Digestion in humans, as in other animals, is the process by which food containing nutrients such as proteins, fats, and carbohydrates is eaten and broken down to its components. These components are absorbed from the small intestine and dispersed into the circulation for use by various organs and cells. The body is thus provided with the molecules from which energy, as calories, is used for metabolism--the chemical processes by which the body builds and recycles bones, blood, muscles, nerves, and organs. These nutrients also provide certain components that the body is unable to make, such as vitamins and minerals, salts, and certain essential amino acids to build proteins and fatty acids required for cell function that the body does not make. Failure to provide any of these leads to deficiency diseases. In the United States, the average diet provides about 11 percent of calories as protein, 46 percent as fat, and 43 percent as carbohydrate. Government health agencies recommend that the risk for various diseases involving heart, blood vessels, and certain cancers could be reduced by lowering the quantity of fat consumed to 30 percent of calories and increasing intake of unrefined carbohydrates, vegetables, and fiber.
Food enters the digestive tract by way of the mouth, where it undergoes physical breakdown through chewing. Enzymes such as ptyalin, which initiates sugar digestion, are introduced in salivary secretions, which also provide lubrication to facilitate chewing and swallowing.
The food bolus (soft mass) passes through the esophagus and is retained in the stomach. There food is liquefied by a mixture of hydrochloric acid and pepsin, which is secreted by the stomach wall. Simultaneously, various gastric enzymes, such as pepsin (which initiates protein digestion), are secreted into the stomach. Secretions of mucus protect the stomach from its digestive enzymes. The stomach's contents are then metered out by the muscular pumping motion of peristalsis, passing through the pyloric valve into the duodenum, the first portion of the small intestine.
In the average adult the small intestine is about 5 to 6 m (16.4 to 19.7 ft) long, with an enormous absorbing surface, the mucosal epithelium, which represents the interface of the body with the outside world. The mucosa area is increased by small fingerlike projections called villi, which protrude into the intestine. The surface of each absorbing cell of the mucosa also has microscopic brushlike projections called microvilli. These factors increase the absorbing surface 600-fold or equivalent to the surface of half a basketball court.
Nutrient absorption occurs essentially in the small intestine. A common duct from the pancreas and the gallbladder into the duodenum serves as a conduit to introduce bicarbonate (to neutralize hydrochloric acid), pancreatic enzymes (for degradation of proteins and carbohydrates), and bile salts (for fat absorption).
Peristalsis moves the semiliquid food mass, or chyme, into the next portion of the small intestine, the jejunum, where the bulk of digested carbohydrate, protein, water, water-soluble vitamins, electrolytes, and minerals are absorbed.
Remaining nutrients are propelled to the last third of the small intestine, the ileum. Here fat; fat-soluble vitamins A, D, E, and K; vitamin B(12); and bile salts are absorbed. Some fluid, indigestible residues, and cellular debris pass through the ileocecal valve into the colon, which is the major reservoir for intestinal bacteria.
Additional water is extracted, potassium is excreted to maintain electrical neutrality, and the chyme is thickened to form stool. This, in turn, is propelled by peristalsis into the rectum for evacuation. Normal stool weight is approximately 250 g (9 oz) daily, of which 10 to 20 percent is bacteria. It contains indigestible fiber, metabolic end products, water, fats, electrolytes, and small amounts of protein that is excreted as feces.
Dietary fats (lipids) comprise saturated fats (generally solid at room temperature, such as beef fat) and polyunsaturated fats (liquid at room temperature, as in vegetable and fish oils). During digestion, fats are broken down partially to free fatty acids and molecules with one, two, or three attached fatty acids--mono-, di-, and triglycerides.
To digest fats, after a meal the gallbladder contracts and discharges bile salts, which are made in the liver and stored in the gallbladder. The bile salts emulsify the fatty acids, enabling fat to be "dissolved" in water to be absorbed. During fat absorption the bile salts are reabsorbed and recirculated by the liver about six times daily. The bile salt-coated fat is able to travel through the water in the intestine to the intestinal cells in the last portion of the small intestine, the ileum, where it dissolves in the membranes of intestinal cells.
After absorption, lipids are repackaged with proteins as chylomicrons and sent to the liver. Here they are repackaged again in a coat of cholesterol and protein. This coating, which allows the fat to be dissolved in the blood, enables the fats to be transported to various parts of the body where fatty acids may be removed to provide energy for cellular components. The demands to make cholesterol by the liver for the coatings are greater for saturated fats than for polyunsaturated fats, which causes the former to contribute to higher blood cholesterol levels than the latter.
Of the carbohydrates consumed daily, about 60 percent is starch, 30 percent is sucrose, and 10 percent is lactose and incidental amounts of other sugars. Starch is a polysaccharide made up of glucose molecules arranged in long chains with branches. Depending on the way the glucose molecules are joined, the polysaccharides may be amylose, amylopectin, or cellulose. (The linkage of glucose molecules in cellulose is not broken by humans, so cellulose is considered indigestible fiber.)
Digestion of both types of starch--amylose and amylopectin--is initiated by salivary amylase during chewing and is stopped by gastric acid in the stomach. Starch is further digested in the duodenum by pancreatic amylase to maltose and isomaltose. These branch chain components are broken down to glucose molecules by enzymes called maltases, located in the microvilli of the intestinal absorbing cell. At the same time, ingested sucrose is broken down by sucrase to glucose and fructose.
Similarly, lactose is broken down by lactase to glucose and galactose. Glucose and galactose are transported across the intestinal cell on a glucose carrier in combination with a sodium ion. In the cell interior, sodium is removed from the carrier and pumped out of the cell, leaving the sugar within the cell and freeing the carrier to repeat the process. The driving force from this sodium pump enables the cell to accumulate high quantities of glucose, while the amount in the intestine decreases as sugar is absorbed. This process is called active, or uphill, transport.
Fructose, released when sucrose is hydrolyzed, is absorbed by a diffusional process that is not energy dependent. Sugars exit the intestinal cell by diffusion into the capillaries to the blood circulation. Deficiency of one or more of the disaccharidases--most commonly lactase, which causes lactose intolerance--may occur. If it does, sugars remain undigested in the gut cavity and accumulate water, which leads to bloating and pain. In the colon, bacteria utilize the sugars to produce acid and gas, all contributing to diarrhea.
Proteins are large molecules composed of chains of amino acids. Protein digestion is initiated in the stomach by acid and pepsin secretion. In the duodenum and upper jejunum the pancreatic enzyme trypsin breaks down most of the undigested proteins to smaller units containing short chains of amino acids, small peptides containing two to six amino acids. Some of these small peptides are absorbed intact.
The bulk of the degradation products, the amino acids, are transported across the intestinal cell by a sodium-ion-dependent active process similar to that for glucose. For a short period in the newborn, absorption of ingested intact protein occurs by a process, called pinocytosis, in which the absorbing cell of the gut engulfs large peptide fragments. In some infants who are given cow's milk, the milk protein absorbed acts as a foreign protein, resulting in a hypersensitivity reaction, or milk allergy.
Some individuals lack a digestive enzyme that normally breaks down a wheat protein called gluten. This defect is known as celiac disease or sprue, in which undigested gluten may cause severe and chronic allergic response of the small intestine.
Approximately 5 to 10 liters (1.3 to 2.6 gal) of water, derived from food and drink as well as salivary, gastric, pancreatic, biliary, and intestinal secretions, circulate through the digestive tract daily. All but 500 ml (15 fl oz) are absorbed in the small intestine. Of the remainder, slightly more than half is absorbed in the colon.
Water transport in the small intestine occurs in response to osmotic gradients, that is, as the concentration of nutrients increases intracellularly during active transport, the osmotic pressure increases, causing an influx of water into the cell. Some electrolytes dissolved in this water enter the absorbing cell by "solvent drag."
The remaining electrolytes--bicarbonate, sodium, potassium, and chloride--are absorbed along the length of the small intestine by diffusion. In addition, some sodium enters the cell during active transport of sugars and amino acids.
In the United States adults ingest up to 20 mg of iron daily. Of this amount, about 0.5 to 1.0 mg is absorbed in healthy individuals. This may be increased up to fourfold in individuals who are iron deficient.
Iron in food may exist primarily as inorganic (ferrous) iron and a smaller portion as ferric iron. After uptake by the intestinal cell, iron is stored as protein-bound ferric iron or eventually transferred from the cell.
Another source of dietary iron exists in the form of hemoglobin iron, which is hydrolyzed to heme and globin and then further degraded to release free ferrous iron.
Absorption of both these forms of iron is regulated by iron that preexists in body pools and in the intestine. When those sites are adequately filled, absorption is inhibited; if the store is deficient, uptake and transfer are augmented.
Absorption of calcium occurs mainly in the duodenum. Vitamin D facilitates calcium absorption as much as four times more than that in vitamin D deficiency states. It is believed that a calcium-binding protein, which increases after vitamin D administration, binds calcium in the intestinal cell during absorption, followed by calcium transfer from blood circulation for storage in bone.
Fat-soluble vitamins are absorbed by the same mechanism described for the absorption of fat. At present, it is believed that most of the water-soluble vitamin absorption occurs by diffusing across intestinal cells. At least two water-soluble vitamins that are important in blood cell maturation are known to have specialized absorption mechanisms--vitamin B(12), as described above, and folic acid.