Compounds in living organisms that are insoluble in water are referred to as lipids (fats), and make up the bulk of fat in organisms.
Lipids are important in the functioning of the human body. Apart from cholesterol, the best-known lipid, there are several other biologically important lipids in the body.
- Fatty acids, which can be used as an energy source and which are transported in the blood bound to a protein called albumin. They resemble soaps – in this way, they disrupt cell membranes. Fatty acids are thus not found at high concentrations, but are linked to glycerol to form triglycerides and phospholipids.
- Triglycerides are a type of fat that consist of three fatty acids linked to a chemical substance called glycerol. Similarly to cholesterol, they’re carried in the blood as complexes with proteins called lipoproteins.
- Phospholipids. These are more complex molecules containing two fatty acids and a phosphate end. The phosphate end is water-soluble and the lipid end is not. Phospholipids form part of the cell walls – one part blends with water and the other with the non-phosphate part of another phospholipid.
Phospholipids and triglycerides are transported in the blood in the form of lipoprotein complexes in which cholesterol is also present, either as cholesterol or as a cholesterol ester. These lipoproteins are graded according to size and lipid content.
There are six lipoproteins that are clinically important:
Increased over above
Increased over above
Decreased from above
Very-low density lipoproteins (VLDL)
Intermediate- density lipoproteins (IDL)
Low-density lipoproteins (LDL)
High-density lipoproteins (HDL)
These lipoprotein “shuttles” have specific functions. Of relevance are:
- Chylomicrons. These are large particles that carry dietary fats from the intestine through the blood circulatory system. In muscle and adipose tissue, there’s an enzyme that will remove a large portion of the triglycerides in chylomicrons for burning as energy in the muscle or storing as energy in adipose tissue (fat). The remnants of chylomicrons are taken up in the liver.
- VLDL. Very-low-density lipoprotein. This is similar to chylomicrons in that it’s rich in triglyceride. Between meals, the supply of triglyceride to organs is done by this lipoprotein and its remnants (intermediate-density lipoprotein, IDL) are also recycled to the liver. Here a portion of VLDL loses much more triglyceride, and becomes LDL.
- LDL. This is a cholesterol-rich lipoprotein that delivers cholesterol to tissues, where it’s used by growing cells that need cholesterol. Alternatively, it may be deposited in arteries when it’s in excess of the removal capacity. This has earned it the name of “bad” cholesterol.
- HDL. This is a cholesterol-rich lipoprotein that’s known as “good” cholesterol, because its main function is to remove cholesterol from cells and tissues and carry it back to the liver for excretion.
Lipoproteins are made up of lipids and protein. They all contain a mixture of lipids but in different proportions. The proteins within lipoproteins also vary. The liver and intestine use apolipoprotein B to make VLDL and chylomicrons respectively. HDL doesn’t have apolipoprotein B but contains predominantly apolipoprotein A1. LDL is a good source of cholesterol for cells and the importation happens through the binding of apolipoprotein B to a special receptor that’s expressed when the cell needs cholesterol.
The total cholesterol in the blood is the sum of the cholesterol in all the particles. In a desirable healthy state, the total is less than 5mmol/L with less than 3mmol/L in LDL and more than 1.2mmol/L and 1.4mmol/L in HDL in men and women.
Reviewed by Prof David Marais, FCP(SA), Head of Lipidology at Groote Schuur Hospital and the University of Cape Town
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