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.
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".
Not true. Calories are a very crude metric of metabolic energy.
Wood (and anything else that burns) has a bunch of calories, yet we cannot process it as energy.