What is cholesterol and how does arteriosclerosis develop?

There are two main types of cholesterol, based on the kind of lipoprotein that is involved:

• LDL cholesterol (LDL-C): “LDL” stands for “low-density lipoprotein.” In this form, cholesterol is transported from the liver to the rest of the body. High LDL cholesterol levels are associated with a higher risk of cardiovascular disease, which is why it is frequently referred to as “bad” or “unhealthy” cholesterol.
• HDL cholesterol (HDL-C): “HDL” stands for “high-density lipoprotein.” In this form, cholesterol is transported back to the liver from the body’s tissues. It is broken down there and then flushed out of the body with the bile fluid. A high HDL cholesterol used to be considered beneficial for cardiac (heart) health. That is why HDL cholesterol is often called "good" cholesterol, but recent studies have not confirmed any preventive effect. It is not known exactly what significance the HDL level has in terms of your health.

The sum of the various types of cholesterol in the blood is called total cholesterol.

Arteriosclerosis is the medical term for the arteries hardening, thickening and narrowing. The arteries are the blood vessels that transport blood rich in oxygen from the lungs to the various organs in the body. Too much cholesterol in the blood can contribute to the development of arteriosclerosis.

People used to think that excess cholesterol was deposited in blood vessels like scale in a pipe. We now know that it is more complicated than that:

• If there is too much LDL cholesterol in the blood, it can oxidize. This oxidized cholesterol can’t be processed in the usual way by our body and it acts differently.
• Our blood vessels have a thin, smooth layer on the inside, which is called the endothelium. It forms a divider between the blood and the other layers of the blood vessel wall. One of the things it does is to regulate the diameter of the blood vessel. Oxidized LDL cholesterol can damage the endothelium, which disrupts its functioning.
• Oxidized LDL cholesterol in the blood is absorbed by phagocytes ("scavenger cells"). These cells can accumulate in the innermost part of the vessel wall (intima) and form deposits there.
• The immune system’s phagocytes release various messenger substances that help to generate inflammation and growth, such as cytokines. The resulting inflammations can then damage the vessels.

The combination of the damage, inflammation and deposits can lead to the vessel walls becoming thicker over time.

There are other risk factors for arteriosclerosis besides LDL cholesterol levels. The vessel walls have more trouble healing if your blood pressure is constantly high. Toxins from cigarette smoke can also increase the amount of inflammation and also attack the vessel walls. Arteriosclerosis can get worse faster if you have diabetes. Being overweight and having certain diseases can also increase the risk of arteriosclerosis.

Arteriosclerosis often already starts in late childhood and gradually develops over a very long period of time. It usually only leads to noticeable symptoms once a blood vessel has been narrowed by more than 70%. That typically affects the flow of blood, and it is not possible to transport enough oxygen to the tissue or organs that the vessel is supplying.

If a vessel wall tears and blood suddenly comes into contact with the cholesterol-rich deposits as a result, a blood clot might form. That is because our bodies try to seal the wound in the blood vessel wall, just like when scabs form if you cut your skin. If this happens, the consequences will depend on various things, including how big the blood clot is. A big clot can completely block the blood vessel, causing a heart attack or stroke. But the blood clots that form are often only small – they fix the damage in the blood vessel wall and do not have any noticeable consequences.

Arteriosclerosis may develop in various parts of the body. It is particularly dangerous in the large arteries that carry blood to the brain and heart. Narrow coronary blood vessels can cause chest pain (pectoral angina) during physical strain. If a coronary blood vessel becomes blocked, blood will no longer flow to part of the heart muscle, which might result in a heart attack. If a blood vessel in the brain becomes blocked, it might lead to a stroke. Problems with blood circulation (peripheral artery disease, PAD) may develop if the vessels that supply the arms or legs become narrowed.

A high LDL cholesterol level is a recognized risk factor for cardiovascular disease. Cholesterol levels can help get an idea of someone’s individual risk. But that does not mean that everything that can improve cholesterol levels, or claims to do so, is automatically good for your health.

For example, one medication called torcetrapib was shown to be very effective at increasing “good” HDL cholesterol levels and reducing “bad” LDL cholesterol levels. But when the manufacturer did a study involving 15,000 people to see whether torcetrapib also prevented cardiovascular disease, the opposite was found to be true: it actually increased the risk of cardiovascular disease. So it was never approved as a medicine.

This example makes it clearer why it is so important to take factors other than cholesterol levels into account when assessing the advantages and disadvantages of treatments. There is a lot of advice out there about what to do about high cholesterol, and many treatments are available. But only some of them have been reliably proven to prevent serious health problems, such as heart attacks, and increase life expectancy.