The principal dietary carbohydrates are polysaccharides, disaccharides, and monosaccharides. Starches (glucose, polymers) and their derivatives are the only polysaccharides that are digested to any degree in the human intestinal tract. Digestion of carbohydrate starts from the mouth itself, where starch is attacked by salivary α amylase, since the pH of this enzyme is lesser than the acidic gastric juices its action is inhibited when it enters the stomach. When starch enters the small intestine, both the salivary and pancreatic α amylase act on the ingested polysaccharides and break them into simpler forms. Finally, the brush border lining of the small intestine secrete a certain enzyme which break polysaccharides into the simplest form, which is absorbed and used in the body. It should be noted here that deficiency of one or more enzyme secreted by the brush border layer of the small intestine may cause diarrhea, bloating, and flatulence after ingestion of too much sugar (as it happens in the case of excessive sweet intake).


The process of utilization of the simplest form of carbohydrate in the body is called glycolysis. In more scientific terms the oxidation of glucose at the cellular level is called glycolysis. Glucose is oxidized as either pyruvate (in presence of oxygen) or lactate (when oxygen supply is limited). Conversion of 1 molecule (actually, it is mole if we talk more precisely) of glucose requires 2 ATP (Adenosine Tri Phosphate, which is like energy coin of the body) and gives 8 ATP of energy.

Glucose + 2 ATP——> pyruvate + 8 ATP

                        (energy)                                         (energy)

The NADH generated during oxidation is used to fuel mitochondrial (mitochondria is the powerhouse of the cell) ATP synthesis via oxidative phosphorylation. This is how each and every cell of the body gets energy.


Protein is made up of amino acids linked with a peptide bond or linkage. Contrary to carbohydrate-protein digestion starts from the stomach, where pepsins cleave some of the peptide linkage (which is called denaturation of protein). Like many other enzymes concerned with protein digestion, pepsin is secreted in the form of inactive precursors (pro-enzyme) and activated in the gastrointestinal tract. The pepsin precursors are called pepsinogen and are activated by gastric acid.

Pepsin hydrolyzes the bonds between two amino acids of all types (that is coming from all food source) so the products of peptic digestion is polypeptides of very diverse sizes. Because pepsins have a pH optimum of 1.6 to 3.2, their action is terminated when the gastric content is mixed with alkaline pancreatic juice in the first and second part of stomach.

In the small intestine, polypeptides formed by digestion in the stomach are further digested by the powerful proteolytic enzymes of the pancreas and intestinal mucosa.


Real digestion starts in the small intestine although a small amount of lipase is secreted by Ebner’s glands on the tongue, and by the stomach, these digestive actions are not significant, as almost no real breakdown of fat occurs until the fats reach the duodenum in the form of gastric chyme.

Lipase and other digestive juices brake down the fat molecule into fatty acid. Fat is digested by the digestive enzyme called lipase, which is secreted by the pancreas. Lipase divides complex lipid molecules into simple fatty acid molecules. However, because fat does not dissolve in water, the fat molecules enter the duodenum in a congealed mass, which makes it impossible for the pancreatic lipase enzymes to attack them since lipase is a water-soluble enzyme and can only attack the surface of the fat molecules. To overcome this problem the digestive system uses a substance called bile, produced in the liver but stored in the gallbladder, which enters the duodenum via the bile duct. Bile emulsifies fats – meaning, it disperses them into small droplets which then become suspended in the watery contents of the digestive tract. Emulsification allows lipase to gain easier access to the fat molecules and thus accelerates their breakdown and digestion.

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