Nutrient Utilization:
Digestion, Absorption, and Metabolism
Ingestion of food initiates a multitude of physical and chemical processes that allow the body to utilize food nutrients for maintenance of body temperature and functioning of its vital organ systems, for making new tissue for growth or repair, and for performing work. These processes are generally kno"'lI1 as digestion, absorption, and metabolism of nutrients.
Although it is customary to study separately each of these phases of nutrient utilization, one should understand that they all go on simultaneously and are highly interdependent; on the level of the individual molecule, however, they are sequential for a specific location and time. To illustrate, while some molecules of starch are being degraded by the digestive enzymes in the intestinal lumen, the digestion products (glucose) of other molecules ingested in the same meal are being absorbed into the epithelial cells of the mucosa. Still others are being metabolized in the mucosal cell to provide energy for continuous absorption, and some have already reached the liver and are being used to replenish the glycogen stores of this organ. Yet others may have been removed from the blood by other'tissues and are being oxidized to provide energy for work 'in the muscle or converted to fat in the adipose tissue.
