Medium chain triglycerides (MCT)
The digestion and absorption of medium chain and short chain triglycerides are similar; therefore, the discussion in this section pertains to both, although only MCTs are referred to. Gastric lipase, which has practically no activity in digesting LCT, can initiate the breakdown of M CT. Though gastric lipolysis is considered insignificant in the digestion of fat in general, it may be important when a sizable proportion oftotal fat intake is in the form ofMCT, as is recommended for certain therapeutic diets and in the digestion of milk fat. In the intestinal lumen, MCTs are rapidly hydrolyzed into monoglycerides and fatty acids by the pancreatic lipase. In contrast to the long chain monoglycerides, a considerable proportion of medium chain monoglycerides normally undergoes further hydrolysis to glycerol and fatty acids before absorption.
Both intact MCTs and their digestion products are readily dispersed in the aqueous intestinal contents; as a result, although the presence of bile salts stimulates their digestion and absorption, these processes proceed relatively rapidly, even in bile-deficiency states in which ingestion of LCT causes severe steatorrheaY .Uptake into the mucosal cell can also proceed at any stage of digestion; even intact MCTs can enter the cell when luminal hydrolysis is incomplete, as in patients with pancreatic insufficiency. Once they are inside the mucosal cell, the hydrolysis of these glycerides (mono di, and triglycerides) is completed by intracellular lipases, which have little activity with long chain glycerides. This intra cellular lipolysis is followed by rapid removal of the fatty acids from the cell. Unlike the LCT fatty acids, which are absorbed into the lymphatics, the bulk of the MCT fatty acids are directly absorbed into the portal circulation and carried to the liver, where they are readily metabolized.
Lauric acid (12 carbons), found especially in coconut oil, is usually classified as a medium chain fatty acid. In intestinal absorption, it seems to be on the borderline beh'leen the long chain and medium chain fatty acids. Although some lauric acid enters the circulation through the portal route, a considerable proportion is reesterified and enters the lymph in chylomicrons. To some extent the same is true with other fatty acids below and above lauric acid in chain length, but proportionally the portal route is the major one for fatty acids with fewer than 12 carbons and the lymphatic system for those \vith more than 12 carbons.
The therapeutic value of MCT preparations is based on their special behavior in digestion, absorption, and transportation. But more recently they have proven beneficial in the rare disorders of lipid transport in which chylomicron removal from the blood is defective.
