Digestion and absorption of carbohydrate-we must know

© 2021 Google

                             Digestion and absorption of carbohydrate

        Keywords: Glycemic index, SGLT-1 secondary active transport,                     symport, GLUT-2., glycosuria.

  The gastrointestinal system is a giant and unique food processor.  Many factors affect the digestion and absorption of carbohydrates such as the food matrix and other foods eaten at the same time. Foods that are less cooked or processed are digested more slowly and have a lower glycemic index(GI) than foods that are more cooked or processed. Fiber slows the digestion of carbohydrates. High fiber foods such as whole-grain bread, oats, beans tend to have a lower GI than low fiber foods for example white bread. Fat and protein eaten with carbohydrates help to slow down the digestion of carbohydrates and reduces GI. Lemon juice or citrus fruits added to foods will lower the GI of the carbohydrates.

During digestion, carbohydrates are broken down:        Mechanically  (through chewing ) and

Chemically (by enzymes) into monosaccharides.-glucose, fructose, galactose.

Digestion of carbohydrates starts in the mouth from saliva.  When food is chewed, it is broken down into small pieces and mixed with saliva secreted from salivary glands.

      Ptylin or salivary α-amylase: it digests only boiled starch as cellulose is removed after cooking. The Optimal pH for the enzyme is neutral or slightly acidic media 6.5. It is inactivated at pH 4.5 in the stomach, but ptyalin present inside the bolus works till it comes in contact with gastric juice.

salivary α -amylase mixes with food and starts the digestion of starch until it comes into the stomach and broken into small pieces and comes into contact with highly acidic media of gastric juice. Salivary α- amylase acts on alpha 1-4 linkage of starch producing alpha limiting dextrins and maltose.

In stomach HCl hydrolases some sucrose -not a significant action.

Digestion of carbohydrates in the small intestine:  Pancreatic α- amylase digest starch and dextrins into maltose –a disaccharide. Its function is optimal in an alkaline medium and bile salts increase its activity. Pancreatic α- amylase hydrolyzes internal α -1-4 linkage. Pancreatic α- amylase cannot cleave terminal α -1-4 linkage, α -1-6  linkage or α -1-4 linkages that are immediately adjacent to α -1-6  linkage as a result starch hydrolysis products  are

starch and dextrins-à Pancreatic α -amylase  -à Maltose, maltotriose, and α-limit dextrins.

Then disaccharidases present in the intestinal juice-succus enterics will split disaccharides into monosaccharides. The human small intestine has enzymes on the brush border of epithelial cells of the jejunum and proximal part of the ileum.

Sucrose-à SucraseàGlucose and Fructose

Maltose à Maltase à 2 molecules of Glucose

Lactoseà  Lactase à Glucose  and Galactose

α –limiting  dextrins-à α –limiting  dextrinase à Glucose

Site of digestion  Digestion by these disaccharidases is the brush border of epithelial cells of the jejunum and proximal part of the ileum.

So monosaccharides are end products of carbohydrate digestion. They are formed in the small intestine lumen or in the brush border of epithelial cells of the jejunum and proximal part of the ileum. Rates of absorption of monosaccharides are variable, it is faster with glucose and galactose, intermediate with fructose, and slowest with pentose. All monosaccharides are absorbed before the terminal ileum. Bacteria in the large intestine convert some glucose into methane, CO_2, and other products. The main monosaccharide is glucose.

Site of absorption of monosaccharides: is in the brush border of epithelial cells of the jejunum and proximal part of the ileum. Some disaccharidases are absorbed as such into the epithelial cells. In the epithelial cells, they are hydrolyzed and converted into monosaccharides. After absorption, the monosaccharides are transported to the liver via the portal vein.

Process of absorption: The monosaccharides are absorbed in a two-step process.Their uptake across the apical membrane into the epithelial cells.

1. Simple diffusion-when the concentration of sugar in the lumen of the small intestine is more than in the blood. The glucose then diffuses down a concentration gradient into capillary blood within the villous.

2. Active transport occurs against concentration gradient –and requires energy which is provided by cellular metabolism.

Mechanism of transport: Glucose absorption occurs in two steps – Glucose transport from the intestinal lumen to the epithelial cell and from the epithelial cell into blood. Both steps are dependent on sodium –secondary active transport. In secondary active transport or coupled transport or cotransport energy is provided to transport molecules across a  cell membrane by the electrochemical potential the difference created by pumping ions in or out of the cell.

From the intestinal lumen to the enterocyte is the sodium-dependent hexose transporter -1  commonly called sodium-dependent glucose transporter -1 (SGLT-1).This molecule transports both sodium ion and glucose into the cell and in fact, will not transport either alone. The glucose and sodium have the same carrier (symport) molecule called SGLT-1. The transport by this involves a series of conformational changes induced by binding and release of sodium and glucose which is as follows:

The SGLT-1 is initially oriented facing the intestinal lumen –and is capable of binding sodium but not glucose.

When sodium binds,  a conformational change occurs that opens the glucose-binding pocket. Now glucose binds and the transporter reorients in the membrane such that the pockets holding sodium and glucose are moved inside the cell. Sodium dissociates into the cytoplasm, causing glucose binding to destabilize.

Glucose also dissociates into the cytoplasm and the uploaded transporter reorients back to its original outward facing position.

Once glucose and sodium are in the cell they must be exported from the cell into the blood.

Sodium is rapidly shuttled out in exchange for potassium by sodium pumps on the basolateral membrane and maintains the electrochemical gradient across the epithelium.

SGLT-1 sodium-glucose transporter is coupled to a source of energy provided by the active transport of Na^+. . Sodium –dependent glucose transporter -1 (SGLT-1) is the membrane protein for glucose and galactose uptake at the apical membrane. The glucose and sodium have the same carrier (symport) molecule called SGLT-1.

 In simple words -Glucose combines with a mobile component of the cell membrane to form glucose- carrier complex. This complex causes the movement of glucose across the cell membrane and glucose is released inside the cell.

                                      Glucose absorption

The intracellular glucose then diffuses across the basolateral membrane through  GLUT-2 into the blood capillaries. The glucose is transported by GLUT-2 ( glucose transporter -2.) 

Galactose absorption is like that of glucose.

The apical step of fructose is transportation is by ‘facilitated diffusion through GLUT 5. Some fructose is converted to glucose in the mucosal cells and absorbed.

Pentoses are absorbed by simple diffusion.

The maximum rate of glucose absorption from the lumen of the small intestine is 120gm/hour.

Glycosuria presence of glucose /sugar in the urine. This is typically seen in diabetes mellitus. When blood sugar level rises over the renal threshold value for sugar (180 mg/dl) blood sugar appears in the urine.

Alimentary glycosuria is glycosuria appearing after food may be normal, but usually seen in mild or latent diabetes mellitus.

Factors affecting glucose absorption:

1. Hormonal effect: Thyroxin acts directly on the intestinal mucosa to increase glucose absorption. So in thyrotoxicosis glucose absorption is increased.

2. Electrolytes: Increase Na+ ion concentration causes increase glucose absorption and an increase in glucose level causes increased absorption of sodium, therefore glucose is added in the ‘ORS’ oral rehydration solutions to increase sodium absorption.

3. Condition of the gastrointestinal tract: in diarrhea, enteritis, celiac disease, abnormal intestinal mucosa, gastrocolic fistula, etc.

Functions of carbohydrates: Carbohydrates are the center of nutrition. And perform many functions in living organisms. They provide energy for the CNS and muscles. The recommended amount of carbohydrate for an average adult is about 130gm or carbohydrates should provide   45% or-65% of total calorie intake.

Polysaccharides serve for the storage of energy.

Ribose the 5 C monosaccharide is an important component of coenzyme ATP, FAD, and NAD.

Ribose is the backbone of the RNA.

 Saccharides and their derivatives include many biomolecules that play key roles in the immune system and fertilization.

Internal link:

https://totalphysology20.blogspot.com/2021/01/diabetes Mellitus-we must know

https://totalphysology20.blogspot.com/2021/02/salivary glands-we must know

https://totalphysology20.blogspot.com/2021/03/small intestine  we must  know

https://totalphysology20.blogspot.com/2021/04/large  intestine  we must  know

https://totalphysology20.blogspot.com/2021/05 /bile- we must  know

https://totalphysology20.blogspot.com/2021/05 /gut-brain axis- we must  know

External link:

https://en.m.wikipedia.org>wiki

https://www.sciencedirect.com>topics

https://www.ncbi.nim.nih.gov

https://www.betterhealth.vic.gov.au>

https://www.healthline.com>nutrition

https://www.nestle.in .nutrients >di

Please follow on email and Twitter,

#english# symport # (SGLT-1) # GLUT-2 # glycosuria#recommended dose # Ptylin #