Alzheimer's is sometimes called "type 3 diabetes." If the body stops responding to insulin signals to pull glucose out of the serum to use for energy (insulin resistance), then the brain might chronically get less energy. By weight, the brain is the organ that uses the most glucose of any other organ. The brain is responsible for about 20% of your body's total glucose use, despite being only 2% of your body's weight.

Intermittent fasting usually kicks you into ketosis. Ketosis is when your body runs out of glucose and it metabolizes fat in the liver to use for energy, including the brain, which starts getting 66% of its energy from fat. You're on HN so you probably already know that. It also changes the krebs cycle, and eventually it changes how your body processes ketones themselves, but the important switch is in your brain.

Nutritional ketosis is a form of "fed" ketosis, where you mainly don't eat glucose, and eat more fat, and your body enters a similar state as fasting, except you don't have to be hungry. Nutritional ketosis is also distinguished from "starvation ketosis" (like from fasting) by the quantity of ketone bodies in the serum.

Alzheimer's patients on a ketotic diet show improvement (https://pubmed.ncbi.nlm.nih.gov/33622392/)

Could a nutritional ketosis diet be preventative against Alzheimers?

I sometimes hear stories of friends who eat generally "healthy," and aren't overweight, shocked to learn they are pre-diabetic.

This is the limit of my knowledge, and I know this is an incomplete picture. From my limited view, this is why I strive to be in nutritional ketosis.