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Absorption and distribution of lipids

In the small intestine, the micelles are important for transporting the hydrophobic core content formed by lipids with low water solubility, for instance; cholesterol, MAGs, and  FFA, to the enterocytes for absorption. When the mixed micelles reach the unstirred layer, the complex will be disabled due to a shift in pH, leading to a release of both the core content and the compounds forming the outer layer, among these the bile salts. The mechanism of uptake and absorption of FA is unclear, but it is suggested to be affected by the chain length of the FA. FAs containing <12 C could be bound to albumin, which is a protein with affinity for FA , making them water soluble. Consequently, these FA are free to passively diffuse through the epithelial cells lining the gut lumen, further diffusing into the blood stream where they are transported to the liver via the hepatic vein. The LC FA (>12 C), which are more hydrophobic, are being transported across the cell membrane by the action of transport proteins.

Inside the cell they will be resynthesized into TAGs within the endoplasmatic reticulum, before they are transported to the Golgi apparatus where they are combined with cholesterol, phospholipids and proteins, making a lipoprotein called chylomicron (CM). Lipoproteins are transporter vehicles responsible for transporting the different amounts of cholesterol and TAGs, through the water rich environment of the blood and lymph from the origin to the site of destination. The solubility of the lipoproteins within the water rich environment of the blood stream is due to the coating of apo-lipoproteins.

These specific proteins are important both as enzymatic cofactors and as markers for receptors. The markers help the different tissues and cells recognize the specific type of lipoproteins, making sure that their core contents are delivered at the right destination. As described above, CMs are responsible for transporting lipids derived from the diet to the cells in the tissue or the liver, dependent of the need for FA.  As the TAGs are being removed and absorbed by the tissue, the CM loses its specific apolipoprotein making a CM-remnant (CM R), which is removed from the blood stream by the liver and further catabolized.

Lipids which are synthesized in the liver is packed into lipoproteins called very-low density lipoproteins (VLDL) and secreted into the peripheral circulation. As the TAGs are removed from the VLDL by the action of the lipases in the tissue, it is reduced to Intermediate-density lipoprotein (IDL) and further to low-density lipoprotein (LDL). LDL contains a small amount of TAGs, making the proportion of cholesterol large. Consequently, the main function of the LDL is to transport cholesterol to the peripheral tissue and the liver. High-density lipoprotein (HDL) is the lipoprotein which is responsible for the ‘reverse lipid transport’, transporting redundant cholesterol from the tissue back to the liver and the steroidogenic tissues for degradation and catabolization. Steriodogenic tissues beeing tissues which produces steroids within the human body. The cholesterol are, among others, used to synthesize bile acids, important for an efficient lipid digestion, as previously described.