Because you need protein. Protein in the main building block of our bodies. It's needed to repair our organs and muscles. Your body is digesting your muscles for protein. I know because I experienced the same thing. I also gained a lot of fat in my belly. Now that I added high quality low-fat protein to my raw diet, the fat is burning away, and I feel wonderful. A naturopath told me you can survive about six months without protein before you do damage to yourself.