Lipoproteins - types and their role in body

Cholesterol is a fat that is made in the body. Our body needs some cholesterol to work properly. In order to get to all cells, cholesterol needs to travel through the bloodstream. But because cholesterol is a fat, it separates from the blood similar to the way that oil separates from water.

 In order to prevent this from happening, proteins form a shell around the cholesterol, creating a "cholesterol complex." It is then released into the bloodstream and travels to where it needs to go. A protein that is linked to cholesterol to form this cholesterol complex is called a "lipoprotein."

Types of Lipoproteins:

There are three main types of cholesterol that are tested

 Low density lipoprotein (LDL)

 High density lipoprotein (HDL), and

Very low density lipoprotein (VLDL). 

Triglycerides, another type of lipid in the blood, are also tested.

Low Density Lipoproteins:

Low density lipoproteins, also referred to as LDL, is known as the "bad cholesterol". LDLs are produced by the liver and carry cholesterol and other lipids (fats) from the liver to different areas of the body, like muscles, tissues, organs, and the heart.

Why LDL-C Known as Bad cholesterol:

LDLs transport cholesterol from liver and deliver it to the tissues that need it. But if a lot LDLs left over after all body tissues have been taken the required amount, the LDLs will "let go" of the extra cholesterol while traveling through blood. This LDL cholesterol can build up on the walls of arteries (blood vessels that carry blood from the heart to other parts of the body). This build up of cholesterol is called plaque and over time, it can cause narrowing or hardening of arteries. This condition is called atherosclerosis.

It is very important to keep LDL levels low, because high levels of LDL indicate that there is much more cholesterol in the blood stream than necessary, therefore increasing your risk of heart disease.

LDLs are calculated by using an equation involving total cholesterol, triglycerides, and HDLs--all of which are measured directly in the blood:

LDL = TC – (triglycerides/5) + HDL)

According to the National Cholesterol Education Program:

·         LDL levels less than 100 mg/dL ( 2.6 mmol/L) are considered optimal.

·         LDL levels between 100 – 129 mg/dL (2.6–3.34 mmol/L) are considered near or above optimal.

·         LDL levels between 130 – 159 mg/dL (3.36–4.13 mmol/L) are considered borderline high.

·         LDL levels between 160 – 189 mg/dL (4.14 - 4.90 mmol/L) are considered high.

·         LDL levels at or above 190 mg/dL (4.91 mmol/L) is considered very high.

High Density Lipoproteins:

High density lipoprotein, also known as HDL, is considered the "good" cholesterol. HDL is produced by the liver to carry cholesterol and other lipids (fats) from tissues and organs back to the liver for recycling or degradation. High levels of HDL are a good indicator of a healthy heart, because less cholesterol is available in your blood to attach to blood vessels and cause plaque formation.

HDL is a Good cholesterol:

HDL-C  is "good" because it picks up the extra cholesterol that was dropped off by the LDLs   and brings it to the liver. Liver can repackage it to use it later or simply get rid of it. Because HDL cholesterol removes excess "bad" cholesterol (LDL), it is good to have high levels of HDL and low levels of LDL.

According to the National Cholesterol Education Program:

·         Any HDL level above more than 60 mg/dL (1.56 mmol/L) is considered high.

·         An acceptable HDL range is between 40- 60 mg/dL (1.04–1.56 mmol/L).

·         An undesirable level of HDL is any level below 40 mg/dL (1.04 mmol/L). In this case, low HDL levels may help to contribute to heart disease.

Very Low Density Lipoproteins:

Very low density lipoproteins, or VLDL, are lipoproteins that carry cholesterol from the liver to organs and tissues in the body. They are formed by a combination of cholesterol and triglycerides. VLDLs are heavier than low density lipoproteins, and are also associated with atherosclerosis and heart disease. This number is obtained by dividing your triglyceride levels by 5.

Purpose of LDL:

VLDL is made in the liver in response to a high-carbohydrate meal. The liver converts the extra carbohydrates into fat triglycerides and puts them into VLDLs to be transported to fat cells and muscle within the body. The liver also puts some cholesterol into the VLDL. A VLDL particle is rather large, carrying a lot of triglycerides relative to the amount of cholesterol.

IDL

Once a VLDL delivers its triglycerides to fat cells or muscle, it is called an intermediate density lipoprotein (IDL). This IDL can return to the liver with its cholesterol so that the liver can repackage it to use it later or simply get rid of it. But in most cases, the IDLs remain in the blood and go through another transformation where they lose most of their remaining triglycerides. At this point, the IDLs are almost all cholesterol and are now known as LDL . About three-quarters of total cholesterol in the blood is contained within LDL particles.