in humans, 2–3 percent of body protein is broken down and rebuilt every day, during which some of the building blocks for protein are lost. humans therefore need to frequently replenish their supply of carbon by absorbing co2 from the air. retain all the nitrogen-15 contained in their food need to eat food rich in fatty acids to rebuild their proteins. use food as a source of amino acids. make all of their required amino acids from the fatty acids stored in their liver.