Carbohydrates are digested into monosaccharides like glucose, fructose, and galactose which are then absorbed in the small intestine. Glucose accounts for about 80% of absorbed monosaccharides and is actively transported into intestinal cells via sodium-glucose transporters, using the sodium gradient as an energy source. Galactose absorption is similar to glucose while fructose absorption occurs via facilitated diffusion without requiring sodium or energy. Absorption rates vary between sugars with galactose absorbing most rapidly, followed by glucose, then fructose and pentoses absorbing slowest. Health of the intestinal mucosa and various hormones can also impact carbohydrate absorption rates.

1. Absorption of carbohydrates

2. Digestion of carbohydrates:
 Carbohydrates are broken down into smaller
polysaccharides in the mouth by the action of salivary
amylase. Once in the small intestine, pancreatic amylase
further breaks the polysaccharide into disaccharides. The
three most common disaccharides are maltose, sucrose and
lactose. These disaccharide are broken down into
monosaccharide by the digestive enzymes found at the
brush border of the enterocytes.

3.  Maltose is obtain via the digestion of starch by
amylase. It consists of two glucose monomers
attached via α-1-4 glycosidic bond, which is broken
down by the brush border enzyme called maltase.
Sucrose consists of glucose and fructose and is
broken down by sucrase while lactose consists of
glucose & galactose and is broken down by lactase.

4. Absorption of carbohydrates:
 The principal monosaccharide's produced by the digestion
of carbohydrates are glucose, fructose and galactose
 Glucose accounts for 80% of the total monosaccharide
 The absorption occurs mostly in the duodenum & upper
jejunum of small intestine
 Only monosaccharaides are absorbed by the intestine

5. Absorption Rates:
Cori study:
 He studies the rate of absorption of different sugars from small
intestine in rate
 Galactose:
absorption rate for galactose is maximum
 Glucose:
absorption rate for glucose is moderate
 Fructose:
absorption rate for fructose is minimum

6. Comparison:
 Glucose absorption as 100, comparative absorption of other
sugars as
Galactose =110, glucose=100, fructose=43,
mannose=19, xylose=15
 Galactose is absorbed more rapidly than glucose
 Pentose are absorbed slowly

7. Mechanism of absorption
 Different sugars posses different mechanisms for
their absorption
Examples:
 Glucose is transported into the intestinal mucosal
cells by a carrier mediated and energy requiring
process

8. Active transport mechanism
 Glucose and sodium share the same transport system
referred to as sodium dependent glucose transporter
 The concentration of sodium is higher in the intestinal
lumen compared to mucosal cells
 Sodium moves into the cells along its concentration
gradient & simultaneously glucose is transported into the
intestinal cells
 Mediated by same carrier system

9.  Sodium diffuses into the cell and it drags glucose along with it
 The intestinal sodium gradient is the immediate energy source for
glucose transport
 This energy is indirectly supplied by ATP
 Since the re-entry of sodium (against the concentration gradient)
into the intestinal lumen is an energy requiring active process
 The enzyme sodium potassium ATPase is involved in the
transport of sodium in exchange of potassium against the
concentration gradient

10. Intestinal absorption of glucose
 At the intestinal lumen, absorption is by SGluT and at the
blood vessel sides absorption is by GLuT2

11. SGuT:
Sodium and glucose co-transport system at luminal side;
sodium is then pumped out

12. Galactose absorption:
 Mechanism for absorption of Galactose is similar to
that of Glucose

13. Absorption of Fructose:
 Fructose absorption is simple
 Does not require energy and Na+ ions
 Transported by facilitated diffusion mediated by a carrier
 Inside the epithelial cell, most of the fructose is converted
to glucose
 The latter then enters the circulation

14. Absorption of Pentose:
 Pentoses are absorbed by a process of simple
diffusion

15. Factors effecting:
 Mucus membrane:
Mucus membrane is not healthy, absorption will decrease
 Thyroid hormones:
Increases absorption of hexoses & act on intestinal mucosa
 Adrenal cortex:
Absorption decreases in adrenocortical deficiency, mainly due to
decreased concentration of sodium
 Anterior pituitary:
It affects mainly through thyroid hormones