With the diet we introduce lipids in the form of:
triglycerides (98%),
cholesterol, phospholipids and fat-soluble vitamins (2%).
A triglyceride consists of a glycerol molecule esterified with three fatty acids.
The digestion of lipids is profoundly conditioned by their poor solubility in water, which is the fundamental element inside the digestive tract. Thus, when they are found in the aqueous environment given by saliva, gastric, intestinal, pancreatic and biliary secretions, the fats aggregate together, separating from the aqueous medium.
In the lumen of the stomach the lipids gather in macromolecules isolated from the hydrophilic component of the chyme, a bit like what happens in the broth where the lipid droplets separate from the aqueous part.
This characteristic greatly complicates the digestive processes, since the enzymes responsible for the digestion of fats, being water-soluble, are able to attack only the surface lipids, without being able to penetrate inside the drop. Their effectiveness is therefore modest.
In the stomach, gastric lipase attacks triglycerides, detaching one of the three fatty acids, resulting in the formation of free fatty acids and diglycerides. The digestive efficacy of this enzyme is greatly reduced by the hydrophobic nature of the lipids and by the strong gastric acidity. In the 2-4 hours in which the food remains in the stomach this enzyme, together with the salivary lipases, digests about 10-30% of the lipids food.
An enzyme called pancreatic lipase is poured into the duodenum (initial tract of the small intestine), which performs the same function as gastric and salivary lipase. Its digestive action is however facilitated by the bile salts present in the bile and by the slight basicity that characterizes the intestinal lumen.
Bile salts are synthesized by the liver from cholesterol and, unlike their precursor, are amphipathic molecules. The bile salts are in fact formed by a fat-soluble component and by a "other water-soluble component, complete with negative charges directed towards the outside (it is defined amphipathic or amphiphilic, a molecule containing a hydrophilic and a hydrophobic group; the most classic example is given by the phospholipids that make up the cell membrane).
After being introduced into the intestine, the bile salts enter the lipid drops with their fat-soluble portion. In this way they reduce the cohesion between the various triglycerides, greatly facilitating the digestive activity of the pancreatic lipases. At the same time, the continuous mixing of the intestinal contents, favored by the peristaltic contractions, contributes to the splitting of the lipid droplets into much smaller molecules.
The whole process, which takes the name of emulsion, is irreversible (thanks to the negative electric charge of the water-soluble component of the bile salts which rejects the various lipid molecules).
When we beat a suspension of oil and water with a fork (intestinal peristalsis), the two phases, after being temporarily associated, quickly return to separate. In the intestine, lipid re-aggregation is inhibited by bile salts and other tensioptive molecules
Thanks to this reduction of the lipid droplets, the contact surface of the lipases with the substrates increases considerably and together with it also the digestive efficacy of these enzymes. The adhesion of the lipases to the fat droplets is hindered by the layer of bile salts that surrounds it. the lipid drop; for this reason the digestion of fats requires the presence of an additional pancreatic enzyme, called colipase, which increases the adhesion of lipase to lipid droplets.
Unlike gastric lipase, pancreatic lipase detaches not one but two fatty acids from the triglyceride, with the formation of monoglycerides and free fatty acids.
The final products of lipid digestion are free fatty acids, monoglycerides and lysophospholipids deriving from the digestion of phospholipids (digested by a phospholipase present in pancreatic juice).
As these compounds are formed, they come out of the drops and collect, together with cholesterol, bile salts and lysophospholipids, in very small soluble structures, called micelles, which carry them to the enterocytes responsible for their absorption. The composition of the micelles does not include short and medium chain fatty acids which, by virtue of their greater water solubility, remain in the aqueous medium.
Bile salts are essential both for the digestion of lipids, thanks to their emulsifying properties, and for their absorption, as they intervene in the formation of lipid micelles.
If the bile were not poured into the intestine, most of the fats consumed with food would pass in the feces in an undigested form (steatorrhea)
Free cholesterol and fat-soluble vitamins are absorbed as such, without undergoing particular digestive processes (to be absorbed, esterified cholesterol is hydrolyzed to free cholesterol + fatty acid by pancreatic esterase).
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