People with high blood sugar may have an increasing risk of Alzheimer’s than those with normal blood sugar—whether or not their blood-sugar level technically made them diabetic — scientists said on the basis of following the health status of over 5,189 people for over 10 years.

In other words, the higher the blood sugar, the faster the cognitive decline, said their longitudinal study which has been published in the journal Diabetologia.

Type 1 diabetes is an autoimmune disease needing intake of insulin injection, and type 2 diabetes is a chronic disease caused by diet.

“Dementia is one of the most prevalent psychiatric conditions strongly associated with poor quality of later life,” said the lead author, Wuxiang Xie at Imperial College London.

“Currently, dementia is not curable, which makes it very important to study risk factors,” Melissa Schilling, a professor at New York University, performed her own review of studies connecting diabetes to Alzheimer’s in 2016.

She sought to reconcile two confusing trends. People who have type 2 diabetes are about twice as likely to get Alzheimer’s, and people who have diabetes and are treated with insulin are also more likely to get Alzheimer’s, suggesting elevated insulin plays a role in Alzheimer’s.

In fact, many studies have found that elevated insulin, or “hyperinsulinemia,” significantly increases your risk of Alzheimer’s. On the other hand, people with type 1 diabetes, who don’t make insulin at all, are also thought to have a higher risk of Alzheimer’s.

How could these both be true? Schilling posits this happens because of the insulin-degrading enzyme, a product of insulin that breaks down both insulin and amyloid proteins in the brain—the same proteins that clump up and lead to Alzheimer’s disease.

People who don’t have enough insulin, like those whose bodies’ ability to produce insulin has been tapped out by diabetes, aren’t going to make enough of this enzyme to break up those brain clumps.

Meanwhile, in people who use insulin to treat their diabetes and end up with a surplus of insulin, most of this enzyme gets used up breaking that insulin down, leaving not enough enzyme to address those amyloid brain clumps.

According to Schilling, this can happen even in people who don’t have diabetes yet—who are in a state known as “prediabetes.”

Diabetes can also weaken the blood vessels, which increases the likelihood that you’ll have mini-strokes in the brain, causing various forms of dementia.

A high intake of simple sugars can make cells, including those in the brain, insulin resistant, which could cause the brain cells to die.

The extra fat in obese people releases cytokines, or inflammatory proteins that can also contribute to cognitive deterioration, doctors say.

Diabetes may also increase the risk of developing mild cognitive impairment (MCI), a condition in which people experience more thinking (cognitive) and memory problems than are usually present in normal aging.