There is a surprising amount of upkeep that can be forgone if needed. Muscle, nail and hair growth, reproductive functions like menstruation or sperm production, reducing energy use in the brain (malnourished people often report brain fog). I've also read about slower healing and being more prone to infections, so there's probably some general system overhead that is cut.
Of course, this is no way to live long term, but in a starvation situation it makes sense to reduce energy demand to the absolute minimum