Cholesterol, Triglycerides, Lipopproteins: Words we hear almost every day, but what do they mean and why should we be interested in them?

Cholesterol is an integral constituent of all our tissues (cell membranes). When pure, it is a white soapy crystalline substance. The name cholesterol comes from the Greek khome - bile, gall + sterols - hard, solid + ol (so-called because it was first found in gall stones). It is found in highest concentration in the brain and other organs. It occurs as cholesterol and cholesterol ester, its storage form.

Triglycerides are a series of substances we commonly call fats. They are composed of three fatty acids attached to a glycerol. Depending primarily on the nature of the fatty acids (saturated, monounsaturated, or polyunsaturated), the triglycerides are liquid at different temperatures, with the saturated triglycerides solid at room temperature. Triglycerides are used in our body for fuel and energy storage. Consuming saturated triglycerides tends to increase the concentration of some lipoproteins.

Neither cholesterol nor triglycerides can be transported in blood alone because they are not soluble (in contrast to sugars and most proteins). They circulate in our blood then as lipoproteins, spherical vehicles of surface protein, phospholipids, and cholesterol with an inner “core” of triglycerides and cholesterol esters. Most of our blood cholesterol is ester cholesterol. The smaller the lipoprotein, the heavier it is: the names in part reflect their densities.

The large lipoproteins contain primarily triglycerides. In most people (before breakfast) triglycerides are carried in very low density lipoproteins (VLDL) and most cholesterol in low density lipoprotein (LDL).

Several risk factors have been defined for development of coronary heart disease. These include cigarette smoking, high blood pressure, diabetes, a family history of heart attack, male gender, and abnormalities in circulating lipoproteins. See figure for definitions. Those lipoproteins associated with an increased risk when present in increased numbers are LDL, LP(a), IDL and VLDL. These lipoproteins collectively transport the cholesterol not in HDL, or non-HDL cholesterol. HDL is usually considered anti-atherogenic, thus higher levels are considered advantageous. Low levels of HDL increase the risk of heart disease and stroke. Chylomicrons are generated in the intestine after a meal. They transport absorbed triglycerides and cholesterol into the blood compartment. The cholesterol not in HDL (non-HDL cholesterol) and it’s ratio to HDL best summarize the risk associated with lipid disorders.

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