CARBOHYDRATES IN DIET

CARBOHYDRATES IN DIET

Human diet contains three types of carbohydrates:

1. POLYSACCHARIDES

Large polysaccharides are glycogen, amylose and amylopectin, which are in the form of starch (glucose

polymers). Glycogen is available in non-vegetarian diet. Amylose and amylopectin are available in vegetarian diet because of their plant origin.

2. DISACCHARIDES

Two types of disaccharides are available in the diet.

i. Sucrose (Glucose + Fructose), which is called table sugar or cane sugar

ii. Lactose (Glucose + Galactose), which is the sugar available in milk.

3. MONOSACCHARIDES

Monosaccharides consumed in human diet are mostly glucose and fructose.

Other carbohydrates in the diet include

i. Alcohol

ii. Lactic acid

iii. Pyruvic acid

iv. Pectins

v. Dextrins

vi. Carbohydrates in meat.

Diet also contains large amount of cellulose, which cannot be digested in the human GI tract so it is not

considered as a food for human beings.

IN THE MOUTH

Enzymes involved in the digestion of carbohydrates are known as amylolytic enzymes. The only amylolytic enzyme present in saliva is the salivary amylase or ptyalin.

IN THE STOMACH

Gastric juice contains a weak amylase, which plays a minor role in digestion of carbohydrates.

 IN THE INTESTINE

Amylolytic enzymes present in the small intestine are derived from pancreatic juice and succus entericus.

Amylolytic Enzyme in Pancreatic Juice

Pancreatic juice contains pancreatic amylase.

Amylolytic Enzymes in Succus Entericus

Amylolytic enzymes present in succus entericus are maltase, sucrase, lactase, dextrinase and trehalase.

FINAL PRODUCTS OF CARBOHYDRATE DIGESTION

Final products of carbohydrate digestion are monosaccharides, which are glucose, fructose and galactose. Glucose represents 80% of the final product of carbohydrate digestion. Galactose and fructose represent the remaining 20%.

ABSORPTION OF CARBOHYDRATES

Carbohydrates are absorbed from the small intestine mainly as monosaccharides, viz. glucose, galactose

and fructose.

ABSORPTION OF GLUCOSE

Glucose is transported from the lumen of small intestine into the epithelial cells in the mucus membrane of small intestine, by means of sodium cotransport. Energy for this is obtained by the binding process of sodium ion and glucose molecule to carrier protein. From the epithelial cell, glucose is absorbed into the

portal vein by facilitated diffusion. However, sodium ion moves laterally into the intercellular space. From here, it is transported into blood by active transport, utilizing the energy liberated by breakdown of ATP.

ABSORPTION OF GALACTOSE

Galactose is also absorbed from the small intestine in the same mechanism as that of glucose.

ABSORPTION OF FRUCTOSE

Fructose is absorbed into blood by means of facilitated diffusion. Some molecules of fructose are converted into glucose. Glucose is absorbed as described above.

METABOLISM OF CARBOHYDRATES

Metabolism is the process in which food substances undergo chemical and energy transformation. After digestion and absorption, food substances must be utilized by the body. The utilization occurs mainly by

oxidative process in which the carbohydrates, proteins and lipids are burnt slowly to release energy. This

process is known as catabolism. Part of the released energy is utilized by tissues for physiological actions and rest of the energy is stored as rich energy phosphate bonds and in the form of proteins, carbohydrates and lipids in the tissues. This process is called anabolism.