Digestion of Lipids

The modest consumption of lipids in our diet is crucial to our health. A class of complex chemical molecules, lipids, are mostly insoluble in water. Lipids are high-energy molecules made up of fats and oils that are primarily composed of carbon, hydrogen, and oxygen. Triacylglycerols, phospholipids and sterols are the three most common forms of lipids in the human body. Being insoluble in water, digestion of lipids is complex, but it takes place with the help of enzymatic action of various lipases in the small intestine. It is important to understand how lipids are absorbed in the small intestine. 

It is extremely challenging to digest and absorb lipids. In contrast to carbs and proteins, triglycerides are large molecules that are not water-soluble. Large droplets form when they are in a fluid environment like the digestive system. Before lipases enzymatically digest lipid molecules, the digestive process must first break down those massive fat droplets into smaller droplets.

Small intestine enzymatic digestion is the most important element of this process, with the mouth and stomach playing minor roles. Upon absorption into the circulatory system and distribution throughout the body, fatty digestion products need further treatment owing to their non-water-soluble nature and inability to mix well with the watery blood.

Let’s go back to the beginning to learn more about how lipids are digested in the digestive system.

How are Lipids Digested in the Mouth? 

The fat digestion process begins in the mouth with a few key processes. Chewing reduces food to a fine powder, which is subsequently dissolved in saliva for digestion. Enzymes on the tongue produce lingual lipase, which breaks down triglycerides into their individual components and begins the enzymatic digestion process.

How are Lipids Digested in the Stomach? 

The mixing and churning that occurs in the stomach assists in the dispersion of food particles and fat molecules. Another lipase, known as gastric lipase, is produced by the cells of the stomach and is involved in the enzymatic breakdown of triglycerides. The digestive enzyme lingual lipase gets functional in the stomach after food and saliva have been consumed. When these two lipases are coupled, they have a minor role in fat digestion, with the majority of enzymatic digestion taking place in the small intestine.

How are Lipids Digested in the Small Intestine? 

The bulk of dietary lipids stays undigested and aggregate in big droplets in the small intestine when the contents of the stomach reach this point. The liver produces bile, which is then stored in the gallbladder and discharged into the duodenum, the first section of the small intestine. Because of their hydrophobic and hydrophilic characteristics, bile salts are drawn to both lipids and water. This makes them very useful in breaking down large fat masses into microscopic droplets. As a result of emulsification, digestive enzymes have more surface area to work with, allowing for more lipid accessibility.

The pancreas secretes lipases into the small intestine to aid in the breakdown of triglycerides. Decomposition of triglycerides results in the formation of fatty acids and monoglycerides. Fat-soluble vitamins and cholesterol can be digested without the need for enzymes.

How are Lipids Absorbed in the Small Intestine?

There is a second step in the fat digestion process that must be completed before the fat digestion products may enter circulation and be used by cells in all parts of the body. This is yet another time where bile plays a role. When fat is digested, bile salts form micelles that help lipids reach the microvilli of intestinal cells, where they may be absorbed. The bile salts that are used to emulsify fat and produce micelles are recycled across the intestinal cell membrane.

All of these nutrients may be immediately absorbed into the bloodstream while they are in the lining of the digestive tract, but fat-soluble vitamins, long-chain fatty acids and monoglycerides are carried through a separate pathway. To make chylomicron-like transport vehicles, dietary fats and monoglycerides are recombined in intestinal cells to generate long- and short-chain triglycerides.

Digestion of a Lipid 

As triglycerides are broken down by lipases in pancreatic juice, they first become diglycerides, then 2 monoglycerides and lastly fatty acids. Tyrosine and triglyceride complexes, or lipoprotein complexes, carry the monoglycerides and fats from the digestive tract into the circulation. They are both hydrolyzed in the same way, and their components are absorbed via passing through the small intestine’s mucous membranes.

Conclusion

Digestion of lipids is a complicated process involving soluble enzymes that takes place mainly in the stomach and small intestine. Once in the circulation, the fatty acids and monoglycerides are resynthesized into triglycerides and delivered in the form of chylomicrons. They are both hydrolyzed in the same way, and their components are absorbed via passing through the small intestine’s mucous membranes.