The idea that Alzheimer's is a form of diabetic disease has been gaining currency in medical circles for almost ten years. The accumulated evidence is now so strong that many specialists are now comfortable referring to Alzheimer's as type 3 diabetes.

This shouldn't come as a surprise. Insulin doesn't merely signal the body's somatic cells to take up glucose; it also governs the brain's uptake of glucose. And glucose is what powers the brain. It's the brain's primary energy molecule.

We've known for some time that the brain itself makes a certain amount of insulin, and various parts of the brain are rich in insulin receptors. It's also well established that cognitive decline is correlated with both obesity and metabolic abnormalities involving insulin. (See, for example, the Whitehall II cohort study.) The connection between mental decline and diabetes was actually observed hundreds of years ago by physician Thomas Willis. (Also, in 1935, American psychiatrist William Claire Menninger posited the existence of "psychogenic diabetes" and described a "diabetic personality.") 

The smoking gun (arguably) for abnormalities in brain insulin as the precipitating factor for Alzheimer's Disease was the publication, in 2011, of the Hisayama Study. This study monitored 1017 initially disease-free patients for 15 years and found:

The age- and sex-adjusted incidence of all-cause dementia, Alzheimer disease (AD), and vascular dementia (VaD) were significantly higher in subjects with diabetes than in those with normal glucose tolerance.

American physician David Perlmutter, in his book Grain Brain (2013, Little, Brown), lays the blame squarely on diet, saying "Brain dysfunction starts in your daily bread." He lays out a detailed case (backed up by numerous references to the scientific literature) for eating more fats and cholesterol (yes, more cholesterol) and cutting gluten from your diet entirely, pointing to (among others) studies that have linked low cholesterol to cognitive impairment.  (The latter is, in fact, from the Framingham Heart Study.)

Not everyone is ready to believe gluten is the root of all evil. But there's no longer any question that obesity, diabetes, and Alzheimer's are now firmly linkedin the scientific literature. To fix one, we will very likely need to fix all three.

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