GI Lipid Digestion

  • The major dietary lipids include triglycerides, phospholipids, cholesterol, and cholesterol esters with triglycerides accounting for the vast majority of calories. Digestive enzymes can only access lipids when they come into contact with aqueous fluid; however, lipids tend to self-aggregate into large droplets with little exposure to the aqueous environment. For sufficient digestion to occur, lipids must first be emulsified, a process which breaks up large lipid droplets into much smaller units, thus significantly expanding the lipid-water surface area.
  • Emulsification is the process of disaggregation of large lipid droplets into smaller droplets that have a higher surface area-to-volume ratio. Emulsification aids in lipid digestion by opening up more surface area where digestive enzymes can attack the droplet. The primary physiological emulsifiers are bile acids and the phospholipid lecithin, both of which are synthesized by the liver and secreted into the bile. These molecules in combination with physical agitation by the GI tract significantly emulsifies large lipid droplets.
  • Overview
    • Digestion of lipids primarily occurs in the small intestine with small contributions by the stomach and salivary glands. The churning action of gastric motility partially emulsifies ingested lipids with the aid of dietary proteins which can act as weak emulsifiers.
    • In addition, the stomach and salivary glands secrete low levels of lipases which begin the process of lipid digestion although this is not physiologically critical.
  • Small Intestine
    • The primary location for lipid digestion is the small intestine where strong emulsifiers synthesized by the liver are present together with strong lipid-digesting enzymes synthesized by the pancreas. Bile secreted into the small intestine provides bile acids and phospholipids, strong emulsifiers which help break lipid droplets into smaller sizes whose large exposed surface area is then exploited by pancreatic enzymes. The exocrine pancreas synthesizes a number of enzymes which digest lipids into smaller chemical constituents.
    • Pancreatic Lipase digests triglycerides into components fatty acids and 2-monoglycerides
    • Pancreatic Cholesterol Esterase digests cholesterol esters into component cholesterol and fatty acid
    • Pancreatic Phospholipase A2 digests phospholipids into their component head groups and fatty acid
    • Importantly, the derived amphipathic fatty acids from all these digestive reactions can also contribute to emulsification of undigested lipids