A Peak at the Digestive Process

Digestion is the process that converts the foods we eat into the individual nutrients our bodies require. It involves breaking down food and nutrients into successively smaller pieces until the nutrients are in a chemical form that the body can absorb.

Water, minerals, and vitamins require minimal processing. Once they are released from foods and beverages during digestion, they can be absorbed directly into the body through the intestinal lining. The energy-containing nutrients carbohydrate, fat, and protein, however, are too big to be absorbed in their original form. After being released from foods, they must first be broken down into their molecular building blocks before they can be absorbed. For them to be absorbed through the intestinal lining, carbohydrates must be broken down into simple sugars, fats and oils converted to fatty acids, and proteins broken into individual amino acids or tiny peptides (short chains of 2 to 4 amino acids linked together).

How is Food Digested?

Many people mistakenly believe that food is digested in the stomach. Digestion actually occurs in the GI tract. It is a continuous tube that includes the stomach and runs through the core of the body from the mouth to the anus.

Digestion requires the integrated action of three major body systems:

Physical Structures-muscles and a specialized mucosal lining make up the GI tract and perform the mechanical aspects of digestion; powerful contractions of the muscles lining the GI tract move food and food residue from the mouth to the anus and help mix the food partials with the digestive chemicals. Villi, fingerlike projections on the surface of the mucosal lining, help stir the contents of the intestine and absorb nutrients. Strong circular muscles called sphincters encircle the top and bottom of the stomach and the small and large intestines. By maintaining constriction, they prevent the backward flow of food and food remnants from one part of the GI tract to another. These sphincters relax briefly to allow food and food residue to pass through the intestine and eventually out of the anus.

The anus is an example of a sphincter.

Digestive Chemicals- are produced by the body to break down food. The mouth makes saliva to moisten and soften food, the stomach makes hydrochloric acid that helps breakdown food particles, and the pancreas makes bicarbonate, which neutralizes the acid in the liquefied food entering the small intestine. The pancreas also makes specific digestive enzymes: saccharidases to digest carbohydrates, proteases to digest proteins, and lipases to digest fats and oils. The liver makes bile and sends it to the gall bladder where it is stored until needed. Bile allows fats to mix (emulsify) with the body’s water-based digestive chemicals.

Regulatory Factors- including hormones and nerve impulses control the rate of digestion and its various steps.

The Basic Steps of Digestion and Absorption

• Food enters the GI tract through the mouth where it is moistened by saliva and softened and broken into smaller pieces by chewing (mastication). An enzyme called amylase, which is present in saliva, begins the breakdown of carbohydrates.
• Swallowing causes the chewed food to travel down the esophagus to the stomach.
• The stomach mixes, liquefies, and stores food. Once in the stomach, food is mixed with hydrochloric acid and other gastric juices produced by the stomach lining and liquefied to form a substance called chyme.
• Chyme slowly passes from the stomach into the upper portion of the small intestine. Once in the small intestine, the chyme is broken down into individual nutrients by digestive chemicals produced in the pancreas and bile produced by the liver. The nutrients are then absorbed through the walls of the small intestine into the blood stream. Once in the blood stream, the nutrients circulate throughout the body and can be absorbed and utilized by various cells.
• Food residue from the small intestine passes into the large intestines where water and a group of minerals known as electrolytes are reabsorbed into the blood stream. The remaining solid residue is stored in the rectum (the lower portion of the large intestine) until it is eliminated through the anus.

Many complex physical and chemical activities must occur in the proper sequence to allow the digestive process to take place with reasonable efficiency and speed. Diet composition, hormones, enzymes, physical activity, emotions, illness, and medications impact the digestive process.