Compounds in living organisms but insoluble in water are referred to as lipids and make up the bulk of fat in the organism. Lipids are important in the functioning of the body. The best known lipid related to health is cholesterol.
Besides cholesterol here are the other important ones:
• Fatty acids. These can be used as an energy source and are transported in the blood bound to a protein called albumin. They resemble soaps and this disrupts cell membranes. Fatty acids are thus not found at high concentrations but are linked to glycerol to form triglycerides and phospholipids.
• Triglycerides consist of three fatty acids linked to a chemical substance called glycerol. Similarly to cholesterol, they are carried in the blood as complexes with proteins called lipoproteins.
• Phospholipids. These are more complex molecules containing 2 fatty acids and a phosphate end. The phosphate-containing end is water-soluble and the lipid end is not. Phospholipids form part of the cell walls as 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 fatty acid-linked cholesterol termed cholesterol ester.
These lipoproteins are graded according to size and lipid content, and there are six of them that are clinically important:
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 is an enzyme that will remove of 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 is rich in triglyceride. Between meals the supply of triglyceride to organs is done by this lipoprotein and its remnants (IDL) are also recycled to the liver. But a portion of VLDL loses much more triglyceride and becomes LDL.
• LDL. Is a cholesterol-rich lipoprotein that delivers cholesterol to tissues, where it is used by growing cells that need cholesterol or may be deposited in arteries when in excess of the removal capacity. This has earned it the name of “bad cholesterol”.
• HDL. This is a cholesterol-rich lipoprotein. This is 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 mixtures of lipid together with protein and all contain a mixture of lipids but in different proportions. The proteins within lipoproteins vary. The liver and intestine use apolipoprotein B to make VLDL and chylomicrons respectively. HDL does not 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 is 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 3 mmol/L in LDL and more than 1.2mmol/L and 1.4 mmol/L in HDL in men and women. When fasting chylomicrons are absent and most of the triglyceride in the blood is in VLDL.
This figure is ideally <1.7mmol/L. After a fatty meal the triglyceride in the blood typically increases by 1 to 2 mmol/L at about 2-4 hours. This makes the plasma or serum turbid.