- Absorption – describes the processes of absorption of food after being digested
- Digestion – describes how the various types of foods are digested
- Enzymes at a Glance
DIGESTION actually begins in the mouth, where two kinds of saliva are produced. If a person be relaxed and ready to eat, the parotid glands produce saliva that is watery and can easily digest the food being chewed because it contains digestive enzymes. Under stress, however, the sublingual glands produce a thick saliva that has no digestive enzymes. It is this saliva that is produced in people who gulp their food rather than religiously chewing each morsel. Chewing stimulates the parotid glands (located in front of the ears – the ones that swell during mumps) to release hormones that stimulate the thymus to create T cells necessary for the immune system. Therefore, a relaxed, peaceful atmosphere is not only conducive to better digestion, but also helps the immune system.
This begins in the mouth with the action of salivary amylase. Amylase breaks down complex carbohydrates into smaller fragments, producing a mixture of di-and tri- saccharides (2 and 3 sugars). Salivary amylase continues to digest the starches and glycogen in the meal for one to two hours before stomach acids render it inactive. In the duodenum, the remaining carbohydrates are broken down further through the action of the pancreatic enzyme amylase.
After a monosaccharide is released through the action of digestive enzymes, it is absorbed by one of several mechanisms. Glucose and galactose are absorbed by active transport and compete for the same active transport system. In addition, they also compete for a secondary transporter (sodium-dependent transporter) found in the contraluminal membrane. Fructose is not absorbed by active transport, but by facilitated diffusion which is fructose-specific and is independent on the sodium ion.
This is caused by a group of conditions. One is a deficiency in one or more of the intestinal disaccharides (lactase, maltase, isomaltase, invertase, or trehalase). Another can be the blockage in the transport mechanism across the gut. Disaccharidase deficiency may be a congenital defect or may come as a result of such diseases as celiac, enteritis, kwashiorkor, or malnutrition because of lesions in the intestinal mucosa. The chief clinical manifestation is diarrhea, which occurs when sugars that cannot be absorbed are introduced into the diet.
Triglycerides and other dietary fats are relatively unaffected by conditions in the stomach and enter the duodenum in the form of large lipid drops. Bile salts emulsify them into tiny droplets that can be attacked by pancreatic lipase. This enzyme breaks the triglycerides apart and the lipids released interact with bile salts to form small complexes called micelles. When a micelle contacts the intestinal epithelium, the enclosed lipids diffuse across the cell membrane and enter the cytoplasm. The intestinal cells use the arriving lipids to manufacture new triglycerides that are then coated with proteins. This step creates a soluble complex known as a chylomicron which are secreted into the interstitial fluids.
This is accomplished through passive diffusion. The percentage of cholesterol absorbed depends on several factors, including the fiber content in the diet, the time it takes to pass through the intestines, and the total amount of cholesterol that requires absorption. Cellulose and lignans are good absorbers of cholesterol, as well as decreasing its transit time. Fatty acid absorption is dependent upon the chain length. Those that have ten or fewer carbons are quickly passed into the portal blood stream without further modification and are carried to the liver bound to albumin.
Proteins are complex structures requiring both mechanical and chemical processes to break them down. The mechanical starts in the mouth, increasing the surface area of the food to be exposed to gastric juices. In the acidic environment, the food is cleansed of microorganisms, the cell walls are broken down, and the environment is set up for the arrival of pepsin. Pepsin does not complete the job, but it does reduce the relatively huge proteins of the chyme into smaller polypeptide fragments. After the acid bath has ended and the pH has risen in the duodenum, the pancreatic enzymes appear. Trypsin, chymotrypsin, and carboxypeptidase work together to complete the disassembly of the fragments into a mixture of short peptide chains and individual amino acids. Enzymes on the surfaces of the microvilli complete the process by breaking the peptide chains into their component amino acids which are then absorbed. Carrier proteins on the inner surface of the cell then dump the absorbed amino acid into the interstitial fluid. Once there, most of the amino acids diffuse into the interstitial capillaries.
Water’s role in digestion:
Drinking one glass of water before taking in food produces a domino effect. Water immediately passes into the intestine and is absorbed. Within ½ hr., almost the same amount of water is secreted into the stomach through its glandular layer in the mucosa swelling from underneath, readying the stomach for food breakdown. The act of digestion of solid food depends upon this action. When acid is poured onto incoming food, enzymes are activated, the food is broken down, and the fluid mass formed can then pass into the intestine for the next phase. Mucus covers the glandular layer of the mucosa, which is the inner most layer of the stomach. Mucous consists of 98% water and 2% “scaffolding” that traps water. It is in the mucus layer where the natural buffer state is established. The cells below secrete sodium bicarbonate trapped in the water layer. As the acid tries to go through this protective layer, the bicarbonate neutralizes it. Without this protective layer, acid would reach below the mucus layer, causing pain and the beginning of an ulcer. The efficiency of this procedure depends solely on water. There must be enough to hydrate the mucus glands; otherwise, there is little protection from the strong stomach acids. Antacids are designed to attach to the acid in the stomach, neutralizing it and taking away the valuable elements needed for the digestive process, ultimately compounding the problem.
Hormones that assist in the digestive process:
Duodenal endocrine cells produce hormones that coordinate the secretory activities of the stomach, duodenum, pancreas, and liver. The stomach releases gastrin which stimulates the production of acids and enzymes and increases motility. With the arrival of acidic chyme in the duodenum, secretin is released. The primary effect is to increase the secretion of water and buffers by the pancreas and liver. Cholecystokinin (CCK) is also released when chyme arrives in the duodenum, especially if it contains lipids and partially digested proteins. This hormone also targets the pancreas and liver. In the pancreas, CCK accelerates the production and secretion of all types of digestive enzymes. In the liver, it causes the ejection of bile from the gallbladder into the duodenum. The presence of either secretin or CCK in high concentrations also reduces gastric motility and secretory rates. Gastric inhibitory peptide is released when fats and glucose enter the small intestine. This peptide hormone inhibits gastric activity and causes the release of insulin from the pancreatic islets.