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Low-density lipoprotein (LDL) is one of the five major groups of . These groups, from least dense to most dense, are:
(IDL), LDL, and
(HDL), all of them, particles far smaller than human cells.
Lipoproteins transfer fats through the bloodstream, then the intracellular () to all cells around the body. Lipoproteins are complex particles composed of multiple
which transport all
molecules () around the body within the water outside cells. They are typically composed of 80-100 proteins/particle (organized by a single
for LDL and the larger particles) and transporting about 3,000 to 6,000 fat molecules/particle. The fats carried include , , amounts of each vary considerably.
LDL particles pose a risk for
when they invade the
and become , since the oxidized forms are more easily retained by the proteoglycans. A complex set of biochemical reactions regulates the oxidation of LDL particles, chiefly stimulated by presence of necrotic cell debris and
in the endothelium. Increasing concentrations of LDL particles are strongly associated with increasing amounts of
within the walls of arteries over time, eventually resulting in
and triggering clots within the artery opening, or a narrowing or closing of the opening, i.e. , , and other
complications.
LDL particles (though far different from cholesterol per se) are sometimes referred to as bad cholesterol because they can transport their content of fat molecules into artery walls, attract , and thus drive . In contrast,
particles (though far different from cholesterol per se) are often called good cholesterol or healthy cholesterol because they can remove fat molecules from
in the wall of .
A hereditary form of high LDL is
(FH). High LDL is termed
(after the dated ).
Each native LDL particle enables emulsification, i.e. surrounding/packaging all fatty acids being carried, enabling these fats to move around the body within the water outside cells. Each particle contains a single
B-100 molecule (, a protein that has 4536
residues and a mass of 514 ), along with 80 to 100 additional ancillary proteins. Each LDL has a highly hydrophobic core consisting of polyunsaturated fatty acid known as
and hundreds to thousands (about 1500 commonly cited as an average) esterified and non-esterified cholesterol molecules. This core also carries varying numbers of triglycerides and other fats and is surrounded by a shell of phospholipids and unesterified cholesterol, as well as the single copy of Apo B-100. LDL particles are approximately 22 nm (0.0;in.) in diameter and have a mass of about 3 million . Since LDL particles contain a variable and changing number of fatty acid molecules, there is a distribution of LDL particle mass and size. Determining the structure of LDL has been a tough task because of its heterogeneous structure. The structure of LDL at human body temperature in native condition, with a resolution of about 16 Angstroms using cryo-electron microscopy, has been recently described.
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LDL particles are formed as
lipoproteins lose triglyceride through the action of
(LPL) and they become smaller and denser (i.e. fewer fat molecules with same protein transport shell), containing a higher proportion of cholesterol esters.
When a cell requires additional cholesterol (beyond its current internal
production pathway), it synthesizes the necessary , and inserts them into the plasma membrane. The LDL receptors diffuse freely until they associate with -. LDL particles in the bloodstream bind to these extracellular LDL receptors. The clathrin-coated pits then form vesicles that are endocytosed into the cell.
After the clathrin coat is shed, the vesicles deliver the LDL and their receptors to early , onto late endosomes to lysosomes. Here the cholesterol esters in the LDL are hydrolysed. The LDL receptors are recycled back to the plasma membrane.
lipoproteins interfere with the
system that upregulates genes required for invasive
infection. The mechanism of antagonism entails binding , to a S. aureus
pheromone, preventing signaling through its receptor. Mice deficient in apolipoprotein B are more susceptible to invasive bacterial infection.
LDL particles vary in size and density, and studies have shown that a pattern that has more small dense LDL particles, called Pattern B, equates to a higher risk factor for
(CHD) than does a pattern with more of the larger and less-dense LDL particles (Pattern A). This is thought to be because the smaller particles are more easily able to penetrate the . Pattern I, for intermediate, indicates that most LDL particles are very close in size to the normal gaps in the endothelium (26 nm). According to one study, sizes 19.0–20.5 nm were designated as pattern B and LDL sizes 20.6–22 nm were designated as pattern A.
Some in the medical community have suggested the correspondence between Pattern B and CHD is stronger than the correspondence between the LDL number measured in the standard lipid profile test. Tests to measure these LDL subtype patterns have been more expensive and not widely available, so the common lipid profile test is used more often.
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, less dense (a.k.a. "buoyant") LDL.
With continued research, decreasing cost, greater availability and wider acceptance of other lipoprotein subclass analysis assay methods, including , research studies have continued to show a stronger correlation between human clinically obvious cardiovascular events and quantitatively measured particle concentrations.[]
commonly report LDL-C: the amount of cholesterol which is estimated to be contained with LDL particles, on average, using a formula, the . In clinical context, mathematically calculated estimates of LDL-C are commonly used as an estimate of how much low density lipoproteins are driving progression of . The problem with this approach is that LDL-C values are commonly discordant with both direct measurements of LDL-particles and actual rates of atherosclerosis progression.
Direct LDL measurements are also available and better reveal individual issues but are less often promoted or done due to slightly higher costs and being available from only a
in the United States. In 2008, the
recognized direct LDL particle measurement by
as superior for assessing individual risk of cardiovascular events.
Chemical measures of lipid concentration have long been the most-used clinical measurement, not because they have the best correlation with individual outcome, but because these lab methods are less expensive and more widely available.
The lipid profile does not measure LDL particles. It only estimates them using the Friedewald equation by subtracting the amount of cholesterol associated with other particles, such as
and , assuming a prolonged fasting state, etc.:
where H is HDL cholesterol, L is LDL cholesterol, C is total cholesterol, T are triglycerides, and k is 0.20 if the quantities are measured in mg/dl and 0.45 if in mmol/l.
There are limitations to this method, most notably that samples must be obtained after a 12 to 14 h fast and that LDL-C cannot be calculated if plasma triglyceride is &4.52 mmol/L (400 mg/dL). Even at triglyceride levels 2.5 to 4.5 mmol/L, this formula is considered inaccurate. If both total cholesterol and triglyceride levels are elevated then a modified formula, with quantities in mg/dl, may be used
This formula provides an approximation with fair accuracy for most people, assuming the blood was drawn after fasting for about 14 hours or longer, but does not reveal the actual LDL particle concentration because the percentage of fat molecules within the LDL particles which are cholesterol varies, as much as 8:1 variation.
However, the concentration of LDL particles, and to a lesser extent their size, has a stronger and consistent correlation with individual clinical outcome than the amount of cholesterol within LDL particles, even if the LDL-C estimation is approximately correct. There is increasing evidence and recognition of the value of more targeted and accurate measurements of LDL particles. Specifically, LDL particle number (concentration), and to a lesser extent size, have shown slightly stronger correlations with atherosclerotic progression and cardiovascular events than obtained using chemical measures of the amount of cholesterol carried by the LDL particles. It is possible that the LDL cholesterol concentration can be low, yet LDL particle number high and cardiovascular events rates are high. Correspondingly, it is possible that LDL cholesterol concentration can be relatively high, yet LDL particle number low and cardiovascular events are also low. If LDL particle concentration is used to predict cardiovascular events, many other correlates of these clinical outcomes, such as , obesity and smoking, lose most of their predictive accuracy.
In the USA, the , , and
provide a set of guidelines for fasting LDL-Cholesterol levels, estimated or measured, and risk for . As of about 2005, these guidelines were:
Interpretation
Optimal LDL cholesterol, levels in healthy young children before onset of atherosclerotic plaque in heart artery walls
Optimal LDL cholesterol, corresponding to lower rates of progression, promoted as a target option for those known to clearly have advanced symptomatic cardiovascular disease
Optimal LDL cholesterol, corresponding to lower, but not zero, rates for symptomatic cardiovascular disease events
100 to 129
2.6 to 3.3
Near optimal LDL level, corresponding to higher rates for developing symptomatic cardiovascular disease events
130 to 159
3.3 to 4.1
Borderline high LDL level, corresponding to even higher rates for developing symptomatic cardiovascular disease events
160 to 199
4.1 to 4.9
High LDL level, corresponding to much higher rates for developing symptomatic cardiovascular disease events
Very high LDL level, corresponding to highest increased rates of symptomatic cardiovascular disease events
Over time, with more clinical research, these recommended levels keep being reduced because LDL reduction, including to abnormally low levels, was the most effective strategy for reducing cardiovascular death rates in one large , randomized clinical trial of men with
far more effective than coronary angioplasty/stenting or bypass surgery
For instance, for people with known atherosclerosis diseases, the 2004 updated , NIH and NCEP recommendations are for LDL levels to be lowered to less than 70 mg/dL, unspecified how much lower. This low level of less than 70 mg/dL was recommended for primary prevention of 'very-high risk patients' and in secondary prevention as a 'reasonable further reduction'. Lack of evidence for such a recommendation is discussed in an article in the . It should also be noted that statin drugs involved in such clinical trials have
beyond simply the reduction of LDL levels.
It has been estimated from the results of multiple human pharmacologic LDL lowering trials[] that LDL should be lowered to about 50 to reduce cardiovascular event rates to near zero. For reference, from longitudinal 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 , is about 35 mg/dL. However, all the above values refer to chemical measures of lipid/cholesterol concentration within LDL, not measured Low Density Lipoprotein concentrations, the accurate approach.
The feasibility of these figures has been questioned by sceptics, claiming that many members of the AHA and NIH are heavily associated with pharmaceutical companies giving them bias towards lowering cholesterol levels and such guidelines giving rise to increased use of cholesterol lowering medicine such as statins.[]
A study was conducted measuring the effects of guideline changes on LDL cholesterol reporting and control for diabetes visits in the US from 1995 to 2004. It was found that although LDL cholesterol reporting and control for diabetes and coronary heart disease visits improved continuously between 1995 and 2004, neither the 1998 ADA guidelines nor the 2001 ATP III guidelines increased
control for diabetes relative to coronary heart disease.
Moreover, there are publications regarding the risks of low-LDL cholesterol too.
There are several competing methods for measurement of lipoprotein particle concentrations and size. The evidence is that the NMR methodology (developed, automated & greatly reduced in costs while improving accuracy as pioneered by
and associates) results in a 22-25% reduction in cardiovascular events within just a single year, contrary to the longstanding claims by many in the medical industry that the superiority over existing methods was weak, even by statements of some proponents. Direct LDL particle measurement by
was mentioned by the
and , in a 28 March 2008 joint consensus statement, as having advantages for predicting individual risk of atherosclerosis disease events, but the statement noted that the test is less widely available (single vendor which accepts sample from world-wide), is more expensive (about $80.00 US without insurance coverage) and it is "...unclear whether LDL particle size measurements add value to measurement of LDL particle concentration", an issue which LipoScience consultants also tend to agree with (yet also has little to do with the value of measuring LDL particles vs.
vs. ). Since the later 1990s, because of the development of NMR measurements, it has been possible to clinically measure lipoprotein particles at lower cost [under $100 US (including shipping) & versus the previous costs of &$400 to &$5,000] and higher accuracy. There are also other (less expensive) homogeneous assays for LDL, however most only estimate LDL.
Using NMR, as pioneered by researcher
and the North Carolina State University academic research spinoff company , the total LDL particle concentrations, in nmol/L plasma, are typically subdivided by percentiles referenced to the 5,382 men and women, not on any lipid medications, who are participating in the MESA trial.
The LDL particle concentrations are typically categorized by percentiles, &20%, 20–50%, 50th–80th%, 80th–95% and &95% groups of the people participating and being tracked in the MESA trial, a medical research study sponsored by the United States National Heart, Lung, and Blood Institute.
MESA Percentile
LDL particles /L
Interpretation
Those with lowest rate of cardiovascular disease events & low (optimal) LDL particle concentration
1,000–1,299
Those with moderate rate of cardiovascular disease events & moderate LDL particle concentration
1,300–1,599
Those with Borderline-High rate of cardiovascular disease events & higher LDL particle concentration
1,600–2,000
Those with High rate of cardiovascular disease events and even higher LDL particle concentration
Those with very high rate of cardiovascular disease events and highest LDL particle concentration
The lowest incidence of atherosclerotic events over time occurs within the &20% group, with increased rates for the higher groups. Multiple other measures, including particle sizes, small LDL particle concentrations, total and large
particle concentrations, along with estimations of
pattern and standard cholesterol lipid measurements (for comparison of the plasma data with the estimation methods discussed above) are also routinely provided.
Markers indicating a need for LDL-C Reduction
(Per 2004 United States Government Minimum Guidelines)
If the patient's
then the patient should consider LDL-C reduction if the count in mg/dL is over...
and LDL-C reduction is indicated if the count in mg/dL is over...
High, meaning a 20% or greater risk of heart attack within 10 years, or an extreme risk factor
moderately high, meaning a 10-20% risk of heart attack within 10 years and more than 2 heart attack risk factors
moderate, meaning a 10% risk of heart attack within 10 years and more than 2 heart attack risk factors
low, meaning less than 10% risk of heart attack within 10 years and 1 or 0 heart attack risk factors
serves as the basis for the biosynthesis of many molecules, including cholesterol. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase () is an essential component and performs the first of 37 steps within the cholesterol production pathway, and present in every animal cell.
Keep in mind that LDL-C is not a measurement of actual LDL LDL-C is only an estimate (not measured from the individual's blood sample) of how much cholesterol is being transported by all LDL either a smaller concentration of large particles or a high concentration of small particles. Also keep in mind that LDL particles carry many fat molecules (typically 3,000 to 6,000 fat molecules per LDL particle); this includes cholesterol, triglycerides, phospholipids and others. Thus even if the hundreds to thousands of cholesterol molecules within an average LDL particle were measured, this does not reflect the other fat molecules or even the number of LDL particles.
reduce high levels of LDL particles by inhibiting the enzyme
in cells, the rate-limiting step of cholesterol synthesis. To compensate for the decreased cholesterol availability, synthesis of hepatic
is increased, resulting in an increased clearance of LDL particles from the blood.
reduces intestinal absorption of cholesterol, thus can reduce LDL particle concentrations when combined with statins.
PCSK9 inhibitors, in phase 3 clinical trials, by several companies, appear to be far more effective for LDL reduction than the statins, even statins at high dose.
(B3), lowers LDL by selectively inhibiting hepatic diacyglycerol acyltransferase 2, reducing
synthesis and VLDL secretion through a receptor HM74 and HM74A or GPR109A.
Several CETP inhibitors have been researched to improve HDL concentrations, but so far, despite dramatically increasing HDL-C, have not had a consistent track record in reducing atherosclerosis disease events. Some have increased mortality rates compared with placebo.
is effective at lowering cholesterol levels, but has been associated with significantly increased cancer and stroke mortality, despite lowered cholesterol levels. Other, more recently developed and tested fibrates, e.g. fenofibric acid have had a better track record and are primarily promoted for lowering VLDL particles (triglycerides), not LDL particles, yet can help some in combination with other strategies.
Some , especially delta- and gamma-tocotrienols, are being promoted as statin alternative non-prescription agents to treat high cholesterol, having been shown in vitro to have an effect. In particular, gamma-tocotrienol appears to be another HMG-CoA reductase inhibitor, and can reduce cholesterol production. As with statins, this decrease in intra-hepatic (liver) LDL levels may induce hepatic LDL receptor up-regulation, also decreasing plasma LDL levels. As always, a key issue is how benefits and complications of such agents compare with statins—molecular tools that have been analyzed in large numbers of human research and clinical trials since the mid-1970s.
are widely recognized as having a proven LDL cholesterol lowering efficacy. Current supplemental guidelines recommend doses of phytosterols in the 1.6-3.0 grams per day range (Health Canada, EFSA, ATP III,FDA) with a recent meta-analysis demonstrating an 8.8% reduction in LDL-cholesterol at a mean dose of 2.15 gram per day. However, plant sterols and stanols, if absorbed (intestinal cells generally block), greatly accelerate progression of atherosclerosis more than cholesterol delivered into the arterial walls by lipoprotein particles.[]
induces HMG-CoA reductase activity, whereas
diminishes HMG-CoA reductase activity. While glucagon production is stimulated by dietary protein ingestion, insulin production is stimulated by dietary carbohydrate.[] The rise of insulin is, in general, determined by the digestion of
and subsequent increase in serum glucose levels. In non-diabetics, glucagon levels are very low when ins however, those who have become diabetic no longer suppress glucagon output after eating.
may have similar response to taking
(lowered LDL and increased HDL) through beta-hydroxybutyrate, a
body, coupling the niacin receptor (HM74A).
Lowering the blood lipid concentration of
helps lower the concentration of small LDL particles, because fatty-acid rich VLDL particles convert in the bloodstream into small dense LDL particles.[]
The most effective approach has been minimizing fat stores located inside the
(visceral body fat) in addition to minimizing total body fat.[] , which is more metabolically active than subcutaneous fat, has been found to produce many enzymatic signals, e.g. , which increase
and circulating VLDL particle concentrations, thus both increasing LDL particle concentrations and accelerating the development of .
A 2004 pilot study of
saw that test subjects who took Garcinia Cambogia reduced their level of LDL cholesterol by an average of 14%, whilst the control group saw only a 0.8% reduction. The main ingredient in Garcinia Cambogia and other tropical fruits is .
Studies have reported the benefits of green tea in helping to reduce LDL; some studies have focused on the
qualities of the unfermented green tea leaves, others looked at green-tea compounds called catechins which are thought to decrease cholesterol absorption in the gut.
Because LDL particles appear harmless until they are within the blood vessel walls and oxidized by free radicals, it is postulated that ingesting
and minimizing free radical exposure may reduce LDL's contribution to atherosclerosis, though results are not conclusive.
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