Cholesterol Hazard

Beware Of Cholesterol

Cholesterol is a sterol (a combination steroid and alcohol). Cholesterol is a lipid found in the cell membranes of all tissues, and it is transported in the blood plasma of all animals. Because cholesterol is synthesized by all eukaryotes, trace amounts of cholesterol are also found in membranes of plants and fungi.

The name originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix -ol for an alcohol, as researchers first identified cholesterol in solid form in gallstones by François Poulletier de la Salle in 1769. However, it is only in 1815 that chemist Eugène Chevreul named the compound "cholesterine".

Most of the cholesterol is synthesized by the body and some has dietary origin. Cholesterol is more abundant in tissues which either synthesize more or have more abundant densely-packed membranes, for example, the liver, spinal cord and brain. It plays a central role in many biochemical processes, such as the composition of cell membranes and the synthesis of steroid hormones. Cholesterol is insoluble in blood, but is transported in the circulatory system bound to one of the varieties of lipoprotein, spherical particles which have an exterior composed mainly of water-soluble proteins. The main types, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) carry cholesterol from and to the liver.

According to the lipid hypothesis, abnormally high cholesterol levels (hypercholesterolemia) and abnormal proportions of LDL and HDL are associated with cardiovascular disease by promoting atheroma development in arteries (atherosclerosis). This disease process leads to myocardial infarction (heart attack), stroke and peripheral vascular disease. As high LDL contributes to this process, it is termed "bad cholesterol", while high levels of HDL ("good cholesterol") offer a degree of protection. The balance can be redressed with exercise, a healthy diet, and sometimes medication.

Cholesterol is excreted from the liver in bile and reabsorbed from the intestines. Under certain circumstances, when more concentrated, as in the gallbladder, it crystallises and is the major constituent of most gallstones, although lecithin and bilirubin gallstones also occur less frequently.

Abnormally low levels of cholesterol are termed hypocholesterolemia. Research into the causes of this state is relatively limited, and while some studies suggest a link with depression, cancer and cerebral hemorrhage it is unclear whether the low cholesterol levels are a cause for these conditions or an epiphenomenon.

Conditions with elevated concentrations of oxidized LDL particles, especially "small dense LDL" (sdLDL) particles, are associated with atheroma formation in the walls of arteries, a condition known as atherosclerosis, which is the principal cause of coronary heart disease and other forms of cardiovascular disease. In contrast, HDL particles (especially large HDL) have been identified as a mechanism by which cholesterol and inflammatory mediators can be removed from atheroma. Increased concentrations of HDL correlate with lower rates of atheroma progressions and even regression. The relation of cholesterol to CHD, termed the "lipid hypothesis," is still hotly debated.

Elevated levels of the lipoprotein fractions, LDL, IDL and VLDL are regarded as atherogenic (prone to cause atherosclerosis). Levels of these fractions, rather than the total cholesterol level, correlate with the extent and progress of atherosclerosis. Conversely, the total cholesterol can be within normal limits, yet be made up primarily of small LDL and small HDL particles, under which conditions atheroma growth rates would still be high. In contrast, however, if LDL particle number is low (mostly large particles) and a large percentage of the HDL particles are large, then atheroma growth rates are usually low, even negative, for any given total cholesterol concentration.

Multiple human trials utilizing HMG-CoA reductase inhibitors, known as statins, have repeatedly confirmed that changing lipoprotein transport patterns from unhealthy to healthier patterns significantly lowers cardiovascular disease event rates, even for people with cholesterol values currently considered low for adults. As a result, people with a history of cardiovascular disease may derive benefit from statins irrespective of their cholesterol levels, and in men without cardiovascular disease there is benefit from lowering abnormally high cholesterol levels ("primary prevention"). Primary prevention in women is practiced only by extension of the findings in studies on men, since in women, none of the large statin trials has shown a reduction in overall mortality or in cardiovascular end points.

info : wikipedia