What is HDL cholesterol? Definition

HDL stands for high-density lipoprotein and it is a type of cholesterol in combination with soluble conjugated proteins in the blood stream. It makes up 20% of the cholesterol in the body.

HDL cholesterol is the good cholesterol because it protects your heart from the effects of any excess LDL cholesterol you may have.

Cholesterol is defined as an essential fat with an animal sterol (organic molecules) found in different organs, body tissues and fluids for use by the body cells.

It is present in every single cell of the body but is most abundant in the brain, nervous tissue, skin, and adrenal glands.

About 80% of the cholesterol is produced in the liver and the rest of the 20% comes from the foods that you eat. Small amounts are made by the lining of the small intestine and the individual cells of the body.

Since cholesterol is soft and waxy in nature, it cannot dissolve in blood.  It, therefore, needs a medium so that it can move around in the blood stream freely and uniformly.

It combines with the soluble proteins in the blood and piggy rides on it to flow freely in the blood. This combination is called lipoproteins – “lipo” standing for fat. Practically, all serum lipids are bound in this similar manner.

HDL particles contain mostly A-I and A-II proteins, while low-density lipoproteins (LDL) contain B-100 proteins. The type of protein determines the function of the lipoprotein particle and is, therefore, significant.

In LDL (low-density protein), the protein fraction is less dense and there is more of cholesterol in the LDL particle.

Desirable, good and very good levels 

Desirable   — Below 40 mg/dl or Below 1 mmol/L
Good          — 40 to 59 mg/dl or 1 to 1.5 mmol/L
Very good — 60 mg/dl and above or Above 1.5 mmol/L

HDL cholesterol structure

The lipoproteins, HDL, LDL and VLDL are thus variable amounts of free and esterified cholesterol, phospholipids, and triglycerides bound to the protein.

Maintaining good levels of all the cholesterols in the blood is essential for a healthy lipid profile.

HDL Cholesterol has the smallest particle size amongst the lipoprotein particles. HDL carries about 25 to 33% of all the cholesterol in the blood.

It is called high density because the protein fraction is denser than the cholesterol fraction. HDL is made up of 45-50% protein. By weight, high-density lipoprotein particles consist of 20 percent cholesterol and 50 percent protein.

Good blood levels of HDL cholesterol remove excess of LDL, the bad cholesterol from the blood. It even removes the atheromatous particles and carries them both to the liver for removal from the body. This greatly reduces the risk of heart disease.

In contrast, the LDL cholesterol is the bad cholesterol, which increases the risk of heart disease and stroke when its levels rise above the normal range.

Benefits and Functions of HDL: Clinical significance

HDL cholesterol is the lipoprotein, which carries the LDL cholesterol and the phospholipids from the blood to the liver and is called the good cholesterol for all the right reasons.

1. This is so because HDL promotes the uptake of the LDL cholesterol from the tissues, including the vascular wall, and the Atheroma (Cholesterol plaques deposited on the arterial walls) to the liver for excretion into the small intestines through the bile either directly as cholesterol or as bile acids. This is called reverse cholesterol transport and is indicative of the beneficial effect of HDL.

2. HDL along with its protein and the cholesterol content has an inhibitive action on the oxidation of LDL cholesterol. Oxidized LDL is the main culprit that induces the formation of atherosclerosis. HDL prevents the LDL from getting oxidized, thereby preventing atherosclerosis.

3. HDL prevents platelet aggregation and coagulation. By preventing platelet aggregation, HDL helps in the formation of clots in the blood.

4. HDL may help reduce inflammation.

5. Activation of the endothelium. The endothelium is the innermost lining of all the blood vessels of the circulatory system. HDL helps to maintain a healthy endothelium, which thereby performs important functions such as maintaining blood pressure, forming new blood vessels, preventing atherosclerosis and coagulation of blood.
6. According to a 1999 article in “Circulation,” a journal of the American Heart Association, good HDL levels stop the progression of the LDL plaque build-up; it can actually cause plaque recession and change the cellular composition of the pre-existing plaques. However, increasing HDL levels will not clear of the whole plaque.