Today it is really hard to eat not enough proteins during longer period.
Our muscles are build of proteins. In theory if you don't have enough proteins to build cells, your regeneration will take for ever. Your body will try do use proteins from other cells if possible.
Note that you are not eating pure carbs, nor pure fat. There are proteins there. Our body, when forced to the limits, is amazing when it comes to using given sources. So normally you can take, for instance half of proteins from food, since no more is needed. When you limit amount of them, by half... it not necessarily means that you are giving not enough. Despite exact numbers.