Cholesterol is a waxy water insoluble substance produced by the animals. Plants do not produce cholesterol. Cholesterol is classified as a lipid. Chemically, it is a relatively large molecule and contains 27 carbon atoms. It consists of four rings fused together. The fused ring is called the steroid ring. Almost all the steroids in the body are derived from cholesterol. Cholesterol can exist in two forms-the free form and the bound (esterified) form.
Role of lipoproteins
Since cholesterol is insoluble in water, it cannot be transported in the circulation. Hence, proteins called lipoprotein are available to carry cholesterol in the circulation. The major lipoproteins concerned with the movement of cholesterol in the blood are LDL (low density lipoprotein) and HDL (high density lipoprotein).
Importance of cholesterol
Contrary to what people think, cholesterol is not that bad. In fact, survival depends on the presence of adequate quantities of cholesterol. Cholesterol is the precursor for the synthesis of a number of vital biological molecules. These include the bile salts needed for the digestion of fats and the synthesis of vitamin D in the skin. Cholesterol is also used to produce the all important steroid hormones including the sex hormones.
Risk for heart disease
Plenty of statistical evidence is available suggesting that increased blood cholesterol levels pose a risk for acquiring the coronary heart disease. Any increase in the blood cholesterol over 200 mgs per deciliter is a risk. When the blood cholesterol levels are high, cholesterol tends to precipitate in the arteries producing atherosclerosis. Atherosclerosis, in turn, predisposes to cardiovascular disease.
Role of HDL and LDL
HDL carries the good cholesterol because HDL carries cholesterol to the liver for disposal. HDL also stimulates the immune system to reduce the inflammatory reactions which promote plaque formation in the arteries. LDL is supposed to carry the bad cholesterol as it transports cholesterol to the tissues including the arteries. LDL, by itself, is not that bad because tissues do need cholesterol and the function of transporting the cholesterol to the tissues is performed by LDL. Problems crop up only when the LDL levels in the circulation are high.
Free radical menace
More than HDL, it is the oxidized form of LDL which is more dangerous and poses a huge risk for coronary heart disease. This oxidation is usually brought about by free radicals produced in the body. Free radicals highly reactive chemical species produced during aerobic oxidative metabolism which is a routine and normal occurrence. When free radicals increase in the body, the situation is called oxidative stress. Oxidative stress increases the oxidation of LDL. Oxidative stress is usually countered by compounds called antioxidants. Vitamin C and vitamin E are examples of antioxidants. Whenever the antioxidant level in the body goes down, more of LDL is oxidized and risk for heart disease increases. Thus, adequate quantities of vitamin C and vitamin E are important for keeping the LDL in the deoxidized and less harmful form. Increase in HDL is also useful because it is having an additional role as an antioxidant.