DIGESTION AND ABSORPTION OF CARBOHYDRATES

By : Yashoda Amarasekera

WHAT ARE CARBOHYDRATES?

Carbohydrates are sugars known as saccharides. Most carbohydrate foods contain many saccharides linked together, which are known as polysaccharides.

Carbohydrate digestion begins in the mouth and is complete when the polysaccharides are broken down into single sugars, or monosaccharides, which can be absorbed by the blood.

CARBOHYDRATE DIGESTIONMouth Carbohydrate digestion begins in the mouth.

The salivary glands in the mouth secrete saliva, which helps to moisten the food. The food is then chewed while the salivary glands also release the enzyme called salivary amylase, which begins the process of breaking down the polysaccharides in the carbohydrate food.

Stomach After the carbohydrate food is chewed into

smaller pieces and mixed with salivary amylase and other salivary juices, it is swallowed and passed through the esophagus. The mixture enters the stomach where it is known as chyme.

There is no further digestion of chyme, as the stomach produces acid which destroys bacteria in the food and stops the action of the salivary amylase.

Pancreas and Small Intestine After being in the stomach, the chyme enters

the duodenum. In response to chyme being in the duodenum, the pancreas releases the enzyme pancreatic amylase, which breaks the polysaccharide down into a disaccharide, a chain of only two sugars linked together.

The small intestine then produces enzymes called lactase, sucrase and maltase, which break down the disaccharides into monosaccharides. The monosaccharides are single sugars that are then absorbed in the small intestine.

Large Intestine (Colon) Carbohydrates that were not digested and

absorbed by the small intestine reach the colon where they are partly broken down by intestinal bacteria. Fiber, which cannot be digested like other carbohydrates, is excreted with feces or partly digested by the intestinal bacteria.

ABSORPTION OF CARBOHYDRATES INTO THE BLOODSTREAM Digestible dietary carbohydrates are of two

general types: sugars and starch. Sugars can be monosaccharides, or single sugars, such as glucose, galactose and fructose.

starch – composed of many glucose molecules bound together into a large unit – must undergo digestion until only single glucose molecules remain, ready to be absorbed.

Glucose

Once digestion creates a pool of glucose in your gut, absorptive cells lining your small intestine begin to transport the sugar from your gut to your bloodstream. These cells, called enterocytes, lie as a sort of boundary between your gut and your capillaries, and they absorb glucose with the help of both sodium and a protein-based transporter molecule.

The transporter molecule within the enterocyte moves to the cell membrane closest to your gut and first binds sodium, which changes the shape of the transporter so it can now also grab a single glucose molecule from within your gut. The transporter moves to the interior of the cell, where it releases first the sodium and then the glucose. A different transporter grabs the glucose from the interior of the cell and carries it across to the other side of the enterocyte, releasing it into your bloodstream.

Other Sugars

Galactose and fructose are typically much less abundant in your diet than glucose, but their absorption takes place in the same area of your gut.

In fact, galactose moves into the enterocyte using the same process and transporter molecule as glucose, and therefore it is also dependent upon the presence of sodium.

In contrast, fructose travels into the cell with a different type of transporter than that used by glucose and galactose, and the transporter does not require sodium in order to bind the sugar.

However, both galactose and fructose move from the inside of the enterocyte to your bloodstream using the same transporter.

Considerations Although potassium is not directly involved in the

absorption of carbohydrates into your bloodstream, it plays an important role in sodium function and, indirectly, in water absorption. When the transporter releases sugar from the enterocyte into the blood, it also releases sodium.

To maintain the correct osmotic pressure within the cell, potassium enters the cell as sodium leaves. As this happens, the concentration difference allows water absorption into your bloodstream along with other nutrients.