Cholesterol is essential for all animal life with a typical adult human body containing about 35 g. It is an essential structural component of animal cell membranes and a precursor molecule for all steroid hormones and vitamin D. About one gram is synthesised by the cells of the body per day, while some is excreted through the liver.
Cholesterol is transported in blood bound to proteins called lipoproteins. There are two types of lipoproteins – low-density lipoproteins or LDL and high-density lipoproteins or HDL. Cholesterol bound to LDLs is often called the bad cholesterol and when bound to HDLs the good cholesterol.
Most of us know that high levels of LDL cholesterol can narrow the insides of blood vessels by forming plaques on their walls, thus restricting blood flow. This increases the risk of heart disease and stroke. HDL on the other hand carries the cholesterol to the liver. The liver then flushes it from the body, thus decreasing the risk for heart disease and stroke.
Sounds simple enough? Sorry, time to think again as it is more complicated than that.
Different subclasses of LDL
Contrary to normal wisdom, it has been shown that about 75 percent of patients who suffer heart attacks have total LDL levels that give no indication of cardiovascular risk. What’s going on?
Well, let’s complicate things a little bit.
It has been known since the early 1950s that LDLs comprise of three major subclasses, with particles of different sizes and densities. Subclass A contains more of the larger and less dense LDL particles; subclass I comprises an intermediate group; and finally, subclass B with smaller and denser LDL particles.
It has previously been shown that small and dense LDL is strongly associated with increased cardiovascular risk.
Now new research studying the molecular effect of the different LDL subclasses on blood vessel endothelium has confirmed that of the three subclasses that comprise LDL, only one causes significant damage. LDL subclass B was found to be the most damaging to endothelial function and contributed the most to the development of plaques. Therefore, it’s not the total amount of LDL cholesterol, but rather the concentration of subclass B to the other two, subclass A and subclass I, that should be used to diagnose the risk of heart attack.
However, don’t worry too much about the LDL subclasses as they are more of a diagnostic tool at this time.
Reasons for high levels of bad cholesterol
Let’s make clear from the beginning that most of our circulating cholesterol is actually formed by our own body and genetically determined. So we can blame our parents. However, environmental factors, in particular diet and exercise, appear to also be able to influence the expression of LDL subclasses.
It was once thought that eating too much of cholesterol-rich foods (such as eggs) was the main cause of high cholesterol. Sure, some foods are high in cholesterol, but indulging in such foods has little influence on our blood levels of cholesterol as such.
Although typical daily cholesterol dietary intake might be around 300 mg, most ingested cholesterol is esterified and poorly absorbed by the gut. The body also compensates for absorption of ingested cholesterol by reducing its own cholesterol synthesis. For these reasons, cholesterol in food has little, if any, effect on long-term concentrations of cholesterol in the blood.
On the other hand, eating too much of foods high in saturated fats is more of a problem, and this has more impact on blood cholesterol levels. The principle mechanism by which saturated fat intake can influence LDL cholesterol is via decreased LDL receptor activity, which in turn decreases liver clearance and excretion of LDL cholesterol.
Mono- or poly-unsaturated fats have the opposite effect, increasing LDL receptor activity and turn-over of LDL cholesterol.
So what can you do?
People with high levels of LDL cholesterol may thus be able to reduce their cholesterol levels by:
- Limiting foods that have a high saturated fat content (such as many biscuits, cakes and fatty take-away foods)
- Replacing saturated fats in the diet with mono- or poly-unsaturated fats found in nuts, avocados and oily fish
It is also useful to include more fibre-rich foods in the diet such as fruit, vegetables and wholegrain bread and cereals.
Remember to keep active as it is also an important part of keeping cholesterol levels healthy.
Eating foods enriched with plant sterols has been proven to lower cholesterol levels by up to 10 percent.
Equally, cholesterol-lowering medication has a similar effect and might be necessary if lifestyle changes are not sufficient to reach a desirable cholesterol level. Statin drugs targets the first 18 steps of a complex 37-step process in the formation of cholesterol.