Digestion, Absorption and Metabolism of Carbohydrates

Carbohydrates are essential biomolecules that provide us with our major dose of energy. Their intake fulfils most of our body’s daily energy requirements. They are rich in calories. However, just their intake is not enough to provide energy. The digestion, absorption and metabolism of carbohydrates are crucial for their conversion into energy. It must be noted that the digestion of carbohydrates starts in the mouth and they are absorbed in the small intestine. Carbohydrates can be simple or complex, based on the number of saccharide units in them.

Digestion of carbohydrates

When we take food into our mouths, we chew it. Chewing (which is the common word for mastication) breaks down complex carbohydrate molecules into simpler ones. Salivary glands present in the mouth secrete saliva (or salivary amylase). Due to the action of saliva, starch (which is a complex carbohydrate) is broken down into maltose and simpler polysaccharides. The concentration of maltose increases in the mouth. This is why food tastes sweet to us. Maltose is a sugar, whose presence gives food a sugary taste. However, not all food is broken down in the mouth. Only 5% are broken down. It is good for us as a lot of carbohydrate breakdown can lead to tooth decay.

Digestion hardly takes place in the stomach, because salivary amylase is not active in the stomach. From the stomach, it goes to the duodenum, which is the first part of the small intestine. The pancreas secretes pancreatic amylase which breaks down the 𝛼-1,4 bonds of the polysaccharide. It, however, cannot break down 𝛼-1,6 bonds or terminal bonds. 

Now, the small intestinal wall secretes enzymes, known as disaccharidase, that line the villi. Enzymes like maltase, sucrase and lactase are released that break down carbohydrates into monosaccharides. These monosaccharides are absorbed by the small intestine. 

Whatever is not digested and absorbed (for example, fibre) reaches the colon, where it is eliminated by the excretory processes. This is called excretion.

Absorption of carbohydrates

The next step after digestion is absorption. In the small intestine, there are certain cells that contain transporters or transport proteins, which help to transport the carbohydrates into the blood. SGLT-1 transporter is responsible for the absorption of glucose, and GLUT5 transport protein helps in the absorption of fructose. 

The first organ in the body that receives these broken-down carbohydrates is the liver. In the liver, galactose is converted to glucose. Fructose (which is already a simple sugar) is broken down further into carbon units. Glucose is stored as glycogen. Extra glucose is given back to the blood. 

Maintaining the appropriate levels of glucose in the body is of utmost importance. Too less glucose would lead to problems like fatigue and loss of concentration and too much glucose would have other serious implications like diabetes. Luckily for us, the body has a natural mechanism to regulate the levels of glucose via a process of negative feedback. 

When we consume a meal, the glucose level in the body suddenly shoots up. In such cases, the pancreatic beta cells secrete insulin, whose function is to signal the body cells to exclude extra glucose from the blood. Extra glucose is then stored in the cells or used to make macromolecules.

The hormone opposite to insulin is glucagon; this gets activated when blood sugar levels in the body drop. This happens when we indulge in extensive physical work. It signals the cells to stop storing glucose and signals the liver to break down glycogen and release glucose into the blood. 

Carbohydrate metabolism

Carbohydrate metabolism is a process by which a constant amount of energy is supplied to body cells. The breakdown of glucose into smaller molecules via a process called glycolysis is known as carbohydrate metabolism. Glucose enters into Kreb’s cycle (which is also known as the citric acid cycle or CAC). It takes place inside mitochondria, the powerhouse of the cell.

In glycolysis, glucose is ultimately broken down into two molecules of pyruvate (first glucose gets converted to glucose-6-phosphate, which then gets converted to fructose-6-phosphate, which ultimately gets converted to pyruvate). 

The process of respiration and carbohydrate metabolism is linked. When we respire, we inhale oxygen from the atmosphere and exhale carbon dioxide in return. The inhaled oxygen accompanies the conversion of glucose into energy. 

Sometimes, glycogen can be directly converted to glucose-6-phosphate (without the need to convert glycogen to glucose first). The direct conversion of glycogen into glucose-6-phosphate is known as glycogenolysis. 

Conclusion

It is crucial to study the digestion, absorption and metabolism of carbohydrates as they are the major sources of glucose (which provides energy) in the body.We covered the digestion, absorption and metabolism of carbohydrates in these notes. These processes are responsible for providing energy to our body. The goal of digestion is to break down complex carbohydrate molecules into simpler sugar molecules. Digestion starts in the mouth and ends in the small intestine. In the small intestine, the process of absorption takes place. Glucose is transported to blood through the help of transport proteins. Carbohydrate metabolism is the process of further breakdown of glucose to provide energy to the body. When we consume a meal, the glucose level in the body suddenly shoots up. In such cases, the pancreatic beta cells secrete insulin, whose function is to signal the body cells to exclude extra glucose from the blood. Extra glucose is then stored in the cells or used to make macromolecules.