SUMMARY: LIPID DIGESTION AND ABSORPTION
Dietary lipids consist largely of neutral fats also known as triglycerides which contain a glycerol backbone
esterified to three fatty acid side chains. Phospholipids, cholesterol, cholesterol esters, and non-esterified or
free fatty acids are also found in smaller quantities in the typical diet (Ferrier, 2017). Phospholipids and
cholesterol esters contain fatty acids while cholesterol does not. However, cholesterol is derived from fats
and exhibits similar physical and chemical properties, hence also considered a fat (Hall & Hall, 2021).
Digestion in the Stomach
Digestion of small quantities of triglycerides begins in the stomach. The enzymes that catalyze fat digestion
in the stomach are two acid lipases. One is lingual lipase, which comes from glands at the back of the tongue
and is swallowed with saliva, and the other is gastric lipase, which is secreted by the gastric mucosa (Ferrier,
2017). This accounts for only less than 10% of fat digestion (Hall & Hall, 2021).
Emulsification in the Small Intestine
The primary step in fat digestion is a process called emulsification, where fat globules are broken down into
small hydrophobic lipid droplets, thereby increasing their surface areas and allowing water-soluble digestive
enzymes to act on the globule surfaces more effectively. Emulsification is achieved by two mechanisms,
agitation by peristalsis in the stomach and the use of bile in the duodenum (Hall & Hall, 2021).
Bile is a liver secretion containing bile salts and lecithin that each have polar, water-soluble parts and fatsoluble portions. The fat-soluble portions interact with the surface of fat globules while water-soluble parts
interact with surrounding watery fluids. Lipases can only act on the surfaces of fat globules, thus necessitating
this detergent function of bile salts and lecithin to stabilize the lipid droplets produced by peristalsis and
keeping them from coalescing (Hall & Hall, 2021; Ferrier, 2017).
Degradation by Pancreatic Enzymes
Triglycerides are hydrolyzed by pancreatic lipase, producing a mixture of a mixture of 2-monoacylglyceride
and free fatty acids. Cholesterol esters and phospholipids are hydrolyzed by cholesterol ester hydrolase and
phospholipase A2, respectively, producing free fatty acids, as well as free cholesterol. An enteric lipase
produced by enterocytes is also present but is not typically needed (Hall & Hall, 2021).
Absorption by Enterocytes
The end products of fat digestion, free fatty acids, free cholesterol, and monoglycerides along with fat-soluble
vitamins form micelles by action of bile salts. Micelles carry the core of fat digestate to the brush border of
intestine where the digested lipids diffuse into the enterocytes. Short- and medium-chain fatty acids, being
water-soluble do not require micelles for intestinal absorption (Ferrier, 2017).
The monoglycerides and fatty acids are resynthesized into triglycerides in the smooth endoplasmic reticulum
of enterocytes to form chylomicrons. Apolipoproteins, primarily apolipoprotein B, are adsorbed on the outer
surfaces of chylomicrons, increasing suspension stability and preventing adherence to lymphatic wall
vessels. Chylomicrons are largely made up of triglycerides but also contain 9% phospholipids, 3%
cholesterol, and 1% apolipoproteins (Hall & Hall, 2021).
Transport in the Circulation
Chylomicrons are released from enterocytes by exocytosis into the lacteals then through the lymphatic
system into the thoracic duct and finally enters venous circulation at the juncture of the jugular and
subclavian veins. On the other hand, short- and medium-chain fatty acids enter the blood directly (Ferrier,
2017).
Chylomicrons enter the capillary beds of adipose tissue, skeletal muscle, and heart, which synthesize
lipoprotein lipase transported to the surface of capillary endothelial cells. Triglycerides are degraded to free
fatty acids and glycerol by lipoprotein lipase. The free fatty acids can then enter adjacent muscle cells and
adipocytes and are metabolized, such as for fuel or for resynthesis into triglycerides. Chylomicron remnants
use apolipoprotein-E on their surfaces to bind to receptors on endothelial cells in liver sinusoids where they
are endocytosed (Hall & Hall, 2021).
Majority of lipids in the plasma in the postabsorptive state are in the form of lipoproteins composed of
triglycerides, cholesterol, phospholipids, and protein. The four major classifications of lipoproteins include,
a) very low density lipoproteins (VLDLs) that contain high amounts of triglycerides and moderate amounts of
phospholipids and cholesterol, b) intermediate density lipoproteins (IDLs) derived from VLDLs whose
triglyceride content has been reduced, c) low density lipoproteins (LDLs) derived from IDLs whose triglyceride
content is almost completely removed, and d) high density lipoproteins (HDLs) containing high mounts of
protein with small amounts of phospholipids and cholesterol. VLDLs primarily transport triglycerides
synthesized in the liver to peripheral tissues while other lipoproteins facilitate various stages of transporting
phospholipids and cholesterol from the liver to peripheral tissues or peripheral tissues to the liver (Hall & Hall,
2021).
References:
Ferrier, R. (2017). Lippincott’s illustrated reviews: biochemistry (7th ed.). Wolters Kluwer.
Hall, J., & Hall, M. (2021). Guyton and hall textbook of medical physiology (14th ed.). Elsevier.