Digestion and absorption of fat

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                                    Digestion and absorption  of Fat

Keywords: Steatorrhoea, Chylomicrons, Micelle, Monoglycerides, Glycerol.

Table of contents

1.	Introduction
2.	Structure
3.	Types
4.	Digestion
5.	Absorption
6.	Infants
7.	Steatorrhoea

Introduction 

Ester of fatty acids or a mixture of such compounds present in food and living beings are known as fats. Fat often refers to triglycerides -triple esters of glycerol. The triglycerides are the main components of vegetable oils and fatty tissues in animals.

Wilhelm  Bloor of Germany defined lipids in 1916 as substances: 1. not soluble in water, but soluble in lipid solvents such as alcohol, acetone, and ether, and  2. Esters of fatty acids and potential esters of fatty acids. 

According to this definition, fat is triglycerides, diglycerides, monoglycerides, cholesterols(sterols), waxes, phospholipids, and free fatty acids. Fat is a synonym for lipid. 

Fat is one of three macronutrients in human food and carbohydrates and protein.

Types of fat:

1. Simple or Neutral fat:  One molecule of glycerol and three fatty acids form neutral fats.

2. Compound fat:  Glycerol or related substances, fatty acids, and nitrogen-containing bases form the compound fat. Often phosphate groups are present. The primary compound fats are phospholipids or phosphatides, Sphingomyelins Galactolipids.

Compound fats are               1. Integral parts of the general cell structure

                                              2. Present in large amounts in nervous tissue

                                              3. Concerned with fat transport.

Associated fats: Hydrolysis of fats yield associated fats - glycerol, fatty acids, and soaps. Soaps are salts of fatty acids.—hydrolysis of fats in alkali produces soaps. The process is saponification.

BS=bile salt.Fat, lipase, Micelles.

 Digestion of fat: Lipids are large molecules, and unlike carbohydrates and proteins,  they are not water-soluble and do not mix with the watery blood. So they like to cluster together in large droplets when they are in a fluid environment like the digestive tract. So the digestive process has to break those large droplets of fat into smaller droplets and then enzymatically digest lipid molecules using lipase enzymes.

In the mouth: Chewing breaks food into smaller particles and mixes them with saliva. Lingual lipase begins some digestion of triglycerides, cleaving individual fatty acids and glycerol. Lingual lipase remains active in the stomach and can digest fat without bile salts. 

In the stomach, mixing and churning help to disperse the digestion of food and fat molecules. Gastric lipase also contributes to triglycerides digestion. However, lingual lipase and gastric lipase contribute minimal role in fat digestion. So when food comes into the small intestine, lipids are undigested and are in large droplets.

                             

 TG=Triglyceride, DG=Diglyceride, MG=Monoglyceride, FA=Fatty acid, Gly=Glycerol

                       When food enters the duodenum, bile salts and pancreatic juice come into the duodenum through relaxation of the sphincter of Oddi. Bile salts are amphipathic molecules with hydrophobic and hydrophilic ends, so they can attach to fat on the hydrophobic end and water on the hydrophilic end. This feature makes bile salts effective emulsifiers, meaning that they can break large fat droplets into smaller droplets of fat, increasing the surface area for digestive enzymes to act on fat. The conjugated bile salts occupy the lipid/water interface and form micelles. Micelles are the aggregation of bile salts, fatty acids, and monoglycerides. They are water-soluble. They can dissolve hydrophobic compounds in their interior and make them water-soluble.

Critical micellar concentration refers to both an intrinsic property of the bile acid itself and the amount of bile acid necessary to function in micelles' spontaneous and dynamic formation. The conjugated bile salts prevent passive reabsorption in the small intestine; thus, bile acids/ salts in the small intestine are high enough to form micelles and solubilize lipids. 

The pancreatic lipases secreted from the pancreas come into the small intestine and digest triglycerides. Triglyceride is digested into fatty acids, some free glycerol, and monoglycerides. Cholesterol digested is not digested further. Fat-soluble vitamins are absorbed as such. The end products of fat digestion are fatty acids, glycerol, monoglycerides, cholesterol, and fat-soluble vitamins.

Triglyceride=TG ,Diglyceride=DG,Monoglycerid=MG,Golgi apparatus=GA

Endoplasmic reticulum=ER, Exocytic granules=Exy, Chylomicron= Chy 

Fatty acid= FA, Lipase =LP,Cholesterol=Chol.

 Absorption of fat: More than 95% of ingested fat is digested. The digested products of fatty acids are absorbed in the small intestine. Bile salts cluster around the end products of fat digestion and form micelles. Bile acid-containing micelles help lipases digest lipids and bring them near the intestinal brush border membrane, resulting in fat absorption.

The end products of fat digestion diffuse across the intestinal cell membrane, and bile salts are separated. Bile salts are reabsorbed in the terminal ileum by an active process and recycled back-'entero hepatic circulation' through the portal vein.

Once inside the enterocytes, monoglycerides and fatty acids are transported into the endoplasmic reticulum. They synthesize triglycerides—the Golgi pack triglyceride with cholesterol, lipoprotein, and other lipids to form chylomicrons. Chylomicron is a large structure with a core of triglycerides and cholesterol and an outer membrane made up of phospholipids interspersed with apolipoproteins and cholesterol. Chylomicrons contain protein, therefore, called esterified fatty acids. Chylomicrons are secreted into exocytotic vesicles from the Golgi and transported to the enterocyte's basolateral aspect. The vesicles fuse with the plasma membrane and undergo exocytosis. The chylomicrons come from the cells, enter the lymphatics, and enter into the blood via the thoracic duct.

Cholesterol is absorbed directly into the lymphatics. First, the short-chain fatty acids diffuse into the enterocytes. Then short-chain fatty acids will diffuse into the villus blood capillaries and are transported as free fatty acids (FFA) called Non esterified fatty acids(NEFA). They are bound to albumin in the blood.

Site of absorption: Mainly in the upper part of the small intestine and absorbed from the whole small intestine.

In infants' mechanism of fat, absorption is not mature, so 10-15 %  of ingested fat is excreted in feces. Therefore, the lipid is essential in the infant diet. Breast milk has about 4% fat, almost whole cow's milk. Fat provides nearly half of an infant's calories, essential for brain development.

Infants have some particular adaptations that allow them to digest fat effectively.

1. They have plenty of lingual and gastric lipases right from birth and play a more vital role in infants than in adults.

2. Breast milk contains lipase enzymes and is activated in the baby's small intestine.  

Steatorrhoea: In this condition, the stool is bulky, foul-smelling, pale, and greasy due to increased fecal fat up to 50gm/day. Steatorrhoea is due to improper digestion and absorption of fat.  

Internal link:

https://totalphysology20.blogspot.com/2021/03/small intestine. We must  know

https://totalphysology20.blogspot.com/2021/05 /bile- we must  know

https://totalphysology20.blogspot.com/2021/05 /Gall bladder- we must  know

https://totalphysology20.blogspot.com/2021/05/basic concept of digestion-we must know

External link:

https://en.m.wikipedia.org>wiki

https://www.sciencedirect.com>topics

https://www.ncbi.nim.nih.gov

https://www.betterhealth.vic.gov.au>

https://www.healthline.com>nutrition

https://www.nestle.in .nutrients >di

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Read more: 

The basic concept of digestion

Digestion of carbohydrate

Digestion of protein

Dietary fiber