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Absorption and Metabolism of Fatty Acids

Free fatty acids are present in the small intestine as a result of ingestion with the feed or because they are released from ingested lipid molecules under the action of pancreatic lipases in the small intestine. 

According to the chain length, free fatty acids can be divided in three classes: short chain fatty acids (SCFA, with 2-4 carbon atoms), medium chain fatty acids (MCFA, with 6-10 carbon atoms) and long chain fatty acids (LCFA, with more than 10 carbon atoms).

Because of the fact that fats are not hydrosoluble, emulsification is needed in the small intestine to obtain a good action of the pancreatic lipases and to obtain a good absorption. This is achieved by the combination of gastro-intestinal motility (reduction of particle size) and bile salts. Free fatty acids and other products of fat digestion form so-called micelles together with bile salts. 
The micelles migrate from the intestinal lumen towards the absorptive surface of the small intestine. This way, products of fat digestion (a.o. free fatty acids) are absorbed in the enterocyte.
For SCFA’s and MCFA’s however, emulsification is -due to their relative good hydrosolubility- not essential for absorption in the enterocyte. They can be absorbed directly in the enterocyte.

After entering the enterocyte, metabolism of the free fatty acids differs also with the chain length.  
SCFA’s and MCFA’s enter directly the portal vein and are sent straight to the liver where they are, for the most part, burned as fuel. 
LCFA’s are reconverted in the enterocyte into intact fat molecules (triglycerids). Then, the triglycerids are packed together with phospholipids, cholesterol and proteins into a so-called chylomicron. These chylomicrons are absorbed in the lymphatic system, pass the liver, and then enter the bloodstream. During their transport trough the blood, fatty acids and other products of fat degradation are released from the chylomicrons and are used by the muscles as energy source or stored by the adipose tissue. The chylomicrons get smaller and smaller, until there is little left of them. At this time they are picked up by the liver, broken apart, and used to produce energy or, if needed, repacked into other chylomicrons and sent back into the bloodstream to be distributed throughout the body.

As mentioned, an important part of the fatty acids are used for energy production. The energy producing structures present inside each cell are called the mitochondria. Mitochondria are encapsulated in two membranes, which normally require special enzymes to transport nutrients through them. MCFA’s are unique in that way that they can easily permeate both membranes of the mitochondria without the need of enzymes. Thus, they provide the cell with a quick and efficient source of energy. Longer chain fatty acids demand special enzymes to pull them through the double membrane, and the energy production process is much slower and tackling enzyme reserves.

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