Metabolic Process Involved in Digestion of Carbohydrates

What are carbohydrates?

Carbohydrates (saccharides) are the substances present in the food that humans ingest. The classification of carbohydrates is done, based on the number of single carbohydrate molecules in each of a complex molecule.

  • Simple carbohydrates

    • Mono saccharides: These are the carbohydrates with single sugar molecules – with a simple molecular structure that includes glucose, galactose, fructose and lactose. They can be directly absorbed and used by the body. It means hydrolysis (break down) that occurs in case of polysaccharide is not required for digestion of simple carbohydrates.

  • Complex carbohydrates

    • Disaccharides: They contain 2 saccharide molecules; and it is necessary to get these two molecules break down for their absorption, as they include the following substances:

      • Sucrose (glucose & fructose)

      • Maltose (2 molecules of glucose)

      • Lactose (glucose & galactose)

    • Polysaccharides: They contain approximately 60,000 simple carbohydrate molecules arranged in the form of chains or branched structures. They include starch, dextrin, glycogen and cellulose.

Benefits of complex carbohydrates:

The following are the benefits of complex carbohydrates:

  • They are rich sources of fiber that helps in decreasing the blood cholesterol levels and reducing the chances of heart diseases.
  • They help in reduction of gastrointestinal disorders and constipation.
  • By ingesting complex carbohydrates, weight control and diabetes control can be achieved.

Sources of carbohydrates

The following are the rich sources of carbohydrates
Rice: A staple food containing carbohydrates

  • Rice is a dietary food for more than half of the world’s population. It’s a healthy diet, apart from being a good source of complex carbohydrates. Thus, it is a whole and nutritious cereal.

  • Brown rice in which the bran and germ are not removed is highly nutritious than white rice. White rice contains starch (amylose and amylopectin). So, it causes the raise in blood sugar levels, as they get breakdown.
  • Along with carbohydrates, rice contains – fiber, protein, fat, folic acid, thiamin, niacin, iron, riboflavin, vitamin E, calcium, phosphorous, potassium, sodium, etc.

Metabolism of carbohydrates present in rice is as follows:

  • Starch is a complex carbohydrate with amylose and amylopectin that gets hydrolyzed into simpler molecules with the help of an enzyme called ‘amylase’. This enzyme is usually secreted by the salivary glands. It gets reduced to maltose, malt triose, and alpha-limit dextrin.

  • Maltose, malt triose and dextrin are the products of amylose and amylopectin. They get hydrolyzed further into glucose, monomers in the small intestine with the help of an enzyme called sucrase- iso maltase complex.
  • Cellulose and hemicellulose are the other dietary fibers present in the series of glucose molecules linked by glycosidic bonds, accounted for some 10,000 in number. Cellulase and hemi-cellulases are the 2 enzymes which participate in the digestion of the dietary fibers through a process called fermentation.
  • The glucose molecules thus formed get converted into glycogen through a process called glycogenesis. It is stored in the liver in the form of fat for the purpose of energy.

Wheat Pasta

  • It is rich in carbohydrates and comprised of starches that make 78% of the nutrient content. It usually contains low glycemic index – the digestion and metabolism occurs at a slower rate providing steady source of energy.

  • Starches present in pasta breaks down into glucose molecules during the process of digestion. In this process, the simple glucose molecules enter the blood stream for providing energy for the cells present in brain and muscles; and the left out glucose molecules will be converted into glycogen in the liver through glycogenesis.

Oat meal

  • It consists of carbohydrates such as starches and sucrose in small quantities. Sucrose gets break down into simpler molecules like glucose and fructose with the help of the enzyme action by sucrose, where they can be directly assimilated into the blood stream for the purpose of energy.


  • It is a vital dairy product consisting of 5% of carbohydrates. Of this 5% carbohydrates, lactose is present in huge proportion, while oligosaccharides and mono saccharides are the other components present. Lactose is a complex molecule that comprises of glucose and galactose.

  • Lactose helps the process of digestion of lactose; and turns it into glucose and galactose. The glucose thus formed can be directly absorbed into the body, while galactose undergoes further break down into simpler glucose molecules by lactase for it being a complex molecule.

The other sources of carbohydrates include vegetables, fruits, beverages, sugars etc.

Digestion and Metabolism of ingested carbohydrates

Digestion is the process in which complex substances are broken down into simpler substances for further absorption into the body through digestive system. Metabolism of carbohydrates involves absorption of nutrients presented in the digested food. This takes place once the process of digestion is completed. Metabolism ensures direct absorption of simple carbohydrates. However, the complex carbohydrates have to get broken down into simple carbohydrates, before they are absorbed. The 2 processes involved in metabolism are:

  • Anabolism – It is the process in which energy is consumed to build molecules needed by the body for maintenance. The absorbed nutrients are utilized for maintenance and growth.

  • Catabolism – It is the process in which energy is released by breaking down of the molecules into simple substances.

Steps involved in digestion and metabolism

  • Digestion is a combination of both mechanical and chemical processes; and the enzymes that are used in hydrolysis of carbohydrates are the carbohydrates.

  • Breaking down of the complex carbohydrates into simpler carbohydrates with the help of various enzymes.

  • Hydrolysis of the complex molecules and glucose polymers into mono saccharide units by intestinal surfaces.

  • Glucose if present in excess stimulates the pancreas for releasing insulin. Thus, it transfers the glucose into the liver and muscle cells.

  • Glycogenesis (Anabolism) – It is the process of conversion of glucose molecules formed due to the break down into glycogen. It is stored in the liver and muscles, when there is no need of glucose.

  • Glycogenolysis (Catabolism) – It is the process that occurs when there is a need of glucose in the body, wherein the secretion of epinephrine and glucagon hormones stimulates the conversion of glycogen into glucose.

Formation of energy from glucose:

  • The glucose molecule undergoes metabolic process called glycolysis, whenever energy is needed. This is a catabolic process that produces pyruvic acid and Adenine Triphosphate (ATP) which is the storage region for energy.

  • Glycolysis is a series of chemical processes that convert glucose into pyruvic acid by releasing ATP.

  • Pyruvic acid in turn produces ATP, by converting it into acetyl co A and citric acid.

  • Being the product of Pyruvic acid, citric acid undergoes ‘citric acid cycle’, where majority of the energy is produced through oxidation along with the electron transport chain.

  • The ATP formation involves – the glycolysis, pyruvic oxidation and citric acid cycle. They are sent to the mitochondria, where the electron transport chain occurs by which energy is released.

  • The pyruvic acid that formed in the glycolysis process converts back into glucose through a process called gluneogenesis – an anabolic process.

  • Due to the oxidation of proteins, carbohydrates and fats, energy is produced in the form of ATP. This is utilized in active ion transportation, muscle contraction, synthesis of molecules, cell division and growth.

Formation of fat from utilized glucose

By ingestion, glucose gets converted into glycogen or fat, when it is not required immediately after its formation. Liver cells store glucose in the form of glycogen to certain extent of saturation; and after reaching the saturation point, the glucose gets converted into fat in liver. These fat cells are stored as fat deposits in the liver.

Metabolism of carbohydrates is a key process that includes various chemical reactions. They ensure in providing energy to the body and formation of fat bodies, if the glucose produced is not utilized efficiently.

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