Just need to input fewer calories than you use. A body at rest still uses calories to maintain basic life. So just keep them hydrated and add certain nutrients that aren’t in stored fat. Unfortunately your body can also use muscle as stored energy. If you are inactive, it will burn muscle more than you’d like. It’s expensive to keep around. So if you’re on starvation diet and not using them. No reason to keep them. The body burns them up. So in the end you’ll be lighter, but also extremely weak. The effort of rehabilitation may be as bad as trying to exercise and diet not in a coma.