Low-density lipoprotein (LDL) refers to a class and range of lipoprotein particles, varying somewhat in their size and contents, which carry cholesterol in the blood and around the body, for use by various cells. It is commonly referred to as "bad cholesterol" due to the link between high LDL levels and cardiovascular disease.
Generally, LDL transports cholesterol and triglycerides away from cells and tissues that produce more than they use, towards cells and tissues which are taking up cholesterol and triglycerides.
Role in disease
Because LDL transports cholesterol to the arteries, increased levels are associated with atherosclerosis, and thus myocardial infarctions, strokes and peripheral vascular disease. This is why cholesterol inside LDL lipoproteins is called bad cholesterol. Still, it is not the cholesterol that is bad; it is instead how and where it is being transported, and in what amounts over time.
Increasing evidence has revealed that the concentration and size of the LDL particles more powerfully relates to the degree of atherosclerosis progression than the concentration of cholesterol contained within all the LDL particles. Having low concentrations of large LDL particles is the healthy pattern. Conversely, high concentrations of small LDL particles, despite the same total cholesterol content correlates with much faster growth of atheroma and progression of atherosclerosis.
LDL is formed as VLDL lipoproteins lose triglyceride through the action of lipoprotein lipase (LPL), and become smaller and denser containing a higher proportion of cholesterol.
A hereditary form of high LDL is familial hypercholesterolemia (FH). Increased LDL is termed hyperlipoproteinemia type II (after the dated Fredrickson classification).
The American Heart Association, NIH and NCEP provides a set of guidelines for fasting LDL levels and risk for heart disease.
||Optimal LDL cholesterol, corresponding to reduced risk for heart disease
||Near optimal LDL level
||Borderline high LDL level
||High LDL level
||Very high LDL level, increased risk of heart disease
Over time, with more clinical research, these recommended levels keep being reduced. For instance, for people with known atherosclerosis diseases, the 2004 updated American Heart Association, NIH and NCEP recomendations are for LDL levels to be lowered to less than 70 mg/dL, unspecified how much lower. For reference, from longtudinal population studies following progression of atherosclerosis related behaviors from early childhood into adulthood, it has been discovered that the usual LDL in childhood, before the development of fatty streaks is about 35 mg/dL. There is also increasing evidence and recognition of the value of more sophisticated measurements of LDL particle number (concentration) and size.
LDL Subtype Patterns
LDL particles actually vary in size and density, and studies have shown that a pattern that has more small dense LDL particles equates to a higher risk factor for Coronary Heart Disease (CHD) than does a pattern with more of the larger "fluffy" LDL particles. This correspondence has been suggested by some in the medical community as being a much more solid one than the correspondence between the LDL number measured in the standard lipid profile test. The test to determine this LDL subtype pattern is not widely available, so the common lipid profile test is used almost universally. The lipid profile does not directly measure LDL particles but instead calculates their value based on other particles via the Freidwald equation. There has also been noted a correspondence between higher [triglyceride] levels and higher levels of smaller, denser LDL particles and alternately lower [triglyceride] levels and higher levels of the larger fluffier LDL.  
Last updated: 05-13-2005 07:56:04