The team at Duke University, in the US, showed immune cells which start attacking nutrients in the brain may be a trigger for the disease.
They say their findings could open up new avenues of research for a field that has not developed a single drug to slow the progression of the disease.
Experts said the findings offered new hope of a treatment.
The researchers indentified microglia – normally the first line of defence against infection in the brain – as major players in the development of dementia.
They found some microglia changed to become exceptionally adept at breaking down a component of protein, an amino acid called arginine, in the early stages of the disease.