text: In short:  Protein is what your body is made of.  Carbohydrate is the main fuel that keeps it going.\n\nTo keep the car analogy going:  Protein is like the fiberglass, plastic and rubber in your car.  Carbohydrate is the gasoline (Fat is another major fuel source for the body).\n\nProteins are VERY complex.  They are the most complex chemical substances in the universe (that we know of).  They are massive molecules made of thousands of atoms all linked together to form atomic loops and chains and blobs.  These special shapes give the protein its special characteristics.  That's why one rigid protein called KERATIN forms the hair and fingernails of many animals including humans, while another goopy protein called COLLAGEN helps to keep your skin flexible.\n\nAll proteins are made of the same 20 building blocks called amino acids.  Just like the same 26 letters can spell out 50,000 different English words, these same 20 amino acids an spell out all of the different proteins required for life on earth.\n\nCARBOHYDRATE is a fancy made-up Greek word for all the various forms of SUGAR.\n\nTable sugar is a chemical called SUCROSE, while apples taste sweet because of FRUCTOSE and GLUCOSE.  Milk tastes good because of LACTOSE.  These are all carbohydrates:  Forms of sugar.\n\nSugar can take other unexpected forms also:  The stuff that makes wood tough is not a protein but actually a carbohydrate called CELLULOSE.  Don't worry, cellulose won't make you fat, because humans can't digest it.  Yes, carrots have cellulose, but that part of the carrot goes right through you.\n\nBut we love potatoes, which are essentially the stored sugar of a hardy plant that miraculously grew in Ireland and the Baltic region.  Potatoes don't taste sweet because the carbohydrate that makes up potatoes is STARCH, a complex carbohydrate.\n\nYour body CAN burn protein as fuel.  But, protein is very inefficient and your body is programmed to do this only if there are no sugar or fats available.  When your body starts burning protein for fuel, it starts with the major muscles, tearing them down for fuel like the crew of a ship wrecked in the arctic might start burning the planks of the vessel for warmth.\n\nThis is why starving people begin to have a "living skeleton" appearance (except for the distended belly, which is the result of another chemical process).\n\nMoral of the story:  In spite of what athletes and health nuts tell you, you cannot live off just protein or fiber.\n\nYou crave fat and sugar because your body favors those fuels.  If your car had a brain and had to find its own fuel, it would crave gasoline just like you crave burgers and donuts.\n\nOf course, we have more fat and sugar around than our ancestors could possibly imagine.  Our ancestors also got plenty of exercise, with all the hunting and skinning and chopping and chipping and the warfare.\n\nSince our bodies were designed for the hunter-gatherer lifestyle, and were not designed to sit behind a desk or on a couch eating insane amounts of sugar and fat -- we get sick from doing what feels right.