The majority of the 98.6 million American adults who have been given the news that they have high cholesterol generally understand they are at risk for heart disease. According to the American Heart Association, adults with cholesterol levels over 200 mg/dl have one of the primary risk factors for heart disease--the others being obesity, diabetes, sedentary lifestyle, high blood pressure and cigarette smoking. Not all cholesterol is bad, however. In fact, our bodies need a certain amount of cholesterol to properly function.
What Is Cholesterol?
Cholesterol is a waxy-like substance that occurs almost exclusively in animals--it rarely appears in plants. A certain amount of cholesterol is necessary for the proper maintenance of cells, fluid regulation and hormone production. The rest circulates through the blood and accumulates on the interior walls causing plaques that can calcify and either occlude an artery or break off causing occlusion further downstream.
Cholesterol's Function
Cholesterol has several functions in the body. In addition to forming the part of the cellular walls, the cholesterol content controls its membrane function. The ratio of cholesterol to polar lipids affects the cells' stability, permeability and protein mobility. Cell walls with high ratios of cholesterol are the most stable and have low permeability to liquids, functioning like protective barriers. Membranes like those of the mitochondria have low cholesterol ratios and are permeable.
Hormone Production
Cholesterol also plays an important role in steroid production. It is stored in the adrenals, testes and ovaries that is converted into steroid hormones. They produce male androgens, female estrogrens as well as the adrenal corticoids such as cortisol, corticosterone and aldosterone. In the liver, cholesterol is converted to bile acids, important carboxylic acids that aid in digestion.
Metabolism
The ideal concentration of total cholesterol circulating in the blood is less than 200 mg/dl. Any higher puts the person at risk for heart disease. What controls the concentration of cholesterol in the blood is a constant give and take between what the liver makes and the cholesterol we take in through the diet.
Cholesterol from the diet circulates through the blood to the liver in the form of large lipoproteins--lipids surrounded by proteins. The liver secretes low density lipoproteins (VLDL). VLDLs are broken down in the blood into cholesterol and other compounds. Part of the VLDL is converted into fat tissue as low density lipoproteins (LDL, or the bad type of cholesterol). Another form of cholesterol called high density liporoteins (HDL, or the good type of cholesterol) is transported to the liver where it is broken down and excreted, carrying along with it LDL.
