The brain is another source of insulin say the researchers

Unlike other types of diabetes, the form - dubbed type 3 by the US Brown Medical School team - is not thought to affect blood sugar.

Type 3 affects brain insulin levels, and appears to be linked with Alzheimer's disease.

The team's research appears in the Journal of Alzheimer's Disease.

It raises the possibility of a type 3 diabetes Study author Dr Suzanne de la Monte Diabetes linked to Alzheimer's

Type 1 and type 2 diabetes occur when the body is unable to produce or use insulin from the pancreas.

The so-called 'type 3' diabetes refers to lower than normal levels of newly discovered brain insulin, which appears to be associated with Alzheimer's disease in some way.

Scientists have known for some time that people with diabetes have an increased risk of Alzheimer's disease - by up to 65%.

They have also discovered that many type 2 diabetics have deposits of a protein in their pancreas which is similar to the protein deposits found in the brain tissue of people with Alzheimer's disease.

Research has been going on to find out what links the two conditions.

Dr Suzanne de la Monte and colleagues now believe it is down to what they are calling type 3 diabetes.

By looking at rodents and post-mortem brain tissue from people with Alzheimer's disease they have found that insulin and its related proteins are actually produced in the brain, and that reduced levels of both are linked to Alzheimer's disease.

'Brain' insulin

They say this insulin and its related growth factors and receptors in the brain are vital for the survival of brain cells.

If they are not produced at normal levels, the cells die.

In the case of Alzheimer's, the cells that die are located in the part of the brain involved with memory, called the hippocampus.

Dr de la Monte, who is a neuropathologist at Rhode Island Hospital, said: "What we found is that insulin is not just produced in the pancreas, but also in the brain.

I suspect that the brain insulin itself is not very significant Professor Greg Cole of the University of California, Los Angeles

"These abnormalities do not correspond to type 1 or type 2 diabetes, but reflect a different and more complex disease process that originates in the central nervous system."

Not only does this opens the way for targeted treatment to the brain and changes the way we view Alzheimer's disease, "it raises the possibility of a type 3 diabetes", she said.

"The implication is that treating type 1 or type 2 diabetes may have no impact on Alzheimer's disease. We believe that therapeutic agents need to be designed that specifically influence the actions of insulin in the brain," she said.

Cathy Moulton, care advisor at Diabetes UK, said: "So far studies on a potential link between Alzheimer's and diabetes have come up with inconclusive results.

'More research'

"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 firm conclusions."

A spokeswoman from the Alzheimer's Research Trust said: "Researchers have believed for some time that the role of insulin and its growth factors are very important in Alzheimer's disease.

"Scientists have suggested that the link could be down to molecular changes affected by insulin.

"Work funded by the Alzheimer's Research Trust is currently investigating the way insulin acts on the brain and should improve our understanding of Alzheimer's and hopefully lead to way to new treatments.

Professor Greg Cole, from the University of California Los Angeles' Alzheimer's Disease Research Center, said: "This is a new finding. It is interesting that the brain makes very low levels of insulin.

"But its significance is unclear. The levels are so low that they have not been detected with less sensitive methods. I don't think we can say they are high enough to matter.

"I suspect that the brain insulin itself is not very significant and neither is its deficit in Alzheimer's disease and, therefore, I wouldn't call it type 3 diabetes."

But he said there was evidence that diabetes and Alzheimer's are linked in some way.