Exercise increases levels of a brain molecule that may protect against Alzheimer's disease, research has shown.
Scientists believe the protein PGC-1alpha could open the door to new ways of treating Alzheimer's.
The protein has metabolic effects that also appear to guard against type 2 diabetes.
Researchers studied brain samples from dead Alzheimer's patients and compared them with others from healthy individuals. They found there was less PGC-1alpha in the Alzheimer's-affected brains.
Further investigation revealed that cells containing more PGC-1alpha produced less of the toxic amyloid protein that accumulates in the brains of people with the disease.
Since exercise is known to raise levels of PGC-1alpha, the findings may help explain the link between regular physical activity and reduced Alzheimer's risk.
They also provide a clue to why people with diabetes are more likely to develop Alzheimer's.
Magdalena Sastre, from Imperial College London, who led the study, said: "These early results tell us much more about how diabetes and Alzheimer's are linked, but more importantly, they have given us a potential treatment target.
"Research is the only way to defeat dementia, and it is essential that we follow up this work to see whether drugs that raise the levels of the PGC-1alpha protein could help protect against Alzheimer's."
The findings have been published in the Journal of Alzheimer's Disease.