Comprehensive Guide to Lipid Digestion, Absorption, and Transport

The digestion and absorption of lipids differ significantly from the mechanisms used for carbohydrates and proteins.
In carbohydrate and protein digestion, large, complex molecules are typically broken down into monomers. These monomers are then further processed by brush border enzymes and transported into cells via sodium-dependent transporters.
Lipids do not follow this simplified pathway primarily due to their unique chemical structure and solubility properties.

A lipid is composed of a glycerol backbone joined to three fatty acids.
Lipids are completely insoluble in water. This presents a physiological challenge because digestive enzymes are water-soluble.
Consequently, lipids tend to avoid contact with the aqueous environment of the intestinal lumen where enzymes are present.
Unlike carbohydrates, which begin digestion in the mouth, or proteins, which begin digestion in the stomach, lipids remain virtually unchanged until they reach the duodenum, the first section of the small intestine.

When fats enter the duodenum, they come into contact with specialized cells lining the duodenal walls known as enteroendocrine cells.
Interaction between lipids and enteroendocrine cells triggers the release of two essential hormones:
- Cholecystokinin ( $CCK$ )
- Secretin
These hormones enter the bloodstream to perform their regulatory functions.
$CCK$ specifically targets the gallbladder, stimulating it to contract and release bile.

Bile is synthesized in the liver and stored in the gallbladder until the release is triggered by $CCK$ .
Bile serves as the solution to the "oil and water" problem in the digestive tract.
The bile salts found in bile function similarly to detergents; they possess a structure that allows them to break up large fat globules and make them soluble in water.
Bile salts have an amphipathic nature, meaning they have one hydrophobic end and one hydrophilic end.

Once released into the duodenum, bile salts interact with lipids to form microscopic spheres called micelles.
The structure of a micelle includes:
- Hydrophilic glycerol heads positioned on the outside of the sphere.
- Hydrophobic fatty acid tails sequestered in the interior of the sphere.
Bile salts embed themselves into the micelle. Because bile salts have both hydrophobic and hydrophilic properties, their presence makes the entire micelle water-soluble.

Once the micelle is rendered water-soluble, it can be accessed by the enzyme lipase, which is released from the pancreas.
Lipase attacks the micelle to liberate fatty acids through a step-by-step process:
- One lipase reaction removes a fatty acid from a triglyceride, leaving a diacylglycerol.
- A subsequent lipase reaction removes another fatty acid, resulting in a monoacylglycerol.
While some references suggest lipase can remove all three fatty acids, in practice, lipase mainly accesses the two fatty acids located on the outside of the molecule.
Liberated fatty acids are more water-soluble than triglycerides because they possess an alcohol group ( $-OH$ ) at the end of the acid chain.

Fatty acids are taken up by the enterocytes through facilitated diffusion.
While there is no universal scientific consensus on the exact mechanisms or the specific proteins involved in this transport, the field currently accepts facilitated diffusion over simple diffusion.
Data shows that fatty acids are absorbed approximately $1000$ times faster than triglycerides, despite the fact that triglycerides are theoretically more membrane-soluble. This discrepancy suggests a protein-mediated (facilitated) process rather than simple diffusion through the lipid bilayer.
Malfunctions in this fat transport system are associated with specific disease states.

Once inside the cell, the fatty acids are reformed into triglycerides.
This process of resynthesis is likely performed to maintain a concentration gradient. Since lipids do not use sodium-dependent transport to move into the cell, reforming triglycerides ensures that the concentration of free fatty acids inside the cell remains low, encouraging more fatty acids to enter from the lumen.
The reformed triglycerides are packaged into chylomichrons.
A chylomicron is a ball of lipid molecules similar to a micelle, with a few key differences:
- It contains a small amount of proteins that help maintain its specific solubility.
- It does not contain bile salts.

Chylomicrons are released from the base of the enterocyte via exocytosis.
Unlike other nutrients that enter the bloodstream directly, chylomicrons are taken up into the lymphatic system through specialized vessels called lacteals.
Lacteals are extensions of the lymphatic system characterized by very leaky cell membranes, which allow the large chylomicron particles to enter.
The chylomicrons travel through the lymph to the thoracic duct.
Following a meal high in fat, the normally clear lymphatic fluid becomes thicker and whiter in appearance (similar to milk) due to the high concentration of chylomicrons.
The chylomicrons eventually reach the liver, where they are repackaged into different forms of lipoproteins.
These lipoproteins include:
- Very Low Density Lipoprotein ( $VLDL$ )
- Low Density Lipoprotein ( $LDL$ )
- High Density Lipoprotein ( $HDL$ )
These lipoproteins are released into the bloodstream, where they circulate through the vascular system to be taken up by cells that require fats.