.

He said that reductions in mitochondrial DNA may be one of the earliest signs of Alzheimer's and suggests the pathology of the disease begins far earlier than had been previously thought.

They found they could detect the reductions in mitochondrial DNA up to a decade before symptoms of dementia appeared.

Other biochemical markers of Alzheimer's disease, such as proteins that form tangles in the brain cells, appear much later.