When your body is faced with starvation it must deal with several dilemmas. The first priority is to preserve glucose-dependent tissue: Choose...intestinal cellsthe lungsthe heartred blood cells, brain cells, and the central nervous system. After carbohydrate reserves are depleted within a few hours, circulating Choose...carbon atomshydrogen ionsamino acidsATPsare used to make glucose and Choose...ketone bodiesATPwaterfatty acids. The second priority is to maintain muscle mass. Because little glucose can be made from triglycerides stored in the adipose tissue, the brain cells adapt so that they can use Choose...glycogentriglyceridesketone bodiesNADH for fuel.

When glucose levels drop to baseline levels after several hours of fasting, the hormone Choose...glucagonlipasepyruvatepyrophosphate stimulates the breakdown of liver glycogen to glucose. During the first few days of starvation glucogenic amino acids, especially alanine, furnish 90% of the brain’s glucose supply and Choose...ketone bodiesglycerolfatty acidsamino acids provide(s) the remaining 10%. Eventually protein breakdown must slow down or the body will not survive for more than three weeks without food. Fat catabolism rates double to supply fatty acids for fuel and glycerol for glucose. The average person has Choose...one weektwo weeksthree weeksone month of fat stores, then the body must turn back to protein for energy. Protein breakdown slows drastically and gluconeogenesis drops by two-thirds or more. A starving person can survive until Choose...25%40%50%75% of his or her proteins are broken down.