Depletion of Food Stores in the Body Tissues During Starvation.
Even though the tissues preferentially use carbohydrate for energy over both fat and protein, the quantity of car-bohydrate normally stored in the entire body is only a few hundred grams (mainly glycogen in the liver and muscles), and it can supply the energy required for body functions for perhaps half a day. Therefore, except for the first few hours of starvation, the major effects are progressive depletion of tissue fat and protein. Because fat is the prime source of energy (100 times as much fat energy is stored in the normal person as carbohydrate energy), the rate of fat depletion continues unabated, as shown in Figure 71–3, until most of the fat stores in the body are gone.
Protein undergoes three phases of depletion: rapid depletion at first, then greatly slowed depletion, and, finally, rapid depletion again shortly before death. The initial rapid depletion is caused by the use of easily mobilized protein for direct metabolism or for conver-sion to glucose and then metabolism of glucose mainly by the brain. After the readily mobilized protein stores have been depleted during the early phase of starvation,
the remaining protein is not so easily removed. At this time, the rate of gluconeogenesis decreases to one third to one fifth its previous rate, and the rate of depletion of protein becomes greatly decreased. The lessened availability of glucose then initiates a series of events that leads to excessive fat utilization and conversion of some of the fat breakdown products to ketone bodies, producing the state of ketosis. The ketone bodies, like glucose, can cross the blood-brain barrier and can be used by the brain cells for energy. Therefore, about two thirds of the brain’s energy is now derived from these ketone bodies, principally from beta-hydroxybutyrate. This sequence of events leads to at least partial preservation of the protein stores of the body.
There finally comes a time when the fat stores are almost depleted, and the only remaining source of energy is protein. At that time, the protein stores once again enter a stage of rapid depletion. Because proteins are also essential for the maintenance of cellular func-tion, death ordinarily ensues when the proteins of the body have been depleted to about half their normal level.
Vitamin Deficiencies in Starvation. The stores of some of thevitamins, especially the water-soluble vitamins—the vitamin B group and vitamin C—do not last long during starvation. Consequently, after a week or more of starvation, mild vitamin deficiencies usually begin to appear, and after several weeks, severe vitamin defi-ciencies can occur. These deficiencies can add to the debility that leads to death.