Alzheimer’s disease has occasionally been referred to as “type 3 diabetes even though the link between sugar, insulin and memory loss was not clearly understood. The only loose association may be that type 1 diabetes is an autoimmune disease, type 2 diabetes is a chronic disease caused by diet and Alzheimer’s may be both–it has an underlying inflammation and may be a side effect of a sugary, Western-style diet. But going by recent studies, that link may not be loose after all.
A longitudinal study, published last month, 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. 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 the risk of Alzheimer’s.
What’s even more intriguing is that people with type 1 diabetes, who don’t make insulin at all, are also thought to have a higher risk of Alzheimer’s. Researchers believe that this happens because of IDE-insulin-degrading enzyme, which 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 in type 1 diabetes, aren’t going to make enough of IDE to break up those brain clumps. Meanwhile, in people who use insulin to treat their diabetes and end up with excess of insulin, most of this enzyme gets used up breaking that insulin down, leaving insufficient amounts to clear those amyloid brain clumps.
According to researchers, the above phenomenon can happen even in people who don’t have diabetes yet—who are in a state known as “prediabetes.” Researchers segregated nearly 1,000 people into four groups based on how much of their diet came from carbohydrates. The group that ate the most carbs had an 80 percent higher chance of developing mild cognitive impairment (MCI)—an early stage of Alzheimer’s—than those who ate the smallest amount of carbs. Thus, the suggestion that a Mediterranean diet, for example, may be good for brain health.
Diabetes can also weaken blood vessels and reduce cerebral blood flow which increases the likelihood of ministrokes 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. Meanwhile, eating too much in general can cause obesity. The extra fat in obese people releases cytokines, or inflammatory proteins that can also contribute to cognitive deterioration. Thus, what we eat is a big factor in managing our memories.
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Fanfan Zheng, Li Yan, Zhenchun Yang, Baoliang Zhong, Wuxiang Xie. HbA1c, diabetes and cognitive decline: the English Longitudinal Study of Ageing. Diabetologia, April 2018, 61 (4) 839–848.