Digestion and Absorption of Nutrients

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Chapter: Anatomy and Physiology for Health Professionals: Digestive System

Typical meals contain carbohydrates, lipids, proteins, water, and vitamins.


Digestion and Absorption of Nutrients

Typical meals contain carbohydrates, lipids, proteins, water, and vitamins. The digestive system handles each of these components differently. Digestion involves breaking down large organic molecules before absorp-tion can occur. Water, electrolytes, and vitamins can be absorbed without preliminary breakdown but may require special transport mechanisms. Discussion of the various types of nutrients is essential in under-standing their actions within the digestive system.

As various types of food move through the GI tract, chemical processing via enzymatic breakdown occurs. Even after a short time in the stomach, the appearance of various foods has changed dramatically. This is due to mechanical breakdown. Digestion then breaks them down into their basic chemicals, each with extremely different molecules. These molecules are small enough that they can be absorbed across the small intestine’s walls.

Digestion is a catabolic process, in which large food molecules are broken down into chemical build-ing blocks or monomers. Enzymes secreted into the alimentary canal lumen from intrinsic and accessory glands are used for this process. Enzymatic break-down of food molecules is hydrolysis, since water is required to break down the molecular bonds. Most digestion occurs in the small intestine. Pancreatic enzymes break down polymers and some other large chemicals into smaller pieces. The intestinal or brush border enzymes break these down into individual components. The alkaline pancreatic juice neutral-izes the acidic chyme entering from the stomach. The enzymes can then operate efficiently in this environ-ment. Pancreatic juice is the main source of lipases, and along with bile, is necessary to break down fats.

Absorption moves substances from the GI lumen into the body cells. Substances usually do not move between cells due to the tight junctions that join epi-thelial cells of the intestinal mucosa at their apical surfaces. The materials must pass through the epi-thelial cells instead, entering via an apical membrane and exiting through a basolateral membrane into the interstitial fluid. Then, substances can diffuse into blood capillaries to be transported via the hepatic por-tal vein to the liver. However, certain lipid products of digestion enter the lacteal in the villus and are carried through the lymphatic fluid to the blood.

The plasma membrane’s structure allows for passive absorption of nonpolar substances. These can dissolve in the lipid core of the plasma membrane. All other types of substances require a carrier mechanism. The majority of nutrients utilize active transport that is directly or indi-rectly driven by metabolic energy in the form of ATP.

Every day, as much as 10 liters of foods, drinks, and GI secretions enter the alimentary canal. However,­ only 1 liter or less will reach the large intestine. Nearly all foods, 80% of electrolytes, and most of the water will be absorbed in the small intestine. Absorption occurs all along the small intestine, with most being com-pleted by the time chyme moves into the ileum. The ileum’s major role is absorption of bile salts, which will be recycled back to the liver to be resecreted. The over-all absorptive ability of the small intestine is amazing.

Carbohydrate Digestion and Absorption

Carbohydrates include sugars and starches and are organic compounds. Energy from carbohydrates mostly is used to power cellular processes. They are ingested in forms that include grains; vegetables; glycogen from meats; disaccharides from cane sugar, beet sugar, and molasses; and monosaccharides from fruits and honey. Digestion breaks carbohydrates down into monosac-charides, which include fructose, galactose, and glucose, for easy absorption. Liver enzymes convert fructose and galactose into glucose, which is the form of carbohy-drate most commonly oxidized for use as cellular fuel.

Some excess glucose is changed to glycogen, which is stored in the liver and muscles. Glucose can be rapidly mobilized from glycogen, but only a certain amount of glycogen can be stored. Excess glucose is usually converted into fat and stored in adipose tissue. For energy, the body first metabolizes glucose, then glycogen into glucose, and finally fats and proteins.

There are two steps in the digestion of complex car-bohydrates, which include starches and simple polysac-charides. The first step utilizes carbohydrases from the salivary glands and pancreas. These are known as sali-vary amylase and pancreatic alpha-amylase. The ­second step utilizes brush border enzymes from the microvilli, which break disaccharides and trisaccharides into sim-ple sugars known as monosaccharides before absorp-tion. The enzyme called maltase breaks the bonds of the two glucose molecules of the disaccharide maltose. The enzyme sucrase breaks down the disaccharide sucrose into glucose and another six-­carbon sugar called fructose­. The enzyme lactase hydrolyzes the disaccharide called lactose into one molecule of glucose and one molecule of galactose. The main carbohydrate in milk is lactose, which provides essential functions in infancy and early childhood, since it breaks down lactose. When the intestinal mucosa stops making­ lactase, the person becomes lactose intolerant. If this person consumes milk or other dairy products, many ­different digestive problems can occur, including gas, lower abdominal pain, diarrhea, and vomiting.

Lipid Digestion and Absorption

Lipids include fats, fat-like substances, and oils; cholesterol;­ and phospholipids. They supply energy for body processes and building of certain structures. The small intestine is the primary site of lipid diges-tion via the presence of lipase from the pancreas.

The digestion of lipids requires lingual lipase from the glands of the tongue as well as pancreatic lipase. Triglycerides are the most important and numerous dietary lipids. They are made up of three fatty acids, attached to one molecule of glycerol. Lingual and pan-creatic lipases break off two of the fatty acids, leaving monoglycerides. The lipases are water-soluble enzymes. Lipids usually form large drops that exclude water molecules. Therefore, lipases are able to attack just the exposed surfaces of lipid drops. Lingual lipase starts to break down triglycerides in the mouth and continues for varying amounts of time within the stomach.

Chemical digestion is improved by bile salts when they emulsify lipid drops into tiny emulsion drop-lets. This provides increased access for pancreatic lipase. Emulsification only occurs after chyme has been mixed with bile inside the duodenum. Pancre-atic lipase can then break triglycerides apart, form-ing a mixture consisting of monoglycerides and fatty acids. When these molecules are released, they react with bile salts in the chyme, forming tiny micelles, which are lipid–bile salt complexes, each only about 2.5 nanometers (0.0025 micrometers) in diameter.

As a micelle contracts the intestinal epithelium, lipids diffuse across the plasma membrane, entering the cytoplasm. New triglycerides are synthesized from the monoglycerides and fatty acids. These triglycerides along with absorbed fat-soluble vitamins, phospholip-ids, and steroids become coated with proteins. They are then known as chylomicrons. Most bile salts within micelles are reabsorbed via sodium-linked cotransport. Approximately, 5% of bile salts secreted by the liver enter the colon and only about 1% are lost in the feces.

Chylomicrons transport dietary fats to muscle­ and adipose cells. Very low-density lipoprotein (VLDL) molecules, formed in the liver, transport tri-glycerides from excess dietary carbohydrates. When VLDL molecules reach adipose cells, the enzyme lipo-protein lipase helps to convert VLDL to low-density lipoprotein (LDL). Because most triglycerides have been removed, LDL molecules have higher cholesterol than VLDL. Cholesterol is obtained for the body’s needs by peripheral tissue cells, which use endocytosis to remove LDL from the plasma. FIGURE 24 -18 shows digestion, absorption, and transportation of lipids.


High-density lipoprotein (HDL) has high ­levels of protein and low levels of lipids. It removes ­cholesterol from tissues and sends it to the liver.

HDL molecules containing cholesterol enter liver cells via receptor-mediated endocytosis. The liver excretes the cholesterol into the bile or uses it to create bile salts. The intestine reabsorbs much of the cholesterol and bile salts and repeats the cycle. Each time, some cholesterol and bile salts reach the large intestine and are excreted in the feces. The intestinal villi also absorb electrolytes via active transport and water via osmosis.

Protein Digestion and Absorption

Proteins are created from amino acids and include enzymes, plasma proteins, the muscle components actin and myosin, hormones, and antibodies. After digestion breaks proteins down into amino acids, they may also supply energy. They are transported to the liver, where deamination occurs, which is the loss of their nitrogen-containing portions. They react to form the waste urea, excreted in urine.

Proteins supply the essential amino acids and provide nitrogen and other elements. Protein require-ments differ based on body size, metabolism, activity levels, and other factors. Nutritionists recommend a daily protein intake of 0.8 g/kg of body weight; there-fore, most average adults should consume 60–150 g of protein per day.

Protein digestion is a complicated process that requires a long time because of the complicated struc-tures of proteins. Proteolytic enzymes must attack individual proteins while stomach acids disrupt pro-tein structures to expose peptide bonds. The acidity of the stomach allows for pepsin, the proteolytic enzyme secreted in its inactive form from the chief cells. Pep-sin is effective when the pH is between 1.5 and 2.0. It breaks down peptide bonds in polypeptide chains.

As chyme enters the duodenum, enteropepti-dase triggers conversion of trypsinogen into ­trypsin. The pH is increased by buffers to between 7.0 and 8.0. Pancreatic proteases begin to work. Trypsin, chymotrypsin, and elastase are able to break certain peptide bonds within polypeptides. Trypsin breaks the bonds that involve amino acids such as arginine and lysine. Chymotrypsin attacks peptide bonds that involve tyrosine and phenylalanine. Carboxypeptidase removes the final amino acid in a polypeptide chain and is not dependent on any specific amino acid. This produces free amino acids while other peptidases generate different short peptides­.

Several peptidases, especially dipeptidases, are con-tained within the epithelial surfaces of the small intes-tine. This type of enzyme breaks short peptide chains into individual amino acids, which diffuse through cells to their basolateral surfaces. They are then released into interstitial fluid via facilitated diffusion and cotransport. When reaching the interstitial fluids, amino acids diffuse into intestinal capillaries and are transported to the liver via the hepatic portal vein.

Vitamins

Vitamins are other organic compounds required for normal metabolism. Body cells cannot synthesize adequate amounts of vitamins, so they must come from foods. Vitamins are classified by their solubil-ity. Fat-soluble vitamins include A, D, E, and K, and water-soluble vitamins include the B vitamin group and vitamin C.

Bile salts in the small intestine promote absorp-tion of fat-soluble vitamins. The water-soluble vita-mins include the B vitamins and vitamin C. They are absorbed in the small intestine via specific active or passive transporters—except for vitamin B12, which is absorbed in the terminal ileum.

Water Absorption

Every day, the small intestine receives about 9 liters of water, mostly from GI tract secretions. The chyme is mostly made up of water and about 95% is absorbed in the small intestine via osmosis. Of the remaining 5%, most is absorbed in the large intestine. Only about 0.1 liter remains as a softening agent in the feces.

Water is normally absorbed at about 300–400 mL per hour. It moves freely back and forth across the intestinal mucosa. However, net osmosis happens when a concentration gradient is created by the active trans-port of solutes—mostly sodium ions—into the muco-sal cells. Therefore, water uptake is highly coupled to solute uptake. Also, water uptake affects absorption of substances that usually pass by diffusion. When water flows into mucosal cells, these substances follow along their own concentration gradients.

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