Low-density lipoprotein

Low-density lipoprotein is also known as LDL. Most people call this the 'bad' cholesterol, as it carries cholesterol from the liver to the rest of the body.

Let me explain this in a little more detail. Since cholesterol is required by every cell in the body, it needs to have a transport vehicle. Cholesterol cannot move around the bloodstream on its own, as water is a large component of blood, and cholesterol is like a fat; the two of which do not mix.

To get around this problem, cholesterol travels with special carrier proteins, called lipoproteins. There is only one form of blood cholesterol, however there are a few types of lipoproteins, the main ones being ldl and hdl.

So basically, LDL fulfils the task of cholesterol transportation to the rest of the body from the liver.


My analogy of LDL….

I like to think of the whole system as a big distribution centre (the liver), which needs its stock (cholesterol) delivered by trucks (LDL particles) to lots of smaller stores (body cells) all across the country.

If there is too much stock (cholesterol), then more trucks (LDL) will be required to transport it, increasing the likelihood of an accident on the road (artery), with the goods (cholesterol) being dumped in a place where we really don’t want them (e.g. arterial roads / arteries!). Does that make sense?

I suppose I’m just trying to highlight that fact that there really isn’t a problem with having LDL in the body, the main concern is having too much low-density lipoprotein and the wrong type of LDL.

If we have too much LDL, some excess will be 'dumped' onto the artery walls, restricting blood flow and contributing to atherosclerosis. In this way, excess LDL increases the risk of heart disease.


HDL

HDL is also known as the “good cholesterol”. It is another type of lipoprotein, which is involved in returning cholesterol back to the liver from the rest of the body.

So if we think of the truck example, the HDL might act as a special type of truck-tower that detects the roads for too much LDL, carrying excess back to the distribution centre (the liver).

In this way, HDL cholesterol helps protect you against heart disease because it is helping reduce LDL build up in arteries.


How is LDL created?

Low-density lipoproteins are created from very low-density lipoproteins (VLDL). When VLDL loses some triglyceride (fat), it becomes smaller and more dense, essentially converting it from “very low density” to a “low density” lipoprotein. LDL contains a higher percentage of cholesterol than VLDL.


Types of LDL

LDL cholesterol can be modified in size and density. Larger and less dense LDL particles are considered to be safer, because they aren’t as small so are less likely to squeeze into the walls of our arteries.

Conversely, small and dense LDL’s are considered to be particularly dangerous, because they are more likely to penetrate our artery walls purely because of their compact size. The more small dense LDL’s we have, the greater our risk of coronary heart disease.


Apolipoproteins

Blood tests that measure different types of apolipoproteins are being increasingly used in conjunction with standard cholesterol testing.

The apolipoproteins most commonly measured include apoA1, apoB and apo(a).

Apo A1 and apo B can be used to assess risk of atherosclerosis and heart/vascular disease, and has even been considered to be a better predictor of risk than LDL and HDL cholesterol.

This is mainly because HDL and LDL cholesterol tests do not give information about the amount of small dense LDL’s; the ones considered more dangerous to heart health.

There is a condition called hyperapobetalipoproteinaemia, which is associated with having small dense LDL’s. Normally, Apo B studies as well as LDL cholesterol can be used to determine whether a person has this condition.

There is generally an increased risk of heart disease with low levels of apoA1 and increased apoB or apo(a).


Measuring LDL

When people have a cholesterol test, the low-density lipoprotein (LDL) cholesterol is not usually measured directly, rather it is estimated using the following equation.

For mmol/l measurement:
LDL = total cholesterol – HDL – (0.45 × triglycerides)

For mg/dl measurement:
LDL = total cholesterol – HDL – (0.20 × triglycerides)

This is why some people don’t get an LDL cholesterol result after having cholesterol tests, with an explanatory note often saying that LDL can’t be estimated if triglycerides are too high.


Familial hypercholesterolemia

A genetic condition characterised by a high level of LDL is called familial hypercholesterolemia.

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