The point of keto is to force your body to deplete its glucose (and the stored form, glycogen) so it will have to use body fat as a fuel source. It's capable of making ketone bodies from your fat, which can replace glucose as an energy-storing molecule if necessary. To do that, you have to break apart fat molecules thus 'burning' the fat off. But here's the thing: your body really really doesn't want to run out of glucose. No glucose means starvation as far as it's concerned—even if you're not feeling hungry, your body is still missing one of its key macronutrients. And when you're (nutritionally) starving, your body will start to break down protein just to get those sweet, sweet carbs. Of course, you have a source of protein in your body already: your own muscles. "When in starvation mode, your body breaks down muscle in your body," says Giancoli. "Ketosis is a way of trying to preserve that protein. It's not ideal, but it's your body's way of saving you."