Absorption of glucose entails transport from the intestinal lumen, across the epithelium and into blood. The transporter that carries glucose and galactose into the enterocyte is the sodium-dependent hexose transporter, known more formally as SGLUT As the name indicates, this molecule transports both glucose and sodium ion into the cell and in fact, will not transport either alone. The essence of transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose, and can be summarized as follows:.
Fructose is not co-transported with sodium. Rather it enters the enterocyte by another hexose transporter GLUT5. Once inside the enterocyte, glucose and sodium must be exported from the cell into blood.
We've seen previously how sodium is rapidly shuttled out in exchange for potassium by the battery of sodium pumps on the basolateral membrane, and how that process maintains the electrochemical gradient across the epithelium. The energy stored in this gradient is actually what is driving glucose entry through the sodium-dependent hexose transporter described above.
Dairy foods are the best source of calcium , a mineral that's important for bone growth. Because teens need about 1, milligrams mg of calcium each day, experts recommend that even those with lactose intolerance include some dairy in their diet. Talking to a registered dietitian is a good idea. They're trained in nutrition and can you come up with eating alternatives and develop a well-balanced diet that provides lots of calcium for developing strong bones.
Some teens might need calcium and vitamin D supplements. Reviewed by: J. Fernando del Rosario, MD. Larger text size Large text size Regular text size. What Is Lactose Intolerance? What Happens in Lactose Intolerance? Besides age, people can become lactose intolerant due to: Ethnic background. People of Asian, African, Native American, and Hispanic backgrounds are more likely to develop lactose intolerance at a young age.
Other problems with the digestive tract. People who have inflammation of their upper small intestine, such as celiac disease or Crohn's disease, have less of the lactase enzyme. Some antibiotics can trigger temporary lactose intolerance because they affect how the intestine makes lactase. After a bout of infectious diarrhea, some people can develop a temporary lactose intolerance that usually improves after a few days or weeks.
Usually within 30 minutes to 2 hours after eating, someone with lactose intolerance will have: nausea stomach cramps bloating gas diarrhea How Is Lactose Intolerance Diagnosed? How Is Lactose Intolerance Treated? What About Calcium? You also can eat non-dairy products like: calcium-fortified juice or soy milk green, leafy vegetables like broccoli, collard greens, kale, and turnip greens beans salmon almonds soybeans dried fruit tofu Talking to a registered dietitian is a good idea.
What Else Should I Know? Here are some tips for dealing with lactose intolerance: Choose lactose-reduced or lactose-free milk. If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete.
By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in Figure 3.
Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme.
The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface.
Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein. Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins.
Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine. Figure 4. Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum. The final step in digestion is the elimination of undigested food content and waste products.
The undigested food material enters the colon, where most of the water is reabsorbed. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum. Diarrhea and constipation are some of the most common health concerns that affect digestion.
Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.
Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning.
This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla. Digestion begins with ingestion, where the food is taken in the mouth.
Digestion and absorption take place in a series of steps with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids.
Elimination describes removal of undigested food contents and waste products from the body. While most absorption occurs in the small intestines, the large intestine is responsible for the final removal of water that remains after the absorptive process of the small intestines. The cells that line the large intestine absorb some vitamins as well as any leftover salts and water.
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