If you’re over 45 the chances are you’ve had your cholesterol levels checked. You’ll be told how much “bad”, “good” cholesterol and triglyceride you have circulating in your blood. But what does all this mean? What is the difference between “good” cholesterol and “bad” cholesterol? And what are triglycerides.
The chances are that everything you think you know about this is wrong. Here’s the real story.
Triglycerides
Triglycerides serve as the body’s energy store. During digestion fats and carbohydrates from food are converted into triglycerides and transported to fat cells where they are stored until needed. Fatty acids from the liver are also converted to triglycerides which are stored in fat cells.
Think of triglycerides as fuel and fat cells as fuel storage depots.
Per gram triglycerides store twice as much energy as carbohydrates. In fact the “energy density” of triglycerides is comparable to that of petrol. 1 kilogram of triglycerides will yield about 37 MJ versus 45 MJ per kilogram from petrol. In other words, triglycerides are a very potent fuel indeed.
Cholesterol
Cholesterol is a basic building block for many chemicals and structures that the body makes. Our bodies utilise cholesterol to build cell membranes and to synthesise a variety of hormones including testosterone and estrogen. The liver and the brain both synthesise cholesterol.
Think of cholesterol as a raw material for the body’s construction and chemical industries.
Typically a human body contains less than 100 gm of cholesterol. That’s less than 4 oz. However cholesterol is constantly being ingested, synthesised and recycled.
You can see what a cholesterol molecule looks like here.
LDLs and HDLs
LDLs (low density lipoproteins) are often called “bad cholesterol” while HDLs (high density lipoproteins) are called “good cholesterol”. This is not strictly accurate. HDLs and LDLs are not different types of cholesterol. There is no such thing as “bad cholesterol” and “good cholesterol”.
Here’s how it works. Recall that:
- Triglycerides are fuel that is stored in the body’s fat cells until needed; and
- Cholesterol is a raw material used in the construction of cell membranes and the synthesis of various hormones.
Neither triglycerides nor cholesterol can travel through the bloodstream unaided. So how do they get to where they are needed? How do triglycerides get from fat cells to muscle cells. How does cholesterol get from the liver to, say, the kidneys?
The answer is that, as well as having an energy, construction and chemical industry the body has a transport and logistics industry. LDLs, HDLs and VLDLs (very low density lipoproteins) are the “trucks” that transport triglycerides and cholesterol. You can think of them as little particles that carry cholesterol and triglycerides through the body.
A picture is worth a thousand words and this Encyclopaedia Britannica graphic of a lipoprotein illustrates the concept. What you see is a lipoprotein “shell” enclosing cholesterol and triglycerides.
The workings of the body’s triglyceride and cholesterol transport systems are complex. A detailed description is beyond the scope of this website. In outline here’s what happens.
- LDLs (low density lipoproteins) and VLDLs (very low density lipoproteins) ferry triglycerides and cholesterol to the cells that need them.
- HDLs (high density lipoproteins) carry cholesterol to the liver for excretion or recycling (Yes, the body also has a recycling industry)
- In addition the HDL particles appear to have an anti-inflammatory effect. I shall be writing more about this later.
The LDL level you see in a cholesterol panel is actually a snapshot of the amount of cholesterol circulating in your blood at the time the sample was taken. The correct term for this is LDL-C.
It is very important to understand the distinction between LDLs and cholesterol.
- Low density lipoproteins (LDLs) are not cholesterol.
- Your LDL level is your LDL-C level.
- Your LDL-C level is a snapshot of the amount of cholesterol circulating in your blood inside the LDL particles.
Similarly your high density lipoprotein (HDL) level is a snapshot of the amount of cholesterol circulating in your blood inside HDL particles.
For reasons which I shall discuss later elevated LDL-C levels are associated with increased risk of heart attack and stroke. The true story is however more complex than most people think. Your heart attack risk may depend as much on the size of your LDL particles as it does on your LDL-C levels.
Postscript
The terms “bad cholesterol” for LDLs and “good cholesterol” for HDLs has become deeply embedded in the language. You should however always remember that these terms are verbal shortcuts for the cholesterol inside LDL and HDL particles.
Tags: cholesterol, HDL, LDL, triglyceride