One person eats nothing but vegetables, fruits, and nuts; another, nothing but meat, milk, and potatoes. How is it that both people wind up with essentially the same body composition? It all comes down to the body rendering food—whatever it is to start with—into the basic units that make up carbohydrate, fat, and protein. The body absorbs these units and builds its tissues from them.
To digest food, five different body organs secrete digestive juices: the salivary glands, the stomach, the small intestine, the liver (via the gallbladder), and the pancreas. These secretions enter the GI tract at various points along the way, bringing an abundance of water and a variety of enzymes. Each of the juices has a turn to mix with the food and promote its breakdown into small units that can be absorbed into the body.
Digestion in the Mouth
Digestion of carbohydrates begins in the mouth, where the salivary glands secrete saliva, which contains water, salts, and enzymes (including salivary amylase) that break the bonds in the chains of starch. Saliva also protects the tooth surfaces and linings of the mouth, esophagus, and stomach from attack by molecules that might harm them. The enzymes in the mouth do not, for the most part, affect the fats, proteins, vitamins, minerals, and fiber that are present in the foods people eat.
Digestion in the Stomach
Gastric juice, secreted by the gastric glands, is composed of water, enzymes, and hydrochloric acid. The acid is so strong that it burns the throat if it happens to reflux into the upper esophagus and mouth. The stomach’s strong acidity prevents bacterial growth and kills most bacteria that enter the body along with food. You might expect that the stomach’s acid would attack the stomach itself, but the cells of the stomach wall secrete mucus, a thick, slimy, white polysaccharide that coats and protects the stomach’s lining.
The major digestive event in the stomach is the initial breakdown of proteins. Other than being crushed and mixed with saliva in the mouth, nothing happens to protein until it comes in contact with the gastric juices in the stomach. There, the acid helps to uncoil (denature) the protein’s tangled strands so that the stomach enzymes can attack the bonds. Both the enzyme pepsin and the stomach acid itself act as catalysts in the process. Minor events are the digestion of some fat by gastric lipase, the digestion of sucrose (to a very small extent) by the stomach acid, and the attachment of a protein carrier to vitamin B12.
The stomach enzymes work most efficiently in the stomach’s strong acid, but salivary amylase, which is swallowed with food, does not work in acid this strong. Consequently, the digestion of starch gradually ceases as the acid penetrates the bolus. In fact, salivary amylase becomes just another protein to be digested. The amino acids in amylase end up being absorbed and recycled into other body proteins.
Digestion in the Small and Large Intestines
By the time food leaves the stomach, digestion of all three energy-yielding nutrients has begun, but the process gains momentum in the small intestine. There, the pancreas and the liver contribute additional digestive juices through the duct leading into the duodenum, and the small intestine adds intestinal juice. These juices contain digestive enzymes, bicarbonate, and bile.
Pancreatic juice contributes to enzymes that digest fats, proteins, and carbohydrates. Glands in the intestinal wall also secrete digestive enzymes.
Pancreatic juice also contains sodium bicarbonate, which neutralizes the acidic chyme as it enters the small intestine. From this point on, the digestive tract contents are neutral or slightly alkaline. The enzymes of both the intestine and the pancreas work best in this environment.
Bile is secreted continuously by the liver and is concentrated and stored in the gallbladder. The gallbladder squirts bile into the duodenum whenever fat arrives there. Bile is not an enzyme but an emulsifier that brings fats into suspension in water. After the fats are emulsified, enzymes can work on them, and they can be absorbed. Thanks to all these secretions, all three energy-yielding nutrients are digested in the small intestine.
The Rate of Digestion
The rate of digestion of the energy nutrients depends on the meal contents. If the meal is high in simple sugars, digestion proceeds fairly rapidly. On the other hand, if it is rich in fat, digestion is slower.
The intestines contain bacteria that produce a variety of vitamins, including biotin and vitamin K (although bacteria alone cannot meet the need for these vitamins). The GI bacteria also protect people from infections. Provided that the normal intestinal flora are thriving, infectious bacteria have a hard time getting established and launching an attack on the system.
In addition, the small intestine and the entire GI tract manufacture and maintain a strong arsenal of defenses against foreign invaders. Several different types of defending cells are present in the GI tract, and they confer specific immunity against intestinal diseases.
The Final Stage
The story of how food is broken down into nutrients that can be absorbed is now nearly complete. The three energy-yielding nutrients—carbohydrate, fat, and protein—are disassembled into basic building blocks before they are absorbed. Most of the other nutrients—vitamins, minerals, and water—are absorbed as they are. Undigested residues, such as some fibers, are not absorbed but continue through the digestive tract as a semisolid mass that stimulates the tract’s muscles, helping them remain strong and able to perform peristalsis efficiently.
Fiber also retains water, keeping the stools soft, and carries some bile acids, sterols, and fat out of the body. Drink- ing plenty of water in conjunction with eating foods high in fiber supplies fluid for the fiber to take up. This is the basis for the recommendation to drink water and eat fiber-rich foods to relieve constipation.
The process of absorbing the nutrients into the body is discussed in another article. For the moment, let us assume that the digested nutrients simply disappear from the GI tract as they are ready. Virtually all nutrients are gone by the time the contents of the GI tract reach the end of the small intestine. Little remains but water, a few salts and body secretions, and undigested materials such as fiber. These enter the large intestine (colon).
In the colon, intestinal bacteria degrade some of the fiber into simpler compounds. The colon itself retrieves from its contents the materials that the body is designed to recycle—water and dissolved salts. The waste that is finally excreted has little or nothing of value left in it. The body has extracted all that it can use from the food.
To digest food, the salivary glands, stomach, pancreas, liver (via the gallbladder), and small intestine deliver fluids and digestive enzymes.