Medium chain fatty acids are defined as those with chain lengths of between 6 and 12 carbon atoms. Medium chain triglycerides are therefore three fatty acids of this length bonded through an ester bond to a glycerol molecule. Compared to longer chain fatty acids, medium chain triglycerides have some interesting properties that makes them nutritionally relevant. Natural sources of medium chain triglycerides are limited, but they are found in coconut oil. However, synthetic oils can be created from coconut oil and medium chain triglycerides are available as supplements. Medium chain triglycerides are rapidly absorbed from the small intestine because the hydrolysed fatty acids do not require bile for micelle formation prior to absorption, due to their relatively water soluble nature. Following absorption the medium chain triglycerides then travel straight to the liver where they are used as a source of energy, and in this way provide a form of quickly usable energy, more similar to carbohydrate than longer chain fatty acids.
Because medium chain triglycerides do not require bile for their absorption, the hormonal reaction to their ingestion may be different to that of longer chain fatty acids. For example, researchers1 have compared long chain triglycerides with medium chain triglycerides with regard their ability to cause stimulation of cholecystokinin release and therefore cause contraction of the gallbladder. Ingestion of a 60 nmol solutions of long chain fatty acids (corn oil increased plasma cholecystokinin (from 2.8 to 6.5 pmol/L) and decreased the gallbladder volume from 33.4 to 13.2 cm2. In contrast ingestion of 60 nmol solution of medium chain triglycerides caused no significant effects on plasma cholecystokinin or gall bladder volume. These results show that because medium chain triglycerides are absorbed in a different way to long chain triglycerides, the hormonal response to their presence differs. Cholecystokinin is a potent stimulator of gallbladder contraction and without the need for bile, medium chain triglycerides do not stimulate cholecystokinin release.
Therefore the cholecystokinin release stimulated by fatty acids is dependent on the chain length. This has a wider relevance than just to gallbladder contraction, because cholecystokinin has other physiological effects during digestion. In particular cholecystokinin causes feedback inhibition of the pyloric sphincter, which slows the release of chyme from the stomach and therefore has a moderating effect on digestive speed. Only long chain triglycerides containing fatty acid of over 12 carbon atoms in length may therefore be able to slow digestive rates. This has implications for appetite regulation as the delay of the chyme leaving the stomach distends the walls of the stomach and induces satiety. Interestingly, the amount of cholecystokinin released following long chain triglyceride ingestion is dependent on the length of the intestine exposed to the subsequent digested fatty acids. As medium chain triglycerides are absorbed rapidly in the proximal part of the ileum, the release of cholecystokinin may reflect a lack of exposure of the intestine to fatty acids.
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