Absorption and Assimilation of Fats

Introduction

Digestion and absorption of fats present some special challenges. As large molecules, triglycerides are not soluble in water, unlike carbohydrates and proteins. In a watery environment like the digestive tract, they like to cluster together in large droplets. Lipids are enzymatically digested by lipases mainly in the small intestine, with some involvement from the mouth and stomach. From there, lipid digested products are absorbed into the bloodstream and transported throughout the body, again requiring special handling because lipids are not water-soluble and cannot mix with the watery blood.

Lipids

Derived lipids – some derivative lipids are required for the human body such as carotenoids; in plants, they function as accessory photosynthetic pigments, as attractants for animals to perform pollination and fruit dispersal. In animals, these are converted into vitamin A (forms visual pigment rhodopsin) and steroids – These are a group of lipids having a common multiple four fused ring structure, present in the human body.

are complex processes. Some lipids are-

• Triacylglycerols or triglycerides constitute the majority of dietary lipids. Each molecule of glycerol is esterified with three fatty acids, usually long-chain fatty acids. Their energy density, 3.75 kcal/g, is twice that of carbohydrates (9 kcal/g) and amino acids. The fatty acids in them must be released to generate energy

• A phospholipid is a lipid molecule that contains two fatty acids at the sn-1 and sn-2 positions and phosphoric acid at the sn-3 position. It is the main constituent of biological membranes. This phosphate group, in turn, is connected to a hydrophilic group, such as choline, serine, or inositol through an ester bond

• Cholesterol and its esters, unlike the lipids found in plants, cholesterol, and its esters are found only in animal sources

Digestion of Lipids

Here is a brief description of the path taken by lipids through the digestive tract.

1. Lipid digestion in the mouth

During chewing, food particles are mechanically broken down into smaller ones and mixed with saliva. By producing lingual lipase, the tongue’s cells begin to digest triglycerides, breaking them down into individual fatty acids.

2. Lipid digestion in the stomach

Mixing and churning in the stomach disperses fat molecules and food particles. As part of the enzymatic digestion of triglycerides, the stomach produces another lipase called gastric lipase. When swallowed with food and saliva, the lingual lipase stays active in the stomach. However, both of these lipases together do not play a significant role in fat digestion, and the majority of enzymatic digestion takes place in the small intestine.

3. Lipid digestion in the small intestine

Most of the dietary lipids remain undigested in the small intestine after the stomach contents are swallowed. As part of the digestive process, the liver makes and stores bile, which enters the duodenum, the first section of the small intestine. Bile is both hydrophobic and hydrophilic, so they are drawn to fats and water. The result is that they work as good emulsifiers, i.e. they break down fat globules into smaller droplets. Emulsification increases the surface area on which digestion enzymes can act on lipids.

Biochemistry of Digestion and Absorption of Lipids

Emulsification, Hydrolysis and Micelle Formation

Assimilation of lipids begins with the action of bile acids, which promote emulsification. Bile acids are derived from cholesterol and have both hydrophobic and hydrophilic domains. If exposed to a lipid aggregate containing a large fraction of triglycerides, the hydrophobic part of bile acids will intercalate into the lipid and the hydrophilic part will remain on the surface. Bile acids, when coated on aggregates or droplets, aid in breaking them down into smaller pieces.

The hydrolysis of triglycerides forms monoglycerides and free fatty acids. In addition to clipping the fatty acids at positions 1 and 3 of the triglyceride, this enzyme also leaves a 2-monoglyceride after doing so. 

In the small intestine, lipase is very abundant shortly after a meal, but it only has the capability of acting on the surface of triglyceride droplets. A smaller droplet size has more surface area compared to larger droplet size, enabling more lipase molecules to get to work on a given volume of lipid.

By acting on lipase, monoglycerides and fatty acids are released and remain bound to bile acids. These fatty acids then join with other lipids to form micelles. A micelle is a small aggregate of mixed bile acids and lipids (4-8 nm diameter) contained within the intake. Upon mixing the ingesta, micelles bump into brush borders on small intestinal enterocytes, allowing lipids including monoglycerides and fatty acids, to enter epithelial cells.

Absorption of Lipids

As a result of fat digestion, fat-soluble vitamins, monoglycerides, glycerol, cholesterol, and monoglycerides need to enter the circulatory system, where they can be used by cells around the body. This process is also aided by bile. 

• Bile salts bond with the products of fat digestion to form micelles, which help absorbed fats reach the intestinal microvilli. By diffusing across the intestinal cell membrane, fat digestion products are transformed into bile salts, which are reused to dissolve fat and form micelles

• The bloodstream can directly absorb short- and medium-chain fatty acids and glycerol once these lipids have entered the intestinal cells, but larger lipids such as monoglycerides, cholesterol, and fat-soluble vitamins require more assistance to absorb and transfer into the bloodstream

• With cholesterol and fat-soluble vitamins, long-chain fatty acids and monoglycerides are converted into triglycerides within the intestinal cells, and these triglycerides are then incorporated into transport carriers called chylomicrons

• The chylomicrons are lipoproteins—large structures made up of triglyceride and cholesterol in the core and phospholipids in the outer membrane interspersed with the proteins. Since they are covered by this outer membrane, they can move in the body’s aqueous environment of blood and lymph

• From the small intestine, chylomicrons travel into the lymphatic system before reaching the bloodstream

• Dietary fats, after absorption, appear in the circulation as a chylomicron

Conclusion 

Digestion and absorption of fats are complex processes. Most occur in the stomach and small intestine, and they involve soluble enzymes, substrates with varying degrees of solubility.

In addition to triglycerides and phospholipids, dietary lipids include steroids, particularly cholesterol and its esters, fat-soluble vitamins, such as A, D, E, and K and carotenoids.

The liquid form of lipids is called fats, and the solid form is known as oils.

A very complex multi-step process is involved in the digestion and absorption of fats, starting in the stomach and ending in the small intestine. A crucial step in lipid assimilation is physicochemical remodeling and lipase-driven hydrolysis. Fat and cholesterol play a role in the pathogenicity of the cardiovascular system due to a transient marked postprandial change in plasma lipoprotein levels.