Mark Bittman, The New York Times, September 25, 2012
Just in case you need another reason to cut back on junk food, it now turns out that Alzheimer’s could well be a form of dietinduced diabetes. That’s the bad news. The good news is that laying off soda, doughnuts, processed meats and fries could allow you to keep your mind intact until your body fails you.
We used to think there were two types of diabetes: the type you’re born with (Type 1) and the type you “get.” That’s called Type 2, and was called “adult onset” until it started ravaging kids. Type 2 is brought about by a combination of factors, including overeating, American-style.
The idea that Alzheimer’s might be Type 3 diabetes has been around since 2005, but the connection between poor diet and Alzheimer’s is becoming more convincing, as summarized in a cover story in New Scientist entitled “Food for Thought: What You Eat May Be Killing Your Brain.” (The graphic—a chocolate brain with a huge piece missing—is creepy. But for the record: chocolate is not the enemy.)
The studies  are increasingly persuasive, and unsurprising when you understand the role of insulin in the body. So, a brief lesson.
We all need insulin: in non-diabetics, it’s released to help cells take in the blood sugar (glucose) they need for energy. But the cells can hold only so much; excess sugar is first stored as glycogen, and—when there’s enough of that—as fat. (Blood sugar doesn’t come only from sugar, but from carbohydrates of all kinds; easily digested carbohydrates flood the bloodstream with sugar.) Insulin not only keeps the blood vessels that supply the brain healthy, it also encourages the brain’s neurons to absorb glucose, and allows those neurons to change and become stronger. Low insulin levels in the brain mean reduced brain function.
Type 1 diabetes, in which the immune system destroys insulin-producing cells in the pancreas, accounts for about 10 percent of all cases. Type 2 diabetes is chronic or environmental, and it’s especially prevalent in populations that overconsume hyper-processed foods, like ours. It’s tragically, increasingly common—about a third of Americans have diabetes or pre-diabetes—and treatable but incurable. It causes your cells to fail to retrieve glucose from the blood, either because your pancreas isn’t producing enough insulin or the body’s cells ignore that insulin. (That’s “insulin resistance”; stand by.)
Put as simply as possible (in case your eyes glaze over as quickly as mine when it comes to high school biology), insulin “calls” your cells, asking them to take glucose from the bloodstream: “Yoo-hoo. Pick this stuff up!”
When the insulin calls altogether too often—as it does when you drink sugar-sweetened beverages and repeatedly eat junk food the cells are overwhelmed, and say, “Leave me alone.” They become resistant. This makes the insulin even more insistent and, to make matters worse, all those elevated insulin levels are bad for your blood vessels.