Many people don't like this but it all boils down to calories in calories out. All human bodies work pretty much the same way with some slight variances due to disease and genetics. Fasting works because it restricts your overall intake.
Many people don't get this, but calories aren't a very reliable measure of available energy. Measuring the caloric content of food is a pretty weird process, and is some kind of "one size fits all" measure of energy which we might be able to extract. The digestive system is not an engine, where you put in fuel, add oxygen, combust and use the energy. The digestive system is more like a mining operation which extracts coal from the bedrock that is your food, and which then throws it over the wall where billions of consumers burn the coal for energy.
That process isn't anywhere near 100% efficient. There is a huge amount of complexity in how food is digested and broken down into protein/sugars/fats. Some people can't even break down certain foods. Even more complexity in how those macronutrients are then dealt with.
Sure, in any system energy in == energy out. But without a way of reliably measuring how much available energy there really is in food for a specific body, there's really no way you can say "calories in, calories out".
It's a simple matter of measuring the calorie content of the food going in minus that of the waste going out. There are many things that contain calories that humans don't digest well but the things we tend to eat have pretty consistent calorie digestion to waste ratios, regardless of the eater. Beliefs to the contrary are mostly driven by the food fad industry.
