People who eat diets high in sugar could be at an increased risk of developing Alzheimer's disease, a new study has found.
For the first time scientists have established a "tipping point" link between blood sugar glucose and Alzheimer's disease, prompting warnings to reduce sugar intake.
While it is not known what causes Alzheimer's, the brains of people who suffer from the disease contain abnormal deposits of protein (amyloid plaques) which compact together to form plaques and tangles in the brain.
Researchers from the University of Bath found excess glucose damages a vital enzyme which should prevent against the build up of plaque in the brain.
Dr Omar Kassaar, from the University of Bath, warned: "Excess sugar is well known to be bad for us when it comes to diabetes and obesity, but this potential link with Alzheimer's disease is yet another reason that we should be controlling our sugar intake in our diets."
Abnormally high blood sugar levels, or hyperglycaemia, is a well-known characteristic of diabetes and obesity.
Diabetes patients have an increased risk of developing Alzheimer's, where protein deposits form in the brain, creating plaques and tangles.
It is already known that glucose and its breakdown products can damage proteins in cells through a reaction called glycation in which a sugar molecule bonds to a protein or fat molecule and stops it functioning properly.
However, researchers have now found that in the early stages of Alzheimer's, glycation damages an enzyme called MIF (macrophage migration inhibitory factor).
MIF, which plays a role in immune response and insulin regulation (needed to control blood sugar levels), is involved in the response of brain cells called glia to the build up of abnormal proteins in the brain during the disease.
Scientists believe that the inhibition and reduction of MIF activity caused by glycation could be the "tipping point" in disease progression.
It appears that as Alzheimer's progresses, the glycation of these enzymes increases.
Professor Jean van den Elsen, from the University of Bath's department of biology and biochemistry, said: "We've shown that this enzyme is already modified by glucose in the brains of individuals at the early stages of Alzheimer's disease.
"We are now investigating if we can detect similar changes in blood.
"Normally MIF would be part of the immune response to the build-up of abnormal proteins in the brain, and we think that because sugar damage reduces some MIF functions and completely inhibits others that this could be a tipping point that allows Alzheimer's to develop."
Dr Rob Williams, also from the department of biology and biochemistry, added: "Knowing this will be vital to developing a chronology of how Alzheimer's progresses and we hope will help us identify those at risk of Alzheimer's and lead to new treatments or ways to prevent the disease."
In the study, scientists from the university worked with colleagues at the Wolfson Centre for Age Related Diseases, King's College London, and is published in the journal Scientific Reports.
Clare Walton, research manager at the Alzheimer's Society, said: "We know that diabetes can double a person's risk of developing dementia but we still don't really understand how the two conditions are linked - this study offers a vital clue.
"The researchers have found a specific effect of high blood glucose on an enzyme in the brains of people with Alzheimer's, providing a plausible biological mechanism connecting the two conditions.
"With diabetes on the rise, a better understanding of how it affects brain cells can help us to find ways to help people with diabetes manage their risk of dementia."
Globally, around 50 million people have Alzheimer's Disease and the figure is predicted to rise more than 125 million by 2050.