theatlantic | In recent years, Alzheimer’s disease has occasionally been referred to as “type 3” diabetes, though that moniker doesn’t make much sense. After all, though they share a problem with insulin, type 1 diabetes is an autoimmune disease, and type 2 diabetes is a chronic disease caused by diet. Instead of another type of diabetes, it’s increasingly looking like Alzheimer’s is another potential side effect of a sugary, Western-style diet.
A longitudinal study, published Thursday in the journal Diabetologia, followed 5,189 people over 10 years and found that people with high blood sugar had a faster rate of cognitive decline than those with normal blood sugar—whether or not their blood-sugar level technically made them diabetic. In other words, the higher the blood sugar, the faster the cognitive decline.
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.
Schilling is not primarily a medical researcher; she’s just interested in the topic. But Rosebud Roberts, a professor of epidemiology and neurology at the Mayo Clinic, agreed with her interpretation.