Digestion is a process by which the food particles are broken down into small enough compounds for absorption.  It is important to our body because it processes the food particles, extract nutrients and excrete waste.

A short overview of human diet that takes part in digestion

Carbohydrates: Almost all carbohydrates of human diet are polysaccharides or disaccharides that are combinations of monosaccharides. Starches are the principal polysaccharides of human diet, which present particularly in the grains (wheat, rice). Sucrose known popularly as cane sugar and lactose present in the milk are the main disaccharides of usual human diet. Digestions of all polysaccharides and disaccharides occur in human digestive tract, which finally converted into monosaccharides such as glucose, fructose and galactose.

Proteins: The human dietary proteins are formed by the long chain of amino acids bound together by peptide linkages. The principal sources of proteins in human diet are meat, fish, eggs, cereals and cheese. Within the digestive tract, all dietary proteins are digesting by the action of proteolytic enzymes that split them into their constituent amino acids.

Fats: Almost the entire fat portion of human diet are neutral fats, also called triglycerides found mainly in food of animal origin and much less so of plant origin. The usual human diets also contain small amounts of other fats including phospholipids and cholesterol esters. Fats digestions occur in the digestive tract with the help of digestive enzymes and yielding free fatty acids and monoglycerides.

Digestion time

Digestion time actually depends upon various factors such as the age of the person, natures of the foods, size of the meal. In a healthy adult, the whole digestion process can take range from 24 to 72 hours.

Process of digestion

1. Digestion in the mouth: The digestion process starts in the mouth where the food particles are chewed and mixed with saliva, which contains digestive enzyme salivary amylase secreted by the three pairs of salivary glands including parotid, submandibular and sublingual gland. This digestive enzyme acts on starch (polysaccharide) and chemically breaks into maltose (disaccharide), maltotriose (trisaccharide) and alpha-limit dextrins (4-8 molecules glucose). But food particles remain in the mouth only a short time, and approximately 3 to 5% of all starches are chemically broken down by this time. After undergoing chewing and starch digestion, the food particles are converted into a soft rounded mass called bolus. The mouth propels this food bolus into pharynx and then the pharynx propels into esophagus by the process of swallowing, and finally the esophagus transports the food bolus into stomach by a series of peristaltic contraction where the next process of digestion occurs.

2. Digestion in the stomach:

When the food bolus enters into the stomach, it gradually begins to mixes with gastric juices (stomach secretions that contain mainly hydrochloric acid, pepsin, mucus and intrinsic factor) by the help of mixing contraction of stomach.

Starch digestion continues in the stomach for as long as an hour until the food bolus is mixed with gastric juices. Then the hydrochloric acid of the gastric juices blocked the activity of the salivary amylase. Nevertheless, on average, 30 to 40% of starches will have been broken down mainly into maltose before the food bolus becomes completely mixed with gastric juices.

When food bolus mixes with hydrochloric acid, the pH of the stomach food gradually decreased.  At pH less than 5, the pepsin of gastric juice only begins the process of proteins digestion and breaks it into proteoses, peptones and polypeptides. Pepsin is most active at a pH of 2 to 3. After undergoing mixing of foods with gastric juices, and partial digestion of starches and proteins, the foods are converted into a murky, milky semifluid mixure known as chyme that transports into the small intestine by the peristaltic activity of stomach.

3. Digestion in the small intestine (duodenum, jejunum and ileum):

The majority of digestions occur in the small intestine and the process of digestion is completed here. When chyme enters into the small intestine, it further mixes with three different types of secretions by the help of small intestinal mixing movement and passes downwards by peristaltic movement and completes the digestion process:

(i) Bile, the secretion of liver, stored in the gallbladder and released into the duodenum through the bile duct that contain bile acids and lecithin. The first step of fats digestion begins in the duodenum with the help of this bile acids and lecithin, which breaks the fat globules into smaller sizes fat known as emulsified fat.

(ii) Pancreatic secretions formed by the pancreas that reach into the duodenum through the pancreatic duct, which contain digestive enzymes such as lipase, amylase, trypsin, chymotrypsin and carboxypolypeptidase.

The pancreatic lipase present in enormous amounts in pancreatic secretions that digest all the emulsified fats and produces the end products of fats such as fatty acids and monoglycerides.

The pancreatic amylase digests remaining starches and converts into maltose (disaccharide), maltotriose (trisaccharide) and alpha-limit dextrins (4-8 molecules glucose).

The pancreatic proteolytic enzymes trypsin and chymotrypsin act on partially breakdown products of protein from stomach and split into small ploypeptides.

Finally, carboxypolypeptidase of pancreatic enzymes splits small ploypeptides into tripeptides, dipeptides and amino acids.

(iii) Intestinal secretions made by the intestinal glands in the small intestine, which contain digestive enzymes such as lactase, sucrase, maltase, alpha-dextrinase, multiple peptidases and a very minute amount of lipase.

The last digestion of carbohydrates in the small intestine is achieved by the intestinal digestive enzymes lactase, sucrase, maltase and alpha-dextrinase. Lactase splits lactose into glucose and galactose, sucrase splits sucrose into glucose and fructose, maltase splits maltose and maltotriose into glucose, and alpha-dextrinase splits alpha-limit dextrins into glucose.

The final digestion of proteins in the small intestine is achieved by the action of multiple peptidases. Two important peptidases such as aminopeptidase and dipeptidase split the remaining ploypeptides into tripeptides, dipeptides and amino acids within the intestinal lumen. All the tripeptides, dipeptides and amino acids are transported into the interior of the small intestinal epithelial cells. Finally, within the epithelial cells, all of the last tripeptides and dipeptides are digested into the single amino acids with the help of intracellular peptidases.

Intestinal lipase is unimportant to digest fats because fats are completely digested by the pancreatic lipase with the help of bile.

4. Digestion in the colon

All dietary fibers escape digestion in the small intestine and enter the colon in an essentially unchanged state. Normally numerous bacteria are present in the colon. These bacteria are capable of digesting the dietary fibers and producing acetate, propionate and butyrate (all are short chain fatty acids) that absorbed by the colonic epithelial cells.

 Flow chart of food digestion

 Digestion of food

 

Therefore, digestion of carbohydrates produces its basic components glucose, fructose and galactose, proteins produces amino acids, and fats produces fatty acids and monoglycerides. Finally, these basic components of food produced from digestion process are easily absorbed into blood stream through the intestinal epithelium.