When fasting or eating a very-low-calorie diet for prolonged periods, the body will obtain fuel from its energy stores. one of the last reserves the body will use is?

One of the last reserves the body will use is protein in muscle and organ tissue.
Given that the body is receiving so much less protein from various fatty foods, it will have to break into its reserves of fats and proteins found within itself. It will start from the less important stuff, until it reaches protein in muscle and organ tissue, but you shouldn't fast that much so as to prevent that from happening.