Nutrient absorption in the bloodstream -
Figure 5. Absorption is a complex process, in which nutrients from digested food are harvested. Absorption can occur through five mechanisms: 1 active transport, 2 passive diffusion, 3 facilitated diffusion, 4 co-transport or secondary active transport , and 5 endocytosis. As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration up the concentration gradient.
Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane.
Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher. Finally, endocytosis is a transportation process in which the cell membrane engulfs material.
It requires energy, generally in the form of ATP. Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions.
Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.
In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane. Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct.
The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. The routes of absorption for each food category are summarized in Table 3.
All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of grams per hour. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces. The monosaccharides glucose and galactose are transported into the epithelial cells by common protein carriers via secondary active transport that is, co-transport with sodium ions.
The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts. The monosaccharide fructose which is in fruit is absorbed and transported by facilitated diffusion alone.
The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down. Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids.
Almost all 95 to 98 percent protein is digested and absorbed in the small intestine. The type of carrier that transports an amino acid varies. Most carriers are linked to the active transport of sodium. Short chains of two amino acids dipeptides or three amino acids tripeptides are also transported actively.
However, after they enter the absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and entering the capillary blood via diffusion. About 95 percent of lipids are absorbed in the small intestine.
Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion. Short-chain fatty acids are relatively water soluble and can enter the absorptive cells enterocytes directly. Despite being hydrophobic, the small size of short-chain fatty acids enables them to be absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus.
The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme. However, bile salts and lecithin resolve this issue by enclosing them in a micelle , which is a tiny sphere with polar hydrophilic ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids.
The core also includes cholesterol and fat-soluble vitamins. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells. Micelles can easily squeeze between microvilli and get very near the luminal cell surface.
At this point, lipid substances exit the micelle and are absorbed via simple diffusion. The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides.
The triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat. This new complex, called a chylomicron , is a water-soluble lipoprotein. After being processed by the Golgi apparatus, chylomicrons are released from the cell. Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals.
The lacteals come together to form the lymphatic vessels. The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system. Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol.
These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat. Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.
Figure 6. Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells. The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport.
These products then enter the bloodstream. The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine.
During absorption, co-transport mechanisms result in the accumulation of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion concentration inside the cells.
To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in. In general, all minerals that enter the intestine are absorbed, whether you need them or not. Iron —The ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via active transport.
Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed. When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off. When the body needs iron because, for example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the intestine and accelerated release of iron into the bloodstream.
Since women experience significant iron loss during menstruation, they have around four times as many iron transport proteins in their intestinal epithelial cells as do men. Calcium —Blood levels of ionic calcium determine the absorption of dietary calcium.
When blood levels of ionic calcium drop, parathyroid hormone PTH secreted by the parathyroid glands stimulates the release of calcium ions from bone matrices and increases the reabsorption of calcium by the kidneys. PTH also upregulates the activation of vitamin D in the kidney, which then facilitates intestinal calcium ion absorption.
The small intestine absorbs the vitamins that occur naturally in food and supplements. Fat-soluble vitamins A, D, E, and K are absorbed along with dietary lipids in micelles via simple diffusion.
This is why you are advised to eat some fatty foods when you take fat-soluble vitamin supplements. Most water-soluble vitamins including most B vitamins and vitamin C also are absorbed by simple diffusion. An exception is vitamin B 12 , which is a very large molecule. Intrinsic factor secreted in the stomach binds to vitamin B 12 , preventing its digestion and creating a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by endocytosis.
Each day, about nine liters of fluid enter the small intestine. About 2. About 90 percent of this water is absorbed in the small intestine. Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells.
Thus, water moves down its concentration gradient from the chyme into cells. As noted earlier, much of the remaining water is then absorbed in the colon. For example, corticosteroids, often prescribed to reduce inflammation from sports injuries, decrease absorption of calcium and vitamin D.
On the other hand, grapefruit and grapefruit juice can enhance the absorption of some pharmaceuticals like Tegretol an anticonvulsant and Zocor used to treat high cholesterol , which may result in toxic levels.
Even when nutrient intake approaches the recommended daily amount, alcohol consumption can cause deficiencies, writes Elizabeth Somer, MA, RD, in her book, The Essential Guide to Vitamins and Minerals. According to Somer, alcohol damages the lining of the stomach and small intestines, altering or reducing absorption of vitamins and minerals.
The recommendation? Keep alcohol to a minimum. Iron is one nutrient particularly affected by caffeine, warn Somer and her coauthor Robert Garrison Jr.
They note that caffeine can reduce iron absorption by up to 80 percent. Consider substituting non-caffeinated versions of coffee, tea and other drinks when you can. Intense exercise can be great for body and soul , but a tough workout above 75 percent of your maximal oxygen uptake [VO2 max] can affect how efficiently you soak up nutrients.
And carbohydrate drinks may also be needed during endurance events to prevent fatigue. And micronutrients, such as the B vitamins and other vitamins and minerals, although not used for energy directly, are still used in the metabolism of energy production.
Another common issue is nutrient competition. After all, Mother Nature packaged nutrients together, so is it really necessary for you to separate them?
There are cases where you may need to supplement with a single vitamin or mineral. For example, your doctor may recommend iron supplements to correct anemia; female athletes or older people often need extra calcium; and experts recommend that women of childbearing age take extra folic acid to prevent some types of birth defects.
Slightly boosting your intake of individual vitamins or minerals, however, is OK, according to Mindell. Consider, for instance, that vitamin C greatly enhances absorption of iron and vitamin D improves calcium uptake.
Follow the guidelines outlined here, and you just may find it a whole lot easier to get what you need. For the most part, vitamin and mineral supplements are absorbed the same way as whole food.
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How Does the Body Absorb Nutrients? Knowing how your body absorbs nutrients can help you fuel your workouts — and your life. By Victoria Freeman.
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