Absorption of carbohydrates

The absorption of carbohydrates starts when the digestion of carbohydrates becomes completed. Digestion of carbohydrates completed when all types of carbohydrates present in our diet are converted into monosaccharides.

The digestion of carbohydrates completes in the small intestine from where it is absorbed. The rate of absorption in the small intestine decreases from above downward. For example, the rate of absorption in the jejunum is three-time more than the rate of absorption in ileum.

The absorbed carbohydrates are all monosaccharides because carbs other than monosaccharides cannot enter the bloodstream.  Some disaccharides which escape the digestion are absorbed by the cell lining the intestinal lumen and then hydrolyzed.

Rate of absorption

The rate of absorption in the small intestine is studied by a scientist Cori. They consider glucose as a standard giving a value of hundred to it and then compare other molecules with it.

In the study, they find that if the rate of absorption of glucose=100 than the rate of absorption of carbohydrates present in the small intestine are

Glucose 100
Fructose 43
Galactose 110
Manose     19
Xylose       15
Arabinose  9

Mechanism of absorption of carbohydrates

There are two mechanisms for the absorption of carbohydrates which are

1. Simple diffusion
2. Active transport

Simple diffusion

This type of mechanism depends on the concentration gradient. Most of the carbohydrates present in the small intestine are absorbed through this mechanism.

Active transport

In this type of mechanism, carbohydrates are absorbed from the intestine with the expenditure of energy. The energy required comes from ATP.

Wilson and Crane shows that substances which are absorbed through active transport must have following features

1. They must have 6 carbon ring
2.  1 or more carbon attached to carbon 5.
3.  OH, the group attached to carbon 2 must be in the same configuration as present in D-Glucose.

Active transport occurs through the carrier proteins which are present in the brush border of intestinal epithelial cells.

Carrier proteins which are involved in the active transport posses the following features.

1.  These proteins have 2 binding sites
2.  These are specific for sugars.
3.   These carrier proteins are sodium-dependent.

Facilitated transport

There are some other sugars which are transported through facilitated transport which needs carriers but do not need energy.

Examples
D-fructose
D-Mannose

Facilitated transport vs active transport

Similarities

1. Both involved carrier proteins
2.  Both show specificity
3.  Both possess specific binding sites
4.  Both possess a binding constant for solutes.

Differences

1. Facilitated transport is bi-directional while active transport is unidirectional.

2. Active transport requires energy while facilitated diffusion does not require energy.

Factor affecting the rate of absorption of carbohydrates

1. State of mucosa
2.  Length of contact
3.  Thyroid hormones increase the absorption of hexoses and acts directly on the intestinal mucosa.
4.  Pituitary glands also affect the absorption because of its effect on the thyroids.
5.  Insulin has no effect on the absorption of glucose.
6.   Vitamin B deficiency diminished the absorption of carbohydrates.
7.  Enzymes’ deficiencies also diminished absorption.