The nutrients furnish the body with materials for growth, and for repair of tissues worn out by use; they also furnish fuel substances from which the body obtains its heat and its energy. All three nutrients can serve as fuel, but the proteids alone can furnish materials for growth and repair of tissues. In order to be used by the body for any purpose, nutrients must first go through a series of complicated changes known as digestion,
which renders them soluble so that they can soak through the walls of the intestine.
THE DIGESTIVE PROCESS
Digestion begins in the mouth. There the food is crushed and its fibres separated by the teeth, it is moistened by the saliva, and substances in the saliva begin a chemical action upon the starch. Chewing should be sufficient to reduce the food to a soft mass well moistened with saliva. Slow eating is desirable, but the emphasis should be placed on thorough chewing. For instance, long intervals between bites are of no special benefit if mouthfuls of food are washed down by swallows of water.
After it has been swallowed, the food passes into the stomach and remains there for a variable length of time, while it undergoes further preparation for absorption. It is moved about by the contraction of the muscular walls of the stomach, so that it becomes mixed with the stomach juices and more thoroughly softened. Some digestion of proteids goes on in the stomach, and a little absorption through the walls.
Little by little the food is discharged from the stomach into the small intestine, and the most important part of digestion then begins. It is acted upon chemically by a fluid flowing into the
intestine from an organ called the pancreas; this pancreatic juice acts upon all three nutrients and is of great importance in the digestive process. The bile and other juices that flow into the intestine perform important functions also.
The food masses are moved along by rhythmic contractions of the intestine, and absorption goes on when the food has been so changed that it can soak through the intestinal walls into the blood and lymph vessels. The small intestine is about 20 feet long, and consequently affords a large surface for absorption, as does also the large intestine, into which the small intestine opens. The blood and lymph carry the digested food substances to all parts of the body, and thus the different tissues are provided with the materials they need for growth, repair, and energy. Excess of food substances may be stored as fat or expelled from the body.
As the blood and lymph go through the tissues they take from the tissues the refuse, or the part that remains after the fuel substances have been consumed. This refuse from the tissues may be likened to the ashes from a furnace; it is finally eliminated from the body through the kidneys and lungs, and to some extent through the skin and bowels. The part of the food that is not digested of course never soaks through the intestinal walls;
it merely passes through the small and large intestines and is finally expelled as feces or bowel movements. The characteristic odor of fecal matter results from the action of bacteria upon it while in the large intestine.