Also with weight loss during a fast the bone mass will decrease, but this is not just a loss of bone density, but a restructuring of the skeletal system in relation to the body mass it supports. The skeleton is not just a lump of calcium but an organ in itself providing a specific function and adjusting to the changing needs put on it by the body. It's capable of many changes and adjustments that you wouldn't expect from something so hard and solid. It's alive!