Yes, eat fat. It’s satiating. If you don’t have enough fat with your food you’ll be hungry after your meals, and feeling hungry is what helps to crash metabolism.
If you’re not yet fat adapted, you’ll want to eat fat because your body isn’t good at releasing it’s own fat; and your body hasn’t creates enough new mitochondria in each cell to take advantage of fat-burning metabolism (as opposed to carb burning metabolism). It can take weeks or months until your body creates enough new mitochondria; and it can take a couple of weeks until your fasting insulin levels go low enough that you stop storing fat and start releasing fat.
Dr. Fung warns against fasting for more than 7 days at a time without medical supervision.Your BMR is unaffected by fasting, it goes down in calorie restricted diets not fasting.If you really want to go hard core you could fast 7 days then eat 1 large meal then fast 7 more days etc. If you’re going to do that you need to be taking electrolytes and a daily multi vitamin Good luck