Despite the vast array of different foods - vegetables, fruits, nuts, cereals, breads, cakes, cookies, pies, red meat, poultry, fish, milk, other dairy products and an infinite variety of foods made from them - the end result of digestion produces only a much smaller list of items that your body actually uses.

Carbohydrates

For the most part, all the carbohydrates are made from three single sugars (monosaccharides). These are glucose, fructose and galactose. These are the only sugars that can enter your circulation and be used for calories. Before carbohydrates can be utilized by your body, they must be digested.

Ordinary table sugar (sucrose) is digested into glucose and fructose. The carbohydrate in milk, lactose, is digested to glucose and galactose. The starches and glycogen are larger molecules composed entirely of glucose and must be digested to glucose. Cellulose, which helps make up the walls of cells in plants, is also a carbohydrate, but it can not be digested by man and provides bulk but no energy. An interesting exception in the group is honey. The sugar in pure honey, which has not had something else added, is really predigested sucrose and is present as the single sugars - glucose and fructose.

Proteins

Proteins are made up of amino acids which are chemically bonded together. But proteins cannot enter the circulation and must be digested into their component parts - amino acids, or very small chains of them. There are 20 amino acids, of which 8 are essential, meaning they can't be manufactured by the body from other amino acids.

Fats

Fats are a combination of fatty acids and glycerol, and most fat is stored as neutral fat or triglycerides. Three fatty acids are attached to one glycerol molecule. They cannot be absorbed either, unless they are digested, breaking down the fat molecule into fatty acids and a few monoglycerides (one fatty acid attached to glycerol).

Alcohol needs to be mentioned because it is a source of calories and in that sense has a food value, despite its harmful affects. It does not need digestion and is rapidly absorbed into the circulation. In addition to these main components of food and beverages are the vitamins, minerals - including sodium, potassium, calcium and iron - and water. Much of the composition of many food items is water and bulk.

Down the Digestive Tube

Despite its many complexities, the digestive system is basically a long tube which is really continuous with the outside of your body. Unless nutrients pass through the surface of the digestive tube, into the internal body, they will not have any effect on your energy or other aspects of body function. Un-digestible bulk is a good example. It is swallowed, makes the entire trip down the digestive tube, and is part of the stool, having served no purpose for your internal body. Its functions are limited to inside the digestive tube.

What Happens inside the Mouth & Esophagus?

Digestion actually begins with your brain. When you see something you would really like to eat, the brain may send a signal to start the saliva flowing. Saliva is formed by three glands, the parotid gland, a large flat gland over your cheeks and in front of your ears, the sub-maxillary, just under the edge of your jaw, and the sublingual gland, just under your tongue. Each of these empties through a duct into the mouth. They produce about 1,500ml of saliva a day.

Saliva contains an enzyme called ptyalin (alpha-amylase) which starts the breakdown of starch. Saliva is also a lubricant. It is important for dental hygiene and washes away bacteria. It also contains protein anti-bodies that destroy bacteria. Very little saliva is produced during sleep, so this anti-bacterial action is slowed or almost non-existent.

Since bacteria are the source of odor, they are one cause for morning breath. Taste also affects the flow of saliva. Sour tastes increase the flow of saliva, sometimes to 20 times the normal amount. Salivation is also stimulated by reflexes that originate in the stomach and upper small intestine. Any irritation may cause it. One irritation may be acidity or heartburn. That is one cause for night time excess salivation. The saliva helps to neutralize or dilute the acidity or to wash away an irritant.

Don't overlook the role of teeth in digestion. Early in this century, a food faddist, Horace Fletcher, recommended chewing each mouthful of your food as much as 150 times to lose weight. It became known as Fletcherizing. But the digestive enzymes in the digestive tube only act on the surfaces of food. Chewing your food well provides more surface for enzyme action. That is especially important for fruits and raw vegetables. These often have cellulose membranes around the digestive carbohydrate. It is necessary to physically breakdown this membrane for digestion.

What your Stomach Does

Your stomach is the source for digestive enzymes to start digestion and it mixes the contents of your stomach with the acid digestive juice containing those enzymes with your food. It is the only part of your digestive system that actually produces any significant amount of acid. True acid indigestion results from the hydrochloric acid produced by your stomach. The rest of your digestive tube neutralizes the acid.

The gastric glands in the stomach are the main source of enzymes and hydrochloric acid. They contain mucus cells that produce mucus and peptic cells (chief cells) that produce an important enzyme called pepsinogen. Still a third type of cells, parietal cells, produce both hydrochloric acid and intrinsic factor.

Intrinsic factor is an important substance you must have to absorb Vitamin B12 In your small intestine. Without that absorption you will develop pernicious anemia and have other problems in reproducing cells. Vitamin B12 is important in cell division to manufacture new red blood cells, new cells for your intestinal lining and all the other cells in your body that are constantly reproducing to regenerate your body.

Hydrochloric acid is necessary to convert pepsinogen to pepsin. Pepsin is the active enzyme that starts the digestion of the protein in your food. It does not complete it, but will breakdown protein into smaller units of several amino acids that can then be digested into the basic amino acids in your small intestine. Your stomach plays an important role in digesting protein, but a much smaller role in digesting carbohydrates and fats.

The other glands in your stomach, called the pyloric glands, secrete more mucus and also an important hormone called gastrin. Gastrin stimulates the cells in your gastric glands to produce some pepsinogen, but mainly cause a marked increase in hydrochloric acid, as much as 8 times normal.

There are some important considerations about acid stomach conditions and peptic ulcers related to the acid digestive juice produced by the stomach. Histamine, a derivative of an amino acid, is involved in stimulating acid secretions. And when its action is blocked with medicines such as Zantac, Pepcid and Tagamet, the stomach produces less acid digestive juice. These medicines block the action of gastrin hormone.

The other important point is how your brain and emotions can affect stomach acidity. The vagus nerve from your brain stimulates the cells that form both hydrochloric acid and pepsinogen, causing them to form vast quantities of both. It also stimulates the production of gastrin hormone. When the action of the vagus nerve is blocked, that decreases acid and pepsin secretion. Emotional stress may cause as much as 50ml of highly acid digestive juice to be formed each hour. That makes it easy to understand the importance of emotions and psychic influences on problems of acid indigestion, such as peptic ulcers. During an emotional crisis, it is not surprising that people will be 'sick to their stomach'.

Because of the structure of the stomach, food is not absorbed through its walls. The exception, if you consider it a food, is alcohol. It is fat soluble and an appreciable amount can be absorbed through the stomach into circulation. There is also an enzyme in the wall of the stomach - called gastric alcohol dehydrogenase - that will metabolize alcohol. The longer alcohol is retained in the stomach, the more will be metabolized and the less will be available to affect the brain and other organs.

Liquid, particularly water, will pass rapidly around the food bolus in the stomach and into the duodenum. But solids must be mixed with digestive juices and churned until they become a semi-solid slush, the chyme. Normally the stomach does not pass solids unless they are small, like a kernel of corn or a seed.

Emptying of the stomach is quite variable, depending on the distention of the stomach, hormones from the small intestine and various reflexes. The type of food makes a difference, but they must act through the hormones and reflexes to be described. In general, sweet liquids are rapidly emptied. Bulk must be liquefied. Carbohydrates cause the least delay, proteins the next, and fat may cause a long delay in stomach emptying.

That's why foods that contain lots of fat are not a good source for immediate energy. They cannot supply energy until digested and they are not digested sufficiently for absorption into the internal body until acted upon in the small intestine.

Your stomach normally produces about 1,500ml of digestive juices a day. Psychic influences on seeing, smelling or thinking about food may cause the stomach to pour out about one-tenth of the total fluid. The production of digestive fluid is also affected by reactions from the small intestine. When food enters the stomach, that excites the flow of most of the gastric juice.

Between eating, when the bulk of the digestive process is over (as during sleep), very little acid or pepsinogen is formed. That's why a person with acid indigestion may not note any symptoms on awakening and only starts having problems when the action of the stomach is stimulated by foods and beverages. The more food a person consumes, the more acid digestive juice is formed - an argument for small meals if you have an acidity problem. Also certain substances - partially digested proteins, low concentrations of alcohol and caffeine - cause release of the gastric hormone.