Life and health depend on cholesterol
Cholesterol is a form of lipid or fat. Technically lipids are water- insoluble biomolecules that comprise fatty acids, waxes, fat-soluble vitamins (such as vitamins A, D, E, and K) monoglycerides, diglycerides, triglycerides, phospholipids, and sterols (which brings us back to cholesterol).
When we measure cholesterol in the blood we really measure different sized particles called lipoproteins. We also use terms such as good and bad cholesterol even though cholesterol is really nothing more than just cholesterol. The various labels are actually being applied to the different sizes (densities) of lipoproteins and, rather than tilt at windmills, I too will use the accepted terms (though begrudgingly).
Because cholesterol is a lipid it can’t just enter the watery environment of the bloodstream and end up anywhere useful so the body produces taxicabs out of various proteins and packs the cholesterol into these cabs and sends them on their way. These taxis are called lipoproteins and come in various sizes that are an inverse relationship to density such that big [size] equals low [density].
A lipoprotein is made from one or more apolipoproteins which are made by genes of the same name (i.e. the apoE gene makes apolipoprotein E, while the apoB gene makes apolipoprotein B, etc). Apolipoprotein synthesis in the intestine is regulated principally by the fat content of the diet while apolipoprotein synthesis in the liver is controlled by a host of factors, including diet, hormones (insulin, glucagon, thyroxin, estrogens, androgens), alcohol consumption and various drugs (statins, niacin, and fibric acids).
Apolipoprotein B (apo B48 and apo B100) – form low density lipoprotein particles. Apolipoproteins A, C, D, H, and L are associated with high density lipoprotein. Lipoprotein (a) combines with B-100 to change some key properties of LDL. Finally apoE is needed to form intermediate lipoprotein.
Just to give you an idea of how complex the taxicab and its passengers are let’s look at Low Density Lipoprotein (LDL cholesterol) which is a single apolipoprotein B-100. However, there are also 80-100 other proteins and a core consisting of polyunsaturated fatty acid, roughly 1,500 cholesterol molecules, and some triglycerides and other fats. That core is further rolled up in a shell of phospholipids and unesterifed cholesterol.
Lipoprotein by Density
Lipoproteins come in different sizes, densities and compositions. The least dense and largest is the chylomicron which transport exogenous (dietary) cholesterol from our intestines to the liver. All other forms of lipoprotein transport endogenous (made right at home in our own bodies) forms of cholesterol.
Lipoproteins from largest to smallest (low density to high density):
Chylomicron – transports triglycerides, phospholipids, cholesterol, and proteins. Chylomicrons transport dietary lipids from the intestines to other locations in the body.
Very Low Density Lipoprotein (VLDL) – is also a triglyceride-rich lipoprotein and does much the same job as chylomicrons, but the lipid source is endogenous (produced by the body not the diet). VLDL are large and secreted by the liver.
Intermediate Density Lipoprotein (IDL) – the “intermediate” between low-density and very-low-density. Also an intermediate in the sense that it forms when high density and very low density lipoproteins are degraded by lipoprotein lipase. Some of the IDL particles are rapidly cleared out by the liver, most remain in circulation and are degraded by hepatic lipase into low density lipoproteins (LDL).
Low Density Lipoprotein (LDL) – also delivers fat molecules to the cells and is instrumental in foiling Staphylococcus aureus infection by disrupting its signaling ability.
Lipoprotein (a) – Combines with the ApoB-100 to form a new type of LDL particle with a greatly increased tendency towards clotting.
High Density Lipoprotein (HDL) – removes excess fat molecules (cholesterol, phospholipids, and triglycerides) from cells. HDLs can be further subdivided into five more groups but we won’t!
Dyslipidemia – is an abnormal amount of lipids in the blood and can refer to either high or low levels.
Familial Hyperlipidemia (FH) – an inherited or genetic form of dyslipidemia. Hyper of course meaning lipid levels are elevated. Apo(a) is much more prevalent in this population.