I hope this is actually not new news to you, but an article in this week's The Atlantic lays out the utterly clear and repeatedly confirmed connection between eating too much sugar (it's less than one might think!), keeping too much of it in our blood (happens naturally as we age), and then cluttering up our brain irreparably and developing Alzheimer's.
Titled "The Startling Link Between Sugar and Alzheimer's" author Olga Khazan has done an excellent job reviewing the recent science in a clear and decisive voice.
From new research, it's clear that the path from sugar to Alzheimer's might bypass the intermediate step of type 2 diabetes, meaning you can be damaging your cognitive abilities with just a high blood sugar, even without a diagnosis of actual type 2 diabetes.
People who DO have a diagnosis of type 2 diabetes have double the risk of Alzheimer's compared to those who don't and it's actually the high levels of insulin that cause the problem, in both type 1 and type 2 diabetes. If you're type 1, you're eating carbs and "covering" them with insulin, causing yourself a problem with high levels, and if you're type 2, you're making your own high levels of insulin. The problem is that the enzyme that cleans up amyloid beta (too much gives us Alzheimer's, but we all make some of it as we age) has a first job of cleaning up extra insulin that accumulates in the brain. You can call after it to clean up your amyloid, but if you've distracted it with a lot of insulin, forget it, your amyloid is there to stay.
There are other theories, equally well-discussed in this article: sugars can also damage fragile blood vessels, which could be a cause as well of cognitive impairment.
(Inspirational for type 1 diabetics are those who are managing their condition with a low carb diet: See the New York Times article with a discussion of a type 1 diabetic markedly reducing his insulin need by doing a Whole30. Hasn't quite made it sustainable yet, but I have confidence he'll get there, just like the surgeon he quotes in his article. A surgeon who doesn't have time for blood sugar fluctuations, she manages her diabetes with a low carb diet!)