Having diabetes can increase a person's risk of developing Alzheimer's disease by up to 65%, scientists have found.
Insulin can be a key part of diabetes treatment
The researchers from Chicago's Rush University Medical Center identified the increased risk in a five-year study of more than 800 people.
UK experts said further research was needed to explore exactly how diabetes increased the risk of developing Alzheimer's.
The research has been published in the journal Archives of Neurology.
Diabetes is known to affect around 1.4 million people in the UK. The majority have Type 2 diabetes. Another million are thought to have diabetes but not know it.
The Chicago researchers studied 824 Catholic nuns, priests, and brothers who were taking part in the Rush University Religious Orders Study for an average of five-and-a-half years.
Over the course of the study 151 people were diagnosed with Alzheimer's, including 31 who had diabetes.
The researchers calculated diabetics therefore had a 65% increased risk of developing Alzheimer's, compared to non-diabetics.
They also looked more closely at mental decline by testing diabetics and non-diabetics on facets of memory and perception commonly affected by ageing, Alzheimer's Disease and other dementias throughout the time they were studied.
The tests showed that only speed of perception - being able to tell whether two things are the same or different - declined faster in diabetics than in healthy patients, by around 44%.
However, because common vascular conditions such as stroke have been shown to affect this function, the researchers say more work would have to be carried out before the mental change could be linked to diabetes.
In other areas of cognition, the rate of change over the period people were studied was no different in diabetics and non-diabetics.
Dr Zoe Arvanitakis, who led the research, said: "We found that diabetes was related to decline in some cognitive systems but not in others."
She said all those who had participated in the study had agreed to donate their brains to research so it would be possible to examine them to try to gain more information about how diabetes was linked to cognitive decline.
Professor Clive Ballard, director of research at the Alzheimer's Disease Society, said the study clearly showed an increased risk of Alzheimer's disease in people with diabetes, confirming the findings of several large previous studies.
He added: "Diabetes along with other conditions such as high blood pressure and high blood cholesterol, are well-recognised risk factors for Alzheimer's.
"Further work to determine the mechanism linking diabetes to Alzheimer's disease will be important for people with diabetes, but may also give new insights to some of the processes that are important in the evolution of Alzheimer's disease more generally."
Professor Ballard said eating healthily, ensuring blood pressure and cholesterol levels are checked regularly, taking exercise and watching our weight may all turn out to be important ways of reducing the risk of developing dementia.
Penny Williams, a care advisor for Diabetes UK, added: "We know that persistently raised blood glucose levels can lead to many complications such as heart disease and blindness, so good blood glucose control is essential.
"There is some evidence to suggest that poorly controlled diabetes also affects the functioning of the brain.
"However, far more research on a link between Alzheimer's and diabetes is needed before we can draw any conclusions."