There is currently no cure for Alzheimer's disease
The most common form of dementia may be closely related to another common disease of old-age - type II diabetes, say scientists.
Treating Alzheimer's with the hormone insulin, or with drugs to boost its effect, may help patients, they claim.
The journal Proceedings of the National Academy of Sciences reports insulin could protect against damage to brain cells key to memory.
UK experts said the find could be the basis of new drug treatments.
The relationship between insulin and brain disease has been under scrutiny since doctors found evidence that the hormone was active there.
The latest study, joint research between Northwestern University in the US and the University of Rio de Janeiro in Brazil, looked at the effects of insulin on proteins called ADDLs, which build up in the brains of Alzheimer's patients and cause damage.
They took neurons - brain cells - from the hippocampus, a part of the brain with a pivotal role in memory formation.
These were treated with insulin and a drug called rosiglitazone, given to type II diabetics to increase the effect of the hormone on cells.
After this, the cells were far less susceptible to damage when exposed to ADDLs, suggesting that insulin was capable of blocking their effects.
Professor William Klein, from Northwestern, said that drugs to boost the brain's sensitivity to insulin could provide "new avenues" for treating Alzheimer's disease.
"Sensitivity to insulin can decline with aging, which presents a novel risk factor for Alzheimer's disease - our results demonstrate that bolstering insulin signalling can protect neurons from harm."
His colleague, Professor Sergio Ferreira, from Rio de Janeiro, said: "Recognising that Alzheimer's disease is a type of brain diabetes points the way to novel discoveries that may finally result in disease-modifying treatments for this devastating disease."
A spokesman for the Alzheimer's Research Trust said that the study shed light on how insulin interacted with toxic proteins linked to the disease.
"People with diabetes are at higher risk of developing Alzheimer's. It is well known that insulin affects how the brain works, and this research adds more evidence to the possibility that Alzheimer's could be a type of brain diabetes.
"The most exciting implications are that some diabetes drugs have the potential to be developed as Alzheimer's treatments."
Dr Victoria King, of the charity Diabetes UK, said: "We already know that people with Type 2 diabetes are at a higher risk of developing Alzheimer's disease.
"This study is in its early stages but it is interesting because it suggests that insulin, alongside drugs that help the body use insulin more effectively, may protect against the underlying biological mechanisms associated with the development of Alzheimer's disease.
"This is very intriguing and could potentially help with new treatments for Alzheimer's disease and shed further light on its links with diabetes. We would certainly welcome more research in this area."