Cholesterol is a white, waxy substance found naturally throughout the body, including the blood, and is essential for good health. Technically cholesterol is not a fat but a closely related substance. It belongs to a class of compounds called sterols. But it's often called a blood fat, or lipid.
Like any wax or fat, cholesterol does not dissolve in water. So in the blood, cholesterol is carried around in an envelope of protein. This cholesterol-protein package, called a lipoprotein (of which there are several kinds), does stay soluble in the watery serum portion of the blood.
That's why, when doctors measure cholesterol, they often do it in two ways: First, they measure total cholesterol - all the cholesterol in your blood. And second, they measure each of the different lipoproteins in your blood that, together, make up your total cholesterol. Each amount is measured as milligrams.
Cholesterol does serve a useful purpose. It is needed by the body to make hormones, including sex hormones and adrenal hormones, and to make vitamin D. It is also needed to produce bile acids, which aid fat absorption in the small intestine, and to build cells, especially the fatty membranes that enclose cells and the structures within cells.
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