Cholesterol is waxy substance that is made in the liver and released into the circulation for use by cells. Some cells can make their own cholesterol when needed to.
As cholesterol is insoluble, it is transported within the circulation bubble-wrapped by transporters known as lipoproteins. These lipoproteins are made in the liver and come in a variety of forms, with particles of varying size and density, which all have different functions.
Low density lipoprotein (LDL) binds to cholesterol to transport it from the liver to the cells, and was originally referred to as ‘bad’ cholesterol, as it’s found in the plaques that cause arteries to become hardened and furred up (atherosclerosis).
High density lipoprotein (HDL) binds to cholesterol to transport it from the tissues back to the liver for processing, so HDL-cholesterol is usually referred to as ‘good’ cholesterol.
Very low-density lipoprotein (VLDL) transports triglycerides from the liver to fat tissues for storage, and raised levels of cholesterol.
Intermediate-density lipoprotein (IDL) is formed from the breakdown of VLDL, once it has released its triglycerides, and can then absorb more and more cholesterol until it becomes LDL cholesterol. IDL-cholesterol is also associated with furring up of the arteries.
Lipoprotein(a) is another LDL-like particle which contributes to furring up of the arteries.
As the complicated interaction between these different forms of lipoprotein and cholesterol has become unravelled, the focus has shifted away from LDL- cholesterol as bad cholesterol. Essentially, every lipoprotein that isn’t HDL is now considered a potential risk factor for cardiovascular disease.
Good cholesterol and the new bad cholesterol
HDL-cholesterol remains the ‘good’ form of cholesterol as having a relatively high level is associated with a lower risk of heart disease and stroke.
The sum total of all the other forms of cholesterol – collectively known as non-HDL cholesterol - are now referred to as ‘bad’ cholesterol.
Non-HDL cholesterol is a better, more accurate predictor of the future risk of cardiovascular disease (heart attack and stroke) than low density lipoprotein (LDL) cholesterol. As a bonus, you don’t need to fast before having a blood test to measure non-HDL cholesterol – no more early morning blood tests with a rumbling tummy!
Ideally, your non-HDL cholesterol should be lower than 4mmol/L and your total cholesterol should be 5mmol/L or less.
The most important measure is your ratio of total cholesterol to HDL- cholesterol, as this takes your non-HDL cholesterol into account. Your ratio of total cholesterol divided by HDL-cholesterol should ideally be less than 4. A ratio above 4 is considered a risk factor for heart attack and stroke. A ratio above 6 is considered high risk, so the lower the ratio the better.
This total/HDL ratio is used to estimate your cardiovascular risk, along with other health and lifestyle risk factors to assess your qrisk2 score . Based on this calculation, NICE now recommends lipid-modification treatment (usually a statin) for people whose risk of having a heart attack or stroke over the next 10-years is 10% or greater.