Everyone knows what hunger is. We speak of hunger in dramatic terms: "gnawing hunger," "ravenous hunger," "so hungry could eat a horse," "hunger pangs." When we are hungry, putting an end to it is one of life’s great pleasures. This pleasure is of course married to necessity. Hunger must be satisfied for food and drink are basic necessities of life. Without them we would die. Hunger and thirst are regularly recurring reminders of our needs.
What causes hunger?
Hunger is caused by the rhythmic contraction of the stomach walls. This contraction is going on all the time but it is felt when the stomach is empty. Each pang lasts from thirty seconds to three minutes and recurs in ten-or fifteen-minute cycles. Unsatisfied, the pangs gradually get worse for a day or two. At three or four days without food these pains weaken and eventually go away.
What happens when we eat?
Digestion begins as soon as we begin to chew. This action breaks the food up into digestible portions and mixes it with saliva. It helps to break down starches. Swallowed food moves to the stomach and rhythmic contractions of that organ mix it thoroughly with the gastric juices. The gastric juices contain two important components: pepsin, a proteinase that helps in the digestion of protein, and rennin, an enzyme that coagulates milk. The food in the stomach is reduced to a semiliquid state called chyme.
Food passes from the stomach into the small intestine where it is subjected to the action of bile, pancreatic juice, intestinal juice, and bacteria. These continue to break down the proteins, fats, starches, salts, and so on, by chemical action. The process of absorption takes place in the small intestine. Emulsified fats, in the form of a yellowish fluid called chyle, are taken up by the lymph vessels. Sugars, salts, and amino acids are absorbed directly by the small blood vessels. The remainder, supplemented by substances processed by the liver, is passed on to the large intestine. It is composed of three divisions, the cecum, colon, and rectum, and these compact the food residue into solid waste.
It takes about three hours for a full meal to move out of the stomach into the small intestine. The empty stomach then begins sending hunger signals, setting the stage for the process to begin anew.
The fats, sugars, salts, and amino acids from the first meal are carried by the bloodstream to other parts of the body. This process is called assimilation. Cells take what they need from the blood, allowing them to grow and to repair any damage. Any nourishment not taken up is deposited and stored as fat.
What nourishment is necessary?
Nourishment needs vary widely and depend on many different factors. Climate plays a part; if you live where it's cold your body must devote a certain number of calories just to keeping warm. Occupation makes a difference; doing hard physical labor uses up energy at a quicker rate than sitting at a desk. Age is important; a growing child needs more food, relative to body weight, than an adult. Size is also important; a large adult needs to eat more than the small one.
Adequate quantities of proteins, carbohydrates, and fats are necessary to sustain life. These three types of nourishment are normally replenished regularly. Everyone must also have minute amounts of minerals and vitamins (organic substances that regulate various metabolic processes). A diet lacking one or another of the vital ingredients is said to be deficient, and the condition (malnutrition) that occurs when a person has lived a long time on a deficient diet is called a deficiency disease (pellagra, scurvy, kwashiorkor, rickets, beriberi, etc.).
What happens when eating is stopped for long periods?
When the body is deprived of all food it begins to consume itself. The cells begin by using the extra nourishment stored in the body as fat. This is the goal of dieting to lose weight.
If food deprivation continues after the fat is gone, the cells begin to feed on the protein found in body tissues. Ultimately the body enters a condition known as starvation. A sever lack of nourishment or no nourishment at all for long periods of time leads to a person losing weight and flesh, becoming thinner and thinner, more and more emaciated, and will eventually appear to be, literally, "nothing but skin and bones." When starved, if the body loses one third to one half of its original body weight, death results. The onset of death is partially dependent on the weight of the victim before food deprivation sets it. Heavier people die of starvation at a somewhat greater weight than thin people do.
The worst cases of starvation are those that occur on a wide scale. When communities of people are starving, this is what we call famine. This used to be a widespread and unpredictable problem but one that was all to common. Bad weather, poor planning, and social problems could easily lead to famine. Centuries ago many societies were never far from this state and not much was need to tip things to famine conditions. Today the cause is almost always political in nature, commonly as a result of warfare.