A brilliant but unfortunately-named article, since people can develop elevated blood sugars and insulin from bad stress management alone, not just diet.
Nonetheless, understanding receptor mechanisms in the body will lead someone to get a good handle on this link, regardless of epidemiological proofs. Think of it this way: the receptors which respond to alcohol become numb as you introduce progressively more alcohol into the body. The receptors which respond to caffeine become increasingly deaf to the signal of caffeine as you repeatedly bring in more and more. This is called tolerance/dependency/etc. The cells inside your brain receive energy via different pathways, one of which is insulin knocking on the door to let sugar in. If you repeatedly, persistently, daily, increasingly pound on the door of insulin receptors in the brain, you WILL deafen and deaden the receptors which bring energy into those brain cells. That disrepair IS neural degeneration. How much? However much you decide to do throughout your life.
There was a case study several years ago about a man with Alzheimer's reversing his affliction with the introduction of coconut oil into his diet. A lot of people picked up on this tactic with varying degrees of success or failure. Once again, this is where a little bit of knowledge is dangerous. To re-upregulate receptors will take more than bringing in some ketones from coconut oil. It will require heavy and intense (and INCREASINGLY heavier and more intense) exercise to create specific enzymes which can allow the brain cells to grow again, clear cell waste again, reverse the degeneration, etc. It will require starving the already-chemical-dependent brain cells of their drug - sugar - for a long enough period of time to reset the down-regulation. It will require a fair bit of fasting efforts so as to amplify autophagy and clear the "rust" on aged and damaged cells. It will require commitment to solid sleep and reliable sleep and wake times. It will require some serious processes to reverse the accumulated damage which we call Alzheimer's or any other degenerative affliction.
Or... today, before you have Alzheimer's or dementia or some other neural degenerative disease, you can start introducing those things in modest measure so as to lower or eliminate your risk in the first place.