What is cholesterol?
Cholesterol is one of the body’s fats (lipids). Cholesterol and another lipid, triglyceride, are important building blocks in the structure of cells and are also used in making hormones and producing energy.
To some extent, the cholesterol level in blood depends on what you eat but it is mainly dependent on how the body makes cholesterol in the liver.
Having too much cholesterol in the blood is not a disease in itself, but can lead to the hardening and narrowing of the arteries (atherosclerosis) in the major vascular systems.
For the sake of simplicity, there are two sorts of cholesterol: a ‘good’ sort called high density lipoprotein (HDL) and a ‘bad’ sort called low density lipoprotein (LDL).
HDL has a useful effect in reducing tissue cholesterol and taking it back to the liver. HDL actually protects against atherosclerosis. LDL may contribute to diseases of the arteries (cardiovascular disease).
It is the proportion of LDL-cholesterol to HDL-cholesterol that influences the degree to which atherosclerosis is likely to cause problems (cardiovascular risk).
LDL-cholesterol level can be lowered by eating a low fat diet and, if required, taking medication. HDL-cholesterol level can be raised by exercising.
What is the cholesterol count?
When the cholesterol level is measured in a blood sample (taken after the patient has fasted for several hours), the levels of all forms of cholesterol can be calculated.
Cholesterol levels in the blood rise slightly with age, and women generally have a higher HDL-cholesterol level than men.
In the UK, the average total cholesterol level is 5.7mmol/l.
The levels of total cholesterol fall into the following categories:
- ideal level: cholesterol level in the blood less than 5mmol/l.
- mildly high cholesterol level: between 5 to 6.4mmol/l.
- moderately high cholesterol level: between 6.5 to 7.8mmol/l.
- very high cholesterol level: above 7.8mmol/l.
However, it is now recognised that the significance of any particular cholesterol level cannot be assessed without taking into account the ratio between good and bad cholesterol (either total-cholesterol/HDL-cholesterol ratio or LDL-cholesterol/HDL cholesterol ratio) or the presence of other cardiovascular risks, such as smoking, diabetes and high blood pressure (hypertension).
It is possible for someone to have a high level of total cholesterol and still have a relatively low cardiovascular risk because of an absence of other risk factors or because their family history is free from coronary disease.
Anyone with an established track record of cardiovascular disease such as angina pectoris, a previous heart attack, coronary angioplasty or coronary bypass surgery should seek advice to keep their total cholesterol level below 5mmol/l or their LDL below 3mmol/l.
What cause high cholesterol levels?
Cholesterol levels can run in families. If the inherited cholesterol levels are very high, this is called familial hypercholesterolaemia (FH) or familial combined hyperlipidaemia (FCH) where the triglyceride levels are very high as well.
Levels can also be influenced by the part of the world you live in: cholesterol levels in northern European countries are higher than in southern Europe and much higher than in Asia.
It is known that the relationship to food is significant, but there is no doubt that genes also play a part. High cholesterol is also seen in connection with other diseases, such as reduced metabolism (due to thyroid hormone problems, for example), kidney diseases, diabetes and alcohol abuse.
What are the symptoms of high cholesterol in the bloodstream?
You cannot tell if you have a high cholesterol level, but a high level in conjunction with other adverse factors increases the risk of developing atherosclerosis and cardiovascular disease.
Atherosclerosis results in narrowing of the arteries. This does not occur suddenly, but builds up over many years during which cholesterol and fat have been deposited in the artery walls. The result is that the arteries become constricted and hardened, their elasticity disappears and the volume of blood able to travel through them is reduced.
The symptoms are therefore the consequences of cardiovascular disease. They depend on the degree of narrowing, the likelihood that the plaque is going to rupture (vulnerability), and the organ supplied by the affected arteries.
- In the brain, an atherosclerotic carotid or cerebral (brain) artery might block with clotted blood (thrombus) or a smaller intracerebral vessel may rupture causing a local haemorrhage. Both these circumstances result in a stroke (cerebrovascular accident or CVA).
- In the heart, narrowed coronary arteries cause angina, and ruptured plaques cause coronary thrombosis (myocardial infarct or heart attack), which may lead to reduced heart function (heart failure) if a significant amount of heart muscle is damaged.
- Carotid arteries in the neck can become narrowed and may lead to clots forming in the neck and floating downstream into the brain, causing a stroke or recurrent temporary strokes (transient ischaemic attacks, also known as TIAs).
- Leg pain on exertion can be experienced due to atherosclerosis in the arteries that supply the lower limbs (intermittent claudication). If a major peripheral vessel to a lower limb blocks suddenly, an acutely ischaemic leg will occur which may be limb-threatening. In the worst cases of chronic lower limb atherosclerosis, this can lead to a leg so starved of blood that it cannot survive and requires amputation.
- It is common in those people most affected to have the disease in several arteries throughout their circulation including the aorta (the main artery in the chest and abdomen, the renal (kidney) arteries and the mesenteric (intestinal) vessels.
What can be done to prevent vascular disease?
High cholesterol is only one of many risk factors in the development of hardening of the arteries. These factors include:
- smoking: recent research shows that middle-aged women and men who smoke have a much higher risk of suffering a heart attack. The risk drops in the years following giving up smoking.
- diet: food is another important factor. A Mediterranean diet made up of bread, fruits, vegetables and small amounts of lean meat, fish, and olive oil is recommended.
- alcohol: moderate consumption reduces the negative effect of the LDL-cholesterol and increases HDL-cholesterol. Too much raises blood pressure and damages the liver, having an adverse overall effect.
- exercise: even on a small scale this can reduce the chance of coronary artery disease. Hard physical exercise increases the blood’s ability to break up blood clots.
- body weight: it is important to avoid obesity, especially when the fat is around the stomach.
How does the doctor make the diagnosis?
After a 12-hour fast, during which only water can be consumed, a blood sample is taken to measure the level of cholesterol, LDL-cholesterol, HDL-cholesterol and triglyceride in the blood. The cholesterol count rises during pregnancy and drops significantly in connection with a heart attack and other serious illnesses.
The doctor can seek evidence of cardiovascular disease by checking the pulse, the blood pressure, listening to the heart and large arteries, checking kidney function with a blood test and arranging an electrocardiogram (ECG).
In some patients with inherited or familial high cholesterol, it is possible to see deposits of cholesterol in the skin, eyes and tendons.
How is raised cholesterol in the blood treated?
Medical treatment with cholesterol-lowering medicine is needed when there is established heart or arterial disease (cardiovascular disease) and the total cholesterol remains greater than 5mmol/l. Medication may also be needed when lifestyle change fails to reduce high cumulative cardiovascular risk in someone without disease.
Cardiovascular risk may be calculated from a patient’s risk factor profile and family history. In the UK it has been recommended that medical therapy should be initiated where the individual’s risk of coronary disease is greater than 30 per cent over 10 years.
The most effective medications for lowering cholesterol are the statins. Although these are normally very effective, non-statin treatments such as Ezetrol (ezetimibe) are available if there are any problems with statin side effects such as muscle pains.
There are now very useful national guidelines to help your doctor estimate the ‘cardiovascular risk’ associated with your cholesterol levels and blood pressure readings.
