Cholesterol
Cholesterol Information
Cholesterol is a fatty lipid found in the body tissues and blood plasma of vertebrates. It can be found in large concentrations in the liver, spinal cord, and brain.
The name originates from the Greek chole- (bile) and stereos (solid) - researchers first identified cholesterol in solid form in gallstones.
Cholesterol is a steroid that is primarily synthesized from acetyl-CoA in the liver, although other sites include the intestines, adrenal glands and reproductive organs.
Cholesterol is an important component of the membranes of cells, providing stability. It is the major precursor for the synthesis of vitamin D3, of the various steroid hormones, including cortisol, cortisone, and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Cholesterol is excreted from the liver in the form of a secretion known as bile; it sometimes crystallizes in the gall bladder to form gallstones.
Mostly insoluble in water, it travels in the blood stream in the form of lipoproteins. Low-density lipoprotein (LDL) carries cholesterol from the liver and intestines to the body cells, while high-density lipoprotein (HDL) carries it back for excretion.
Cholesterol often forms plaque deposits in the walls of arteries, a condition known as atherosclerosis, which is a major contributor to coronary heart disease when the buildup is such that it inhibits blood flow to the heart. The ratio of HDL to LDL is a commonly performed test, and the higher the ratio of these two, the lower the risk of heart disease.
Cholesterol Guidelines
The American Heart Association provides a set of guidelines for total (fasting) blood cholesterol levels and risk for heart disease:
- Less than 200 mg/dL
- Desirable level corresponding to lower risk for heart disease
- Between 200 and 239 mg/dl
- Borderline high risk
- 240 mg/dl or greater
- High risk
However, as today's testing methods determine LDL ("bad") and HDL ("good") cholesterol separetely, this simplistic view has become somewhat outdated. The desirable LDL level is considered to be 75-130 mg/dL, and a ratio of total cholesterol to HDL-arguably the most useful measure-of less than 5 is thought to be healthy.
The Mechanics of Cholesterol Transport and Cholesterol Effects on the Human Body
In the bloodstream, cholesterol and triglycerides (TG) circulate as part of lipoprotein complexes. With ultracentrifugation, these complexes separate into very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) fractions that contain apolipoprotein B-100 (ApoB-100) and high-density lipoprotein (HDL) fractions.
Cholesterol and TG synthesized in the liver are incorporated into VLDL and secreted into the circulation for delivery to peripheral tissues. TG are removed by the action of lipases, and in a series of steps, the modified VLDL is transformed first into IDL and then into cholesterol-rich LDL. IDL and LDL are removed from the circulation mainly by high affinity ApoB/E receptors, which are expressed to the greatest extent on liver cells. HDL is hypothesized to participate in the reverse transport of cholesterol from tissues back to the liver.
Epidemiologic, experimental, and clinical studies have established that high LDL cholesterol (LDL-C), low HDL cholesterol (HDL-C), and high plasma TG promote human atherosclerosis and are risk factors for developing cardiovascular disease. In contrast, higher levels of HDL-C are associated with decreased cardiovascular risk.
Like LDL, cholesterol-enriched triglyceride-rich lipoproteins, including VLDL, IDL, and remnants, can also promote atherosclerosis. Elevated plasma triglycerides are frequently found with low HDL-C levels and small LDL particles, as well as in association with non-lipid metabolic risk factors for coronary heart disease (CHD). As such, total plasma TG has not consistently been shown to be an independent risk factor for CHD. Furthermore, the independent effect of raising HDL or lowering TG on the risk of coronary and cardiovascular morbidity and mortality has not been determined.
