The theory is that, to lose weight (or get into ketosis or autophagy), you switch from "fat storage" to "fat-burning" mode, and carbohydrate will "break your fast" and get you out of fat burning mode. The Glycaemic Index of a food is (compared to glucose) The speed at which a food stimulated insulin... which stops you burning fat, stores fat, and (by taking glucose out of your bloodstream) stimulate the ghrelin hormone, which makes you hungry.
Here is a video by Dr Eric Berg about "What will break my fast?"
Butter is very calorific - but it is "pure fat" so it will not spike insulin... but it cannot help weight loss!
Diabetics CAN fast