Your body of course doesn't use energy it doesn't have. But it can balance the equation by restricting the output - lowering muscle repair (so post-workout reco

Your body of course doesn't use energy it doesn't have. But it can balance the equation by restricting the output - lowering muscle repair (so post-workout recovery is slower), lowering brain supply (lowering concentration), lowering heating (so you fell cold), etc. Whether the body decides to use stored energy or lowers output (while signalling hunger) is mostly hormonally controlled, by insulin and glucagon - this is why insulin resistance is such a problem. Some research seems to show that portion restriction leads to lowered output while increasing time between meals (i.e. not snacking) lead to use of reserves, but I don't have a nice article to link.