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Digestion Of Food

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Food is a substance taken from outside that nourishes the body, builds tissues, and supplies energy. Essential components of our food are mainly carbohydrates, proteins, and fats. Vitamins and minerals are also needed in small quantities. Water plays a crucial role in metabolic processes and also prohibits dehydration of the body. Our body cannot utilize biomolecules in food in their original form hence they have to be converted into simple substances in the digestive system. 

Digestion is known as the process of conversion of complex food substances into simple absorbable forms. The digestive system carries out the latter through mechanical and biochemical methods. Absorption is the process by which nutrients pass from the alimentary canal into blood and lymph through its mucosa. The absorbed food materials are carried by blood and lymph. The lymph is finally moved to the blood circulation. The blood transports absorbed food materials to different tissues where food materials are utilized for various activities like energy, growth, and repair. This process is called assimilation.

Digestion of Food

Digestion is the conversion of nondiffusible food elements into diffusible constituents. It includes mechanical digestion (mastication, chewing, etc.) and chemical digestion (enzymatic action on food). After eating, it takes about 6 to 8 hours to move through the stomach and small intestine. After that food enters the large intestine (colon) for further digestion, water absorption, and finally undigested food elimination. It takes about 36 hours for food to pass through the complete colon. Depending on the individual the whole process from swallowing food to the time it leaves the body as feces – takes about 2-5 days.

Digestion in Oral Cavity

Digestion of Carbohydrates: The digestion of carbohydrates begins to digest minute the food hits the mouth. The saliva secreted by the salivary glands dampens food as it’s chewed. Saliva releases an enzyme known as amylase, which initiates the process of breakdown of the sugars in the carbohydrates being eaten.

Digestion of Lipids: The digestion of lipids starts in the oral cavity through exposure to lingual lipases, which are secreted by glands present in the tongue to initiate the process of digesting triglycerides. Digestion proceeds in the stomach through the effects of both lingual and gastric enzymes.

Action of Saliva: In the oral cavity, the food is mixed with saliva which is slightly acidic (pH 6.8). It contains electrolytes (Na+, K+, Cl, HCO3) and enzymes salivary amylase (ptyalin) and lysozyme. About 30 percent of starch is hydrolyzed in the oral cavity.

Action of Saliva


The enzyme present in saliva i.e. Lysozyme acts as an antibacterial enzyme.

Digestion in Stomach

Digestion of Proteins: Protein is essential for the growth and replenishment of body cells and tissues. Protein digestion takes place in the stomach with the help of protease and pepsin enzymes, which is responsible for the breakdown of proteins into amino acids. The process is aided by the HCl(Hydrochloric acid) present in the stomach.

Digestion of Lipids: Lipids are water-insoluble, organic compounds consisting of fatty acids. The most familiar instance of Lipids is fats. The insolubility of lipids makes it complicated to digest and absorb fats.

Fats are broken down with the help of bile juice, which comprises bile salts. The broken molecules are then acted upon by pancreatic lipase which is the main fat-absorbing enzyme after that pancreatic lipase breaks down the fats into tiny molecules of free fatty acids and monoglycerides.

Action of Gastric Juice: The stomach normally stocks the food for 4-5 hours. The stomach’s gastric glands secrete gastric juice. It includes Hydrochloric acid and proenzymes such as pepsinogen and prorenin. Hydrochloric acid is known to maintain a strongly acidic pH of about 1.5 to 2.5 in the stomach. The pH of infants’ gastric juice is 5.0. HCI destroys bacteria and other injurious organisms that may be present along with food. HCI converts pepsinogen and prorenin into pepsin and rennin respectively. 

Once the pepsin formation occurs it changes pepsinogen into pepsin. Such activation is called an autocatalytic reaction. Pepsin and rennin are absent in invertebrates. Gastric juice is absolutely mixed with food till it becomes a semifluid mass known as chyme. Various reactions are summarized below.

Action of Gastric Juices


Digestion in Small Intestine

Digestion of Carbohydrates: The three carbohydrate products absorbed by the small intestine are glucose, galactose, and fructose. The major part of carbohydrate digestion occurs in the small intestine. It is known as the primary site of the digestion of carbohydrates. Pancreatic alpha-amylase is the chief carbohydrate-digesting enzyme.

Digestion of Lipids: Lipids are hydrophobic in nature and are badly soluble in the watery environment of the digestive tract. Lipids digestion begins with lingual and gastric lipases, but only 10% of ingested lipids got digested by this. The last lipids are broken down in the small intestine.

Digestion of Proteins: Protein digestion initiates in the stomach with the help of pepsin, which splits protein into amino acids and oligopeptides. The process of digestion of protein gets finished in the small intestine with a brush border and pancreatic enzymes. They break the oligopeptides into amino acids, dipeptides, and tripeptides.

Digestion of Nucleic acids: The food containing nucleic acids (DNA/RNA) is digested in the small intestine with the help of pancreatic enzymes and the enzymes produced by the small intestine itself. Ribonuclease and deoxyribonuclease i.e. the pancreatic enzymes break down RNA and DNA into smaller nucleic acids.

Action of Bile

  • Neutralization of HCl: Its sodium bicarbonate neutralizes the HCl of chyme(semifluid food found in the stomach).
  • Emulsification: Sodium glycocholate and sodium taurocholate break down the large fat droplets into smaller ones.
  • Absorption of fat and fat-soluble vitamins: Its salts aid in the absorption of fat-soluble vitamins and fatty acids and glycerol (A, D, E, and K).
  • Excretion: Bile pigments (bilirubin and biliverdin) are known as excretory products.
  • Prevention of decomposition: Because bile is alkaline, it inhibits the growth of germs in food, which delays food degradation.
  • Stimulation of peristalsis: Bile increases the peristalsis of the intestine.
  • Activation of Lipase: Bile consists of no enzyme but activates the enzyme lipase.

Actions of Pancreatic Juice

Pancreatic juice (alkaline, pH 8.4) contains pancreatic enzymes which act as follows.

Action of Pancreatic Juice


The above reactions initiate the conversion of starch into Maltose with help of amylase. After that Enterokinase converts Trypsinogen into Trypsin Trypsin converts proteins into Dipeptides and also Trypsin converts Chymotrypsinogen into Chymotrypsin. Chymotrypsin converts peptones into Dipeptides. Trypsin converts Procarboxypeptidase into Carboxypeptidase. Carboxypeptidase converts proteoses into Dipeptides. Trypsin converts Proelastase into Elastase. Elastase turns elastin into Dipeptides. Pancreatic amylase converts polysaccharides (Starch) into Disaccharides. Nucleases present in the pancreatic juice, act on nucleic acids and form nucleotides and nucleosides.

Actions of Intestinal Juice or Succus Entericus

Intestinal juice (alkaline pH 7.6) contains intestinal enzymes which act as follows: Enterokinase converts Trypsinogen into Trypsin. Aminopeptides convert large peptides to dipeptides. The enzyme Dipeptidases then converts Dipeptides into amino acids. Maltase converts Maltose into glucose. Isomaltase helps in the conversion of Isomaltose into glucose. Sucrase turns sucrose into glucose and fructose. Nucleotidases help in the conversion of Nucleotides into nucleosides and nucleosides into the nitrogenous base and pentose sugar.

Action of Intestinal Juice


Digestion of Fats

Lipases are fat-digesting enzymes that are soluble in water, but they are not soluble in fats and oils. Lipase is activated by the bile. Though, oils and fats cannot dissolve in water.  Saliva contains no lipase. The stomach also lacks any fat-emulsifying agent. However, gastric juice consists small amount of gastric lipase which converts some fats into monoglycerides and fatty acids. Fat is mainly digested in the small intestine. Bile salts of the bile break fat droplets into several small ones by decreasing the surface tension of fat droplets. This process is called emulsification. Lipase is found in pancreatic juice and intestinal juice. The primary enzyme for the breakdown of fat is pancreatic lipase. Moreover, intestinal lipase is also helpful in the digestion of fat. The pancreatic lipase converts emulsified fats (triglyceride fats), firstly into diglycerides and then into monoglycerides, releasing a fatty acid at each step. The intestinal lipase converts extra fats into monoglycerides and fatty acids. Now all fats are converted into fatty acids, glycerol, and monoglycerides. A white or pale yellow liquid taken up by the lacteals (lymph capillaries) from the intestine during digestion is known as the Chyle.

Digestion of Fat


Digestion in the Large Intestine

The digestion activity is less in the large intestine. In this part, the bacterial action on the leftover food particles occurs. The large intestine is where water, nutrients, and some medications are absorbed. The undigested and unabsorbed waste materials known as fecal matter, are moved to the rectum, from where it is abolished through the anus.

Hormonal Control of Digestion

Hormone Source Target Organ Action
Gastrin Pyloric stomach Stomach Stimulates gastric glands to secrete and release gastric juice. It also stimulates gastric mobility.
Enterogastrone(=Gastric Inhibitory Peptide-GIP) Duodenum Stomach Inhibits gastric secretion and mobility(slows gastric contraction).
Secretin-first hormone discovered by scientists Duodenum




Release bicarbonates in the pancreatic juice. Increases secretion of bile. Decreases gastric secretion and mortality.
Cholecystokinin-Pancreozymin(CCK-Pz) Small Intestine Gall bladder and pancreas Contracts the gall bladder to release bile. Stimulates pancreas to secrete and release digestive enzymes in the pancreatic juice.
Duocrinin Duodenum Duodenum Stimulates the Brunner’s glands to release mucus and enzymes into the intestinal juice.
Enterocrinin Small Intestine Small Intestine Stimulates the crypts of Lieberkuhn to release enzymes into the intestinal juice.
Vasoactive Intestinal Peptide(VIP) Small Intestine Small Intestine and stomach Dilates peripheral blood vessels of the gut. Inhibits gastric acid secretion.
Villikinin Small Intestine Small Intestine Accelerates movements of villi.
Somatostatin(SS) Delta cells of islets of Langerhans of the Suppressespancreas. Pancreas, Gastrointestinal tract Suppresses the release of hormones from the digestive tract.
Pancreatic Polypeptide(PP) Pancreatic Polypeptide cells Pancreas Inhibits the release of pancreatic juice from the pancreas.

Mechanism of Digestive Enzymes

Enzymes are needed for the proper functioning of the digestive system. Digestive Enzymes are a group of enzymes that is responsible for the breaking down of polymeric micromolecules into their simpler parts, in conductive to facilitate their absorption into the cells of body.

There are various types of Digestive enzymes, but the main three types produced in the pancreas are as follows:-

  • Amylase
  • Protease
  • Lipase


The production of amylase is mostly found in the pancreas, but also in the salivary glands and small intestine. Amylase breaks carbohydrates(starch and sugars) into simple sugars(glucose). The beginning of the chemical digestion of food is by salivary amylase. This is the main reason that it is crucial for people to take time while eating and chewing food. The ptlylin (one type of amylase), is made in the salivary glands and begins to act on starches while the food is still present in the mouth. Even after swallowing food, it remains active.

The production of pancreatic amylase is in the pancreas and moved to the small intestine. In this part, it abides to split down starch molecules into sugars, which are digested into glucose by other enzymes. Afterward, it is absorbed into the body’s blood circulation through the small intestine’s wall.


The production site of protease is the stomach, pancreas, and small intestine. It is responsible for the main breakdown of proteins and polypeptides from plants and animals and for proline dipeptides from gluten and casein. The release of protease enzyme is by the pancreas into the proximal small intestine, where it got mixed by gastric secretions and broken down into amino acids, the building blocks of protein, that got absorbed and used by the body.


The lipase enzyme breaks fatty acids into fats and oils. The production site of lipase is the pancreas and small intestine. Amylase and protease do a good job of breaking down carbohydrates and proteins, the other enzyme known to be breakdown of fats, oils, and triglycerides is lipase. Lipase is essential for the whole digestion of fats into their smaller components of fatty acids.

FAQs on Digestion of Food

Question 1: What is Digestion?


The conversion of complex food substances into simpler substances that can be absorbed through chemical and mechanical methods is known as digestion.

Question 2: How is food digested?


The conversion of food into their simplest forms such as glucose, amino acids, or fatty acids is done by the digestive system. The food which is broken down is then absorbed in the bloodstream from the small intestine and the nutrients are transported to each cell in the body.

Question 3: What are the steps of digestion?


The steps of digestion are as follows:

  • Ingestion
  • Propulsion
  • Mechanical digestion
  • Chemical digestion
  • Absorption
  • Defecation.

Question 4: Why is digestion important?


Digestion is crucial for breaking down food into nutrients, which the body uses for energy, growth, and repairing of cells.

Question 5: Where does digestion start and end?


Digestion initiates in the mouth and ends at the anus.

Question 6: What food takes the longest to digest?


Beef, bacon, whole milk hard cheese and nuts take long time to digest. These food take almost 4 hours to digest.

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Last Updated : 10 Jan, 2023
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