Lipid digestion

The average daily intake of dietary lipid is about 70 g. Most ingested fat is triglyceride, some is in the form of phospholipids and sterols. All of these compounds have the difficulty of immiscibility in water that would otherwise hinder their absorption.

Therefore, before absorption there is emulsification of the ingested fat - mixing with water by the physical action of the stomach - and the formation of micelles - very small particles with the hydrophilic portion of the lipid molecule on the outside. This is facilitated by the action of bile acids which reduce surface tension. Micelles have the advantage of having a large surface area to volume ratio permitting greater activity of enzymes at their surface and intimate association with the lipophilic absorptive mucosa.

The next stage of digestion is dependent on the molecule being absorbed and largely occurs in the small intestine:

• triglycerides:
  • broken down by pancreatic lipases
  • lipases originate from the tongue, gastric mucosa and most importantly, from the pancreas
  • lipase requires a co-lipase product of the pancreas to prevent bile salt interference with action of lipase
  • triglyceride digested to free fatty acids and monoglycerides
• phospholipids: digested by phospholipase A2 from pancreas
• cholesterol esters and sterols: digested by non-specific lipase from pancreas

Deficient bile salt production or delivery to the small intestine results in steatorrhoea.