Scientists say they have identified a possible genetic link between diabetes and Alzheimer's disease.
It has been known for some time that people with diabetes have a much higher risk of developing Alzheimer's, but not why this is so.
Now US researchers writing in Genetics say a study of worms has indicated a known Alzheimer's gene also plays a role in the way insulin is processed.
Dementia experts said more work in humans was now needed.
Alzheimer's is the most common cause of dementia, which affects 820,000 people in the UK.
There are medications which can slow the progress of the disease, but none that can halt its progress.
A key indication of Alzheimer's, which can only be seen after death, is the presence of sticky plaques of amyloid protein in decimated portions of patients' brains.
Scientists have already found mutations in a gene involved in the processing of amyloid protein in Alzheimer's which run in families.
'Open new doors'
In this study, a team from the City College of New York looked at a similar gene in the nematode worms (C. elegans).
These worms are often studied because they, perhaps surprisingly, a useful model for human research.
The researchers, led by Prof Chris Li, found the gene in the worms also affected the insulin pathway - the chemical reactions involved in its production and processing.
Prof Li said: "People with type 2 diabetes have an increased risk of dementia.
"The insulin pathways are involved in many metabolic processes, including helping to keep the nervous system healthy."
She said more work was needed to investigate this potential link and its effects further.
Mark Johnston, editor-in-chief of the journal Genetics, said it was "an important discovery".
"We know there's a link between Alzheimer's and diabetes, but until now it was somewhat of a mystery.
"This finding could open new doors for treating and preventing the disease."
Dr Marie Janson, director of development at Alzheimer's Research UK, which has itself funded studies looking at the link between diabetes and Alzheimer's said: "This early-stage study may provide an interesting clue to help scientists unravel how diabetes and Alzheimer's are linked, but questions still remain to be answered.
"As this research looked at the effects of a gene in worms, studies are now needed to discover whether the equivalent gene in people has the same effect, and exactly what mechanisms may be involved."
And Dr Anne Corbett, research communications manager at Alzheimer's UK, added: "There is a growing body of evidence linking the development of diabetes with an increased risk of dementia.
"By identifying a potential relationship between genes involved in diabetes and Alzheimer's disease, this study offers further clues as to what this link could be.
"However, further research is needed to know whether these findings, from research with worms, will be the same in humans."