Digestion and Absorption of Lipids (Fat)



Daily requirement:

The daily dietary intake of fat varies from person to person, depending on the nature and socio-economic condition of the individual. However, the average daily intake of fat is 25-160gm.


Fats in the diet

The most abundant fats in the diet are:

  • Neutral fats (triglycerides)
  • Phospholipids
  • Cholesterol
  • Cholesterol esters


Digestion of fats

In young adults, the digestion of lipids occurs in the small intestine, but during mastication, serous glands of the tongue, known as Ebner's glands (present on the dorsal surface of the tongue and in the stomach), secrete lingual and gastric lipase (enzyme, which can digest fat). They are mixed with food during mastication and become more active in the stomach. 

This indicates that the digestion of lipids starts in the mouth. In the mouth and stomach lipase can digest 25-30% of all triglycerides.

We know that most digestion of fats occurs in the small intestine (duodenum) by the action of pancreatic enzymes, but before this occurs emulsification of fat is necessary.


Emulsification of fat

The emulsification of fat is the first step in fat digestion. In this process, fats are broken down into small fat globules by the action of bile acids and lecithin so that enzymes can act on the globule surface.


Action of the pancreatic enzyme in the digestion of fat

Pancreatic lipase:

This enzyme, present in pancreatic juice, hydrolyzed fat molecules into fatty acids and monoglycerides. These are windup/end products of fat digestion. Lipase removes two of the three fatty acids from the glycerol molecule while the remaining portion is attached to it from monoglycerides.


Pancreatic Phospholipase

This cleaves fatty acids from phospholipids such as phosphatidylcholine.


Cholesterol esterase

This hydrolyzed cholesterol esters leaving free cholesterol.


Digestion of fat

Fat      bile   +    agitation emulsification of fats

Emulsified fat      pancreatic lipase fatty Acids and 2 monoglycerides


Absorption of fats

The absorption of fats occurs as follows:

Micelles formation

Micelles are small (4-5 mm in diameter), spherical aggregates containing 20-30 molecules of lipids and bile salts.

The emulsified products of lipid digestion (e.g.; monoglycerides, cholesterol) from micelles with bile salts before they are absorbed.

The cholesterol and fat-soluble vitamin are located within the fat-soluble interior of the micelles.


Micelles and absorption of lipids and bile salts

  1. Micelles contain free fatty acids and 2monoglycerides. They move along the brush border of the microvilli surface allowing their lipids to diffuse across the microvilli membrane into the enterocytes.
  2. Once the micelles make contact with the microvilli, the lipids, cholesterol, and fat-soluble vitamins are removed rapidly.
  3. The monoglycerides, cholesterol, and fatty acids from the micelles enter the mucosal cells by passive diffusion. The subsequent fate of the fatty acids depends upon their size

The triglyceride and cholesterol esters are then coated with a layer of protein, cholesterol, and phospholipids to form chylomicrons, which leave the cells and enter the lymphatics.


Formation of chylomicrons by enterocytes

The triglycerides in the endoplasmic reticulum aggregate into globules along with phospholipids and cholesterol and are coated with proteins. These masses are known as chylomicrons.