High-density lipoprotein Totally Explained

Everything about Hdl-cholesterol totally explained

High-density lipoproteins (HDL) form a class of lipoproteins, varying somewhat in their size (8–11 nm in diameter), that carry fatty acids and cholesterol from the body's tissues to the liver. About thirty percent of blood cholesterol is carried by HDL. It is hypothesized that HDL can remove cholesterol from atheroma within arteries and transport it back to the liver for excretion or re-utilization—which is the main reason why HDL-bound cholesterol is sometimes called "good cholesterol", or HDL-C. A high level of HDL-C seems to protect against cardiovascular diseases, and low HDL cholesterol levels (less than 40 mg/dL) increase the risk for heart disease. The liver synthesizes these lipoproteins as complexes of apolipoproteins and phospholipid, which resemble cholesterol-free flattened spherical lipoprotein particles. They are capable of picking up cholesterol, carried internally, from cells they interact with. A plasma enzyme called lecithin-cholesterol acyltransferase (LCAT) converts the free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol) which is then sequestered into the core of the lipoprotein particle eventually making the newly synthesized HDL spherical. They increase in size as they circulate through the bloodstream and incorporate more cholesterol molecules into their structure. Thus it's the concentration of large HDL particles which more accurately reflects protective action, as opposed to the concentration of total HDL particles. This ratio of large HDL to total HDL particles varies widely and is only measured by more sophisticated lipoprotein assays using either electrophoresis (the original method developed in the 1970s), or newer NMR spectroscopy methods (See also: NMR and spectroscopy), developed in the 1990s.
   In the stress response, serum amyloid A, which is one of the acute phase proteins and an apolipoprotein, is under the stimulation of cytokines (IL-1, IL-6) and cortisol produced in the adrenal cortex and carried to the damaged tissue incorporated into HDL particles. At the inflammation site, it attracts and activates leukocytes. In chronic inflammations, its deposition in the tissues manifests itself as amyloidosis.
   Men tend to have noticeably lower HDL levels, with smaller size and lower cholesterol content, than women. Men also have an increased incidence of atherosclerotic heart disease.
   Historically, beginning in the late 1970's cholesterol and lipid assays were promoted to estimate total HDL-cholesterol because such tests used to be far less expensive, by about 50 fold, than measured lipoprotein particle concentrations and subclass analysis. Over time, with continued research, decreasing costs, greater availability and wider acceptance of other "lipoprotein subclass analysis" assay methods, including NMR spectroscopy, human studies have continued to show a stronger correlation between human clinically obvious cardiovascular events and quantitatively measured large HDL-particle concentrations.

Epidemiology

Epidemiological studies have shown that high concentrations of HDL (over 60 mg/dL) have protective value against cardiovascular diseases such as ischemic stroke and myocardial infarction. Low concentrations of HDL (below 40 mg/dL for men, below 50 mg/dL for women) are a positive risk factor for these atherosclerotic diseases.
Data from the landmark Framingham Heart Study showed that for a given level of LDL, the risk of heart disease increases 10-fold as the HDL varies from high to low. Conversely, for a fixed level of HDL, the risk increases 3-fold as LDL varies from low to high. Even people with very low LDL levels are exposed to some increased risk if their HDL levels are not high enough. (HDL Cholesterol, Very Low Levels of LDL Cholesterol, and Cardiovascular Events; Philip Barter, M.D., September 27, 2007; NEJM(External Link

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Recommended range

The American Heart Association, NIH and NCEP provides a set of guidelines for male fasting HDL levels and risk for heart disease.

Level mg/dL	Level mmol/L	Interpretation
<40	<1.03	Low HDL cholesterol, heightened risk for heart disease, <50 is the value for women
40–59	1.03–1.52	Medium HDL level
>60	>1.55	High HDL level, optimal condition considered protective against heart disease

More sophisticated laboratory methods measure not just the total HDL but also the range of HDL particles, for example "lipoprotein subclass analysis", typically divided into several groups by size, instead of just the total HDL concentration as listed above. The largest groups (most functional) of HDL particles have the most protective effects. The groups of smallest particles reflect HDL particles which are not actively transporting cholesterol, thus not protective.

Raising HDL

Drugs

As of 2006, randomized clinical trials have demonstrated significant reduction of atherosclerosis progression and cardiovascular events with treatments that increase HDL-cholesterol (nicotinic acid or a fibrate).
Pharmacological therapy to increase the level of HDL cholesterol includes use of fibrates and niacin. Consumption of pharmacologic doses of niacin, an immediate release crystalline form of Vitamin B3, can increase HDL levels by 10–30%* , and is the most powerful agent currently available to increase HDL-cholesterol.
The use of statins is effective against high levels of LDL cholesterol, but it has little or no effect in raising HDL-cholesterol.

Aerobic exercise
Weight loss
Smoking cessation
Removing trans fatty acids from the diet
One to two drinks of alcohol a day:
Adding monounsaturated and polyunsaturated fats to the diet, and reducing or eliminating saturated fats.
Adding soluble fiber to diet

Other suggested lifestyle changes include:

Using supplements such as omega 3 fish oil

Limiting intake of dietary fat to 30–35% of total calories

Taking NiacinFurther Information

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