Digestion and Absorption of Food in the Gut
A typical meal contains carbohydrates, fat, protein, water, electrolytes, and vitamins. The gut handles each component differently. Although water, electrolytes, and vitamins do not require special process-ing, others must be broken down into smaller mole-cules before they can be absorbed. Also, special transport mechanisms are used for absorption of dif-ferent types of food. Carbohydrates are fragmented to simple sugars, proteins to amino acids, and fats (lipids) to fatty acids. Special enzymes secreted in specific parts of the gut break down the bondage be-tween the complex molecules and reduce them to their simplest forms. Some enzymes are so specific that they only break up linkages between certain mol-ecules of the components. For example, some break a link only between two glucose molecules and not be-tween glucose and certain other simple sugars.
Many of the enzymes are located in such glands as the salivary gland, glands in the stomach wall, and pancreas and are secreted into the lumen of the gut. Other important enzymes are located in the mucosa of the small intestine.
A summary of the chemical events in digestion is given in Table 11.4.
The carbohydrates in the diet are broken down by the enzymes in the mouth, pancreas, and intestinal ep-ithelium.
In the mouth, amylase, the enzyme of the saliva, breaks down starches into smaller fragments of two sugars (disaccharides) or three sugars (trisaccha-rides). The enzyme works best at an alkaline pH (that of the mouth); as long as the pH is adequate, it con-tinues to work even after the food reaches the stom-ach. It takes about 1 to 2 hours for stomach acid to inactive the salivary enzymes. Amylase secreted by the pancreas begins to work in the duodenum.
The enzymes located in the intestinal epithelium break the disaccharides and trisaccharides into monosaccharides (simple sugars). The enzyme mal-tase breaks the disaccharide maltose into two glu-cose molecules; sucrase breaks sucrose (the sugar we use in coffee) into glucose and fructose; lactase breaks down lactose (present in milk) into glucose and galactose.
The monosaccharides are absorbed into the in-testinal epithelium and secreted into the interstitial fluid where they enter the capillaries to reach the liver via the portal system. In the liver, they are fur-ther processed and liberated into the blood according to need.
Most fat that is consumed is in the form of triglyc-erides. Triglycerides are three molecules of fatty acids attached to a molecule of glycerol. For absorption to take place, the lipid must be broken into monoglyc-erides and fatty acids. The digestion of fat begins in the mouth and continues in the small intestine by the action of the enzymelipase. Because they are not wa-ter-soluble, the consumed lipids tend to form droplets in the gut. However, because lipases are water-soluble, they are able to only reach the outside of the droplets, with fat molecules deep inside the droplets being un-reachable.
The bile salts present in the bile secreted by the liver are important in fat digestion. They break up the large lipid droplets into small droplets, enabling lipase to di-gest the fat. This process of forming small droplets is known as emulsification. In the small intestine, the bile salts emulsify fat and the lipases secreted by the pancreas digest it. The fatty acids and monoglycerides interact with the bile salts to form complexes called micelles. The lipids diffuse across the intestinal cellmembrane when the micelle comes in contact with the intestinal epithelium. The intestinal cells convert the monoglycerides and fatty acids into triglycerides in the cytoplasm and, after coating them with protein, se-crete them into the interstitial tissue. These particles (chylomicrons) are absorbed into lymphatic vessels. The lymphatic vessels, seen as blind-ended tubes at the center of the villi, easily absorb the chylomicrons via gaps between the cells lining the vessels. Ultimately, the chylomicrons travel via the lymphatics to the tho-racic duct to slowly enter the circulation.
In the absence of bile or pancreatic lipase, fat di-gestion and absorption are significantly reduced and fat appears in the feces. Other than fat deficiency, the body is deprived of the fat-soluble vitamins A, D, E, and K because they also cannot be absorbed.
Protein has a complex structure and protein diges-tion is more time-consuming. The large protein com-plexes are initially broken down into smaller particles by the teeth. The hydrochloric acid in the stomach helps break down plant cell walls and connective tis-sue in animal products. The acid in the stomach maintains the pH at the correct level for the enzyme pepsin, secreted by the stomach, to work efficientlyon the proteins. Pepsin breaks down the large polypeptides into smaller ones. When the food enters the intestine, the protein-digesting enzymes liberated by the pancreas begin to work in the more alkaline pH. Each enzyme breaks up special bonds in the pro-teins to ultimately reduce it to free amino acids.
The surface epithelium of the intestine also has en-zymes that break up peptide bonds. The individual amino acids are absorbed into the intestinal epithe-lium by special transport mechanisms. From the ep-ithelium, the amino acids enter the interstitial fluid where they then enter the blood capillaries to reach the liver for further processing.
The cells of the body cannot absorb or secrete water using active transport; movement of water is solely by osmosis. As previously explained, osmosis de-pends on concentration gradients of solutes across a semipermeable membrane. Therefore, water is ab-sorbed into the intestinal epithelium and then into the interstitial fluid when the solute concentration is higher in the walls of the intestine. It enters the lu-men if the contents of the lumen have more solute. Because the intestines are constantly absorbing solutes, the water moves into the capillaries along the osmotic gradient produced.
About 2–2.5 liters (2.1–2.6 qt) of water are taken in, in the form of food or drink. About 6–7 liters (6.3–7.4 qt) of water enter the lumen of the gut by salivary, gastric, pancreatic, intestinal, and bile secre-tions. It is remarkable that only about 150 mL (5 oz) of this fluid is lost in the feces.
The ion concentration in the blood must be main-tained within a narrow range for proper functioning of various metabolic activities. For example, the cor-rect concentration of sodium and potassium is needed for conduction of impulses along nerve fibers and for muscle contraction, to name just two of the many activities of the body. Calcium, another ion, must be in the right concentration for excitation-contraction coupling to occur. Fluctuation in hydro-gen ion or bicarbonate ion levels can drastically change the pH and make enzyme activity chaotic. The absorption of various ions is regulated individu-ally and many of the regulatory mechanisms are poorly understood. Electrolytes are absorbed by ac-tive transport or by diffusion.
All water-soluble vitamins other than B12 are easily absorbed across the intestinal epithelium by diffu-sion. For adequate quantities of Vitamin B12 to be ab-sorbed, it must combine with a glycoprotein intrinsic factor secreted by the stomach. In individuals who have had part of the stomach removed or whose gas-tric mucosa is atrophied, secretion of intrinsic factor is limited and vitamin B12 deficiency results.
Fat-soluble vitamins A, D, E, and K are absorbed like lipids and, therefore, require normal secretion of bile and lipase for absorption.