Cholesterol - HDL & LDL

This post is from an old email that was written by dear Balaji on February 24th the Tuesday, 2004 @ 7:16:53 PM as a follow up to a discussion about cholesterol in phone...
--------------------------------------------------
Guys,

This is what I have understood from the information I have collected.

What is a cholesterol?
cholesterol is a white, powdery fatty substance being produced by animal cells (not plants). In human beings, most of it is produced in the liver cells. It is also synthesized in other parts of the body but, not in a significant amount. So, liver cells are the manufacturing site of cholesterol for all practical purposes.

What is its function?

• Repairing cell membranes
• Aids manufacturing Vitamin D on the skin's surface
• Production of important hormones like estrogen and testosterone
• (possibly) helping cell connections that are important for memory and learning.

Then, what the heck are HDL, LDL, VLDL?
In order for the cholesterol to do the above mentioned functions, it needs to be transported to various parts of the body. Wonder who could be the transporter? It is none other than our blood.... But, cholesterol and blood are like "kerosene and water" they do not mix with each other which brings us to the point that we need somebody who can act like an agent to transport cholesterol.

Lipoproteins
The agents are nothing but the lipoproteins. Note here that lipoproteins are not meant to transport cholestrol, but also other fatty substances (lipids) like triglycerides too. But, lets see the cholesterol lipoproteins first... The core of the lipoprotein molecule contains the fatty substance (cholesterol, triglycerides) with the outer core of proteins called "apolipoprotein".

Types of Cholesterol carrying Lipoproteins
These lipoproteins are of various types based on the thickness of cholesterol core.

• Very Low density Lipoproteins (VLDL):

The inner core contains 10-15% of total cholesterol and some triglycerides. As the VHDL molecule circulates in the blood, it deposits triglycerides on the bidy tissue and eventually becomes low density lipoprotein (just the cholesterol core with negligible triglycerides).

• Low density Lipoproteins (LDL):

LDL are formed from VLDLs. (smaller in size tha VLDLs)

• High density lipoprotein (HDL):

HDL much smaller in size than LDL. Remember density = mass / volume. Lower the volume, higher the density for the same mass.

Why LDL is the villain and HDL is not?
Most of the cholesterol (75%) synthesized in the liver is carried by the LDLs rather than HDL. LDL in its natural form a harmless soul. But, it is very vulnerable to transform itself into a "villain". The transformation is the "oxidation process". Before going to oxidation, lets see what LDL and HDL actually do ?

Since LDL carries the major amount of cholesterol, abundance of LDL increases the amount of cholesterol floating in the blood. HDL actually does the cleaning process like a "good" boy. It transports the cholesterol from various organs and tissues back to the liver for "recycling". Actually the cholesterol transported back to liver by HDL are transformed into bile juices some of which are excreted via faecal matter and some of which are used for other vital functions (sort of recycling operation). so, having a low HDL and high LDL means "abundance of cholesterol" floating on the blood. Lets "turn" back to the "oxidation".

Sidebar: Oxidation: what is that??
Oxidants are unstable "oxygen-free" radicals. The onset of a chemical reaction with an oxidant is called oxidation.

LDL - villainy
The LDL molecule is prone to oxidation because of its chemical composition. The excess cholesterol (due to LDL) tend to deposit inside the arterial walls (blood vessel) which triggers the release of oxidants by the cell walls. The LDL gets modified and thus the villain is "born".

In reponse to the formation of oxidized LDL, the body releases various immune factors aimed at protecting the damaged arterial walls. Unfortunately, these immune factors are release in excess which causes inflammation and promote further injury. Also, an enzyme produced by the arterial cell wall called "phospholipase A2" binds to oxidized LDL which causes "plaque" on areterial walls. another danger is this "oxidized LDL" reduced levels of nitric oxide that helps relax the blood vessels allowing blood to flow freely.

Remember the good guy HDL - in addition to clean up, he prevents the oxidation of LDL. HDL have anti-oxidant properties of its own.

So, HDL - way to go... LDL - becomes a bad bad bad guy if he is in excess...

HDL, LDL values - what do they mean ?

• HDL - Ideal 60 mg/dL. Less than 40 mg/dL is not a good sign

• LDL - Less than 100 mg/dL is good. Above 100 - 120 mg/dL means borderline risk and above 120 mg/dL is BIG trouble.
  

Tidbits:
Our body gets cholesterol also from the food that we eat (particularly from the meat products). But, only 40% of it actually enters the blood stream. The major culprits are "saturated and transfats". some theory suggests that (not sure, but saw in many web sites) that saturated fats stimulates an increased synthesis of cholesterol thereby contributing indirectly to more LDL. transfats are believed to raise LDL as well as reduce HDL (much bigger culprit). Lets move on to triglycerides.

Triglycerides - What are they?
Triglycerides are other form of fatty substances that are derived from our food. They are used to meet the body's energy needs. Calories ingested in our meal that are not used immediately by our body are transported to tissues to be stored for later use. The transportation again occurs through the blood. The lipoproteins that are used to transport the triglycerides are chylomicrons.

Chylomicrons
Since the digestion takes place in our intestine, these chylomicrons (another type of lipoproteins) transport these triglycerides to the liver, muscle cells and the adipose tissue. The core of chylomicron contains 80% triglycerides and rest are "phospholipids".

Why high triglyceride level is harmful?
It seems that triglycerides interact with HDL in such a way that HDL level reduces in the blood which is not a good sign... Also, regardless of cholesterol levels, they are responsible for blood clots that form and block the arteries. High levels are associated with "overactive immune" system that can cause considerble damage to cells and tissues, including the arteries. Optimal level is 150 mg/dL of blood.

THE END.

Sources:
--------
1. http://www.healthandage.com/Home/
2. http://cholesterol.about.com/cs/cholesteroltypes/a/lipotypes.htm
3. http://www.americanheart.org/presenter.jhtml?identifier=4778
4. http://www.google.com

-Balaji.

P.S: What a long-mail.
--
"Two roads diverged in a wood and I took the one less traveled by and,
that has made all the difference" -- Robert Frost

Balaji Krithikaivasan (PhD Student),
[Co]mputer [N]etworking [Re]search [L]ab (CoNReL)