I'm not a biologist or biochemist, but I am a chemist, and really, the human body is just one giant organic chemistry lab. Simply put, most of the reactions in our body happen based on the shape/composition of the reacting species. So if you change the shape/composition (oxidize it), it's not going to behave the same way in the reactions. Or you can just think of it like building a house with really cheap and crappy building materials. If they break down way more quickly than the good stuff, your house is going to fall apart really quickly.