Digestion and Absorption of Carbohydrates

Let us make an in-depth study of the digestion and absorption of carbohydrates.

Digestion of Carbohydrates:

The dietary carbohydrates comprise of the polysaccharides viz. starch and glycogen, the disaccharides— lactose, maltose, sucrose and the monosaccharide’s like glucose, fructose etc. The complex poly and disaccharides are converted into simple monosaccharide’s which are absorbed by the body.

Digestion in the Mouth:

Saliva of the mouth contains salivary amylase (ptylin), whose optimum pH is 6.9 and requires Ca⁺⁺ and CP as activators. Salivary amylase acts on cooked starch and releases maltose. The digestion of starch in the mouth is not complete as the food stays here for a short duration of time.

However salivary amylase continues its action in the stomach but the production of HC1 and activation of pepsinogen to pepsin hydrolyses and deactivates amylase. There is no digestion of carbohy­drates in the stomach as the enzymes for carbohydrate digestion are absent in the gastric secretions.

Digestion in the Intestine:

The pancreatic juice secreted in the intestine contains pancreatic amylase (diastase or amylopsin). It has an optimum pH of 7.1 and is activated by Cl^– ions. The pancreatic amylase acts both on cooked and uncooked starch and also on glycogen and converts it into erythro-dextrin, achro-dextrin and maltose.

The pancreatic amylase acts only on α-1 → 4, glycosidic linkages, it cannot act on the α-1 → 6, glycosidic linkages, which are present at the branching points of starch and glycogen. These undigested branching points are known as isomaltose which are digested by an enzyme called isomaltase or α-dextrinase.

The disaccharides are hydrolyzed by respective disaccharides, which are secreted by the intestinal mucosa. The disaccharide maltose is digested by the enzyme maltase to yield two glucose units. Lactase splits the disaccharide lactose into glucose and galactose. Sucrose is hydrolyzed by sucrose or invertase into glucose and fructose.

Cellulose cannot be digested by human beings as the enzyme cellulase (β-glycosidase) is absent, yet cellulose is specifically included in the diet so as to increase the bulk (fiber or roughage) of the food and thus help in the mobility of the food through the gastrointestinal tract. The end products of carbohydrate digestion are glucose, fructose, galactose, mannose, ribose etc. some of the sugars are converted into glucose before absorption.

Absorption of Carbohydrates:

The monosaccharide’s are absorbed in the small intestine by three mechanisms:

(1) Simple diffusion

(2) Active transport and

(3) Facilitated transport.

1. Simple Diffusion:

As the digestion proceeds, the concentration of glucose in the intestinal lumen increases more than the blood glucose level. This results in an osmotic difference between the two, due to which glucose simply diffuses downhill from a region of higher concentration of glucose (lumen) to the region of lower concentration of glucose (blood).

2. Active Transport:

Simple diffusion continues till the concentration of glucose in the lumen equals to that of the blood, then glucose is transported by an active transport. Here glucose binds to a carrier protein, situated in the outer membrane of the intestinal wall. This carrier protein also binds two Na ions. When both glucose and Na⁺ are bound to the carrier protein it moves into the cell and releases glucose and Na⁺ into the cytoplasm of the cell, from where glucose simply diffuses into the blood.

To continue the active process Na⁺ ions must be expelled out of the cell, so as to maintain a low concentration of Na⁺ inside the cell, when compared to the lumen. Hence Na⁺ is expelled out of the cell into the blood plasma, in exchange of K⁺ through a Na⁺, K⁺, ATPase pump, which hydrolyses ATP for exchanging Na⁺ with K⁺.

As this overall process requires energy it is known as active transport. So, during active transport glucose, moves against concentration gradient. Galactose is also absorbed from the intestine in a similar manner as that of glucose active transport.

3. Facilitated Transport:

Fructose is transported by a carrier protein which does not require energy (ATP). Hence it is known as facilitated transport. This process is very slow.

Lactose intolerance:

It is a genetic defect in which the enzyme lactase is deficient or absent. This leads to non-digestion of lactose and hence results in intolerance to lactose, thereby to milk. This disease is most common in the Asian population.

Infants with inborn defect in the enzyme lactase are intolerant to milk and show symptoms like diarrhea and vomiting. Such infants are fed with artificial milk powder that is lactose free and contains some other added sweetener. Asian adults develop deficiency in the production of lactase in the later stages of life and thus feel aversion to intake of milk.

Diagnosis of lactase deficiency:

Three tests are available for the diagnosis.

Hydrogen breath test:

In a hydrogen breath test, after an overnight fast, 50 grams of lactose (in a solution with water) are swallowed. If the lactose is not digested, enteric bacteria metabolize it and produce hydrogen. This can be detected in the patient’s breath by a clinical gas chromatograph or a compact solid state detector.

Intestinal biopsy:

An intestinal biopsy can confirm lactose intolerance following discovery of elevated hydrogen in the hydrogen breath test. This test needs a highly specialized laboratory and expertise to measure lactase enzymes or mRNA in the biopsy tissue, hence is not taken up routinely.

Stool acidity:

Is used to diagnose lactose intolerance in small infants, for whom other forms of testing are risky or impractical.