This power point presentation shows how carbohydrates are metabolized and absorbed in gastrointestinal tract. The goal of carbohydrate digestion is to break down all disaccharides and complex carbohydrates into monosaccharides for absorption, although not all are completely absorbed in the small intestine (e.g., fiber). Digestion begins in the mouth with salivary amylase released during the process of chewing. Dietary carbohydrates include starches, sugars, and fiber.
Use of Dietary Carbohydrates as Energy. Glucose is the primary energy source of the body. Major dietary sources of glucose include starches and sugars.
Digestion of Carbohydrates. Dietary carbohydrates are digested to glucose, fructose and/or galactose, and absorbed into the blood in the small intestine. The digestion and absorption of dietary carbohydrates can be influenced by many factors.
Absorption of Carbohydrates. Absorbed carbohydrate molecules are used immediately for energy or stored in various forms in the muscles, liver or adipose tissue for future use. Dietary carbohydrates include starches, sugars and fibers that are mostly found in grain products, vegetables and fruit, milk products, and meat alternatives such as nuts, seeds, and legumes. Starches and sugars are the major dietary sources of glucose, which is the primary energy source in the body:
The brain relies primarily on glucose to function; restricting the brain’s glucose supply can impair memory and ability to focus.
Muscles use glucose for energy, especially during high-intensity exercise. Before the body can use the food that is eaten, it must be “digested” (i.e. broken down) into its basic nutrient components.
The digestive system works like a giant food processor. During digestion, starches and sugars are broken down both mechanically (e.g. through chewing) and chemically (e.g. by enzymes) into the single units glucose, fructose, and/or galactose, which are absorbed into the blood stream and transported for use as energy throughout the body.
Digestion of starches into glucose molecules starts in the mouth, but primarily takes place in the small intestine by the action of specific enzymes secreted from the pancreas (e.g. α-amylase and α-glucosidase). Similarly, the disaccharides sucrose, lactose, and maltose are also broken down into single units by specific enzymes. The end products of sugars and starches digestion are the monosaccharides glucose, fructose, and galactose. Glucose, fructose, and galactose are absorbed across the membrane of the small intestine and transported to the liver where they are either used by the liver, or further distributed to the rest of the body. A number of factors affect carbohydrate digestion and absorption, such as the food matrix and other foods eaten at the same time.Glycemic Index (GI) is a scale that uses a numbering system to rank carbohydrate rich foods as “high GI”, “medium GI”, and “low GI” based on the rate that glucose-containing carbohydrates are digested.
2. CONTENT
S
• Sources of carbohydrates in diet
• Digestion of carbohydrates in oral cavity
• Digestion of carbohydrates in stomach
• Digestion of carbohydrates in duodenum
• Hydrolysis by enterocytes
• Overall summary of digestion
• Absorption of Glucose
• Absorption of Galactose
• Absorption of Fructose
• Summary of absorption
• Bibliography
3. Sources of carbohydrates in diet
• There are only 3 major sources of carbohydrates in the normal
human diet
i) sucrose - disaccharide popularly known as cane sugar
ii) lactose - disaccharide found in milk
iii) starch - large polysaccharides present in plant sources of food
4. Digestion of carbohydrates in oral cavity
• Digestion of carbohydrates begins in the mouth
• Salivary amylase(ptyalin) secreted mainly by the parotid glands hydrolyses
starch into the disaccharide maltose and other small polymers of glucose.
• Since , the food remains in the mouth only a short time, so probably not
more than 5% of all starches become hydrolyzed by the time the food is
swallowed
5. Digestion of carbohydrates in stomach
• Stomach secretes gastric amylase which has a minor role in digestion.
• However, starch digestion continues in the body and fundus of the stomach
for as long as 1 hour before the food becomes mixed with the stomach
secretions.
• Activity of the salivary amylase is then blocked by acid of the gastric
secretions because the enzyme is inactive under the pH of 4.0
• On average, 30% to 40% of the starch will have been hydrolyzed , mainly to
form maltose.
6. Digestion of carbohydrates in duodenum
• Duodenum received pancreatic amylase from pancreas.
• Pancreatic amylase has same function as salivary amylase but is several times
powerful.
• Within 15-30 minutes after the chyme empties in duodenum, all the
carbohydrates will have been digested.
• The carbohydrates are converted into maltose or other small glucose polymers.
7. Hydrolysis by enterocytes
• The enterocytes lining the villi of the small intestine contain four enzymes,
which hydrolyses disaccharides and small glucose polymers
i) lactase : splits lactose into a molecule of glucose and a molecule of galactose
ii) sucrase : splits sucrose into a molecule of fructose and a molecule of glucose
iii) maltase : splits maltose into two molecules of glucose
iv) α-dextrinase : splits glucose into multiple molecules of glucose
9. Absorption of glucose
• Absorption of glucose occurs
through secondary active
transport.
• Firstly, Na+ ion is pumped out of
cell through basal surface against
concentration gradient.
• Then, Na+ re-enters the
enterocyte through SGLT-1 ,
carrying glucose along with it.
• Then, GLUT 2 facilitates diffusion
of glucose through the cell’s
basolateral membrane into the
paracellular space and from there
into the blood.
10. Absorption of galactose
Absorption of fructose
• Galactose is transported by almost exactly the same mechanism as glucose
• Transport of fructose from the intestinal lumen into the cell is facilitated by
GLUT-5.
• Exit of fructose from cell to the paracellular space is facilitated by GLUT-2
• Some of the fructose, upon entering the cell becomes phosphorylated , then
converted to glucose and finally transported in the form of glucose in the rest of
the blood.
Since fructose is not co-transported with sodium, its overall rate of transport is
only one-half of glucose or galactose.
12. BIBLIOGRAP
HY
• Guyton and Hall Textbook of Medical Physiology, International 14th edition
• Ganong’s Review of Medical Physiology, International 26th edition
• www. teachmephysiology.com
• www.media.lanecc.edu.com
