Answer: b, d About 15% of the total body weight is made up of proteins, about half of which are intracellular and half extracellular. In man and other animals, dietary protein is the source of most amino acids. Intestinal absorption is the only physiological pathway by which the body obtains exogenous amino acids. Digestion of ingested protein provides free amino acids that are absorbed by the small intestine and transported to the liver where they can be incorporated into new proteins or other biosynthetic products. Excess amino acids are degraded and their carbon skeleton is oxidized to produce energy or it is incorporated into glycogen or into free fatty acids. In addition to the metabolism of dietary amino acids, the existing proteins in the cell are continuously recycled, such that total protein turnover in the body is about 300 g/day. Vertebrates cannot reutilize nitrogen with 100% efficiency; therefore, obligatory nitrogen losses occur, mainly in the urine. Urinary nitrogen losses will diminish when individuals are fed a protein-free diet, but will never become 0 because of the body’s inability to completely reutilize nitrogen. In stressed patients, this ability to adapt to starvation is compromised such that proteolysis of body proteins continues at a substantial rate. This increases the amount of obligatory nitrogen losses which are accentuated by the catabolic disease states. This results in a negative nitrogen balance in which the amount of nitrogen taken in by the patient is exceeded by the amount of nitrogen lost in the urine, stool, skin, wounds, and fistula drainage