However, while carbohydrates and proteins dissolve easily in digestive fluids, lipids are not only insoluble, but tend to stick together to form large clumps. In this way the digestive action of the lipases is severely limited.
In order to be digested and absorbed, fats must therefore be transformed into water-soluble aggregates. This process, called emulsification, occurs by the action of bile, a substance produced by the liver and poured into the duodenum from the gallbladder.
REMEMBER: the activity of pancreatic lipases is enhanced by the presence of bile
After undergoing the emulsion process, the lipids are attacked by specific enzymes produced by the pancreas (lipase, phospholipase and cholesterol esterase) which separate the glycerol from the fatty acids.
The short and medium chain fatty acids (10-12 carbon atoms) are absorbed directly in the small intestine and reach the liver where they are rapidly metabolized.
The long chain fatty acids are absorbed by the enterocytes (the cells of the intestine) and reesterified to triglycerides. They are then associated with cholesterol giving rise to particular lipoproteins called chylomicrons.
The chylomicrons are released into the circulation and reach the peripheral tissues which retain only fatty acids and glycerol.
The residual chylomicrons, poor in triglycerides and very rich in cholesterol, are captured and incorporated by the liver which metabolizes the residual cholesterol and uses the few remaining triglycerides for metabolic processes.
ENDOGENOUS SYNTHESIS OF TRIGLYCERIDES: hepatocytes (liver cells) are able to synthesize triglycerides starting from different precursors (glucose and carbonaceous skeleton of amino acids).
After synthesizing the triglycerides, the liver releases them into the circulation by incorporating them into protein molecules. In this way, very low density lipoproteins or VLDL are formed, very similar in composition to chylomicrons.
REMEMBER: Chylomicrons are secreted by enterocytes while VLDLs are produced by hepatocytes
Peripheral tissue cells retain fatty acids progressively depleting VLDL of triglycerides. This is how IDLs, also known as medium density lipoproteins, are formed. VLDLs can also donate triglycerides directly to HDL (high density lipoprotein) and receive cholesterol in return.
At the end of these processes, the IDLs are further depleted of triglycerides and become LDL, lipoproteins with a very high cholesterol content.
The LDL are picked up by the tissues which, in case of need, take up the cholesterol.
If cholesterol is present in excess, it is taken up by hepatocytes which pour it into the bile and inhibit its endogenous production. This is made possible by HDL (high density lipoproteins) which allow the so-called reverse transport of cholesterol (while VLDL and LDL transport it from the liver to the tissues, the HDL transport it from the tissues to the liver).
It is no coincidence that HDLs are also known as good cholesterol and the higher their content in the blood, the lower the risk of developing cardiovascular diseases.
If, due to an excess of LDL or a reduced function of the receptors, the hepatocytes are unable to metabolize the excess cholesterol, they remain in circulation longer, increasing the plasma concentration of cholesterol and predisposing the subject to various diseases of cardiovascular origin.
Beta oxidation and biosynthesis of fatty acids
