Most people know that high cholesterol is “bad;” but don’t know what cholesterol is, what it does, why it’s needed and how high cholesterol causes health problems. I will attempt to explain and answer these questions. However, keep in mind that this is a simplification of cholesterol and the way I understand it now – if you are an organic chemist or lipidologist; please forgive me!
Q. What is cholesterol?
A. Cholesterol is a fatty, wax-like organic chemical compound composed of carbon, hydrogen, and oxygen whose chemical formula is C 27 H 46 O. It belongs to a group of compounds called lipids but is not a fat (AKA triglycerides). All fats are lipids, but not all lipids are fat.
Q. Why is cholesterol important?
A. Cholesterol is essential to animal life. Thirty percent (30%) of every cell’s membrane is made of cholesterol. This allows the cell membrane to have structure as well as fluidity. Cholesterol is also a precursor to other essential compounds like steroids (think cortisol and aldosterone), hormones (think estrogen, progesterone, and testosterone), bile acids (think bilirubin/bile) and Vitamin D.
A. Though every animal cell can make cholesterol using amino acids and sugars; liver cells manufacture a significant amount of it. An enzyme in liver cells called HMG-COA reductase is an important step in the production of cholesterol. Cholesterol is also consumed when animal products are eaten and absorbed into our bodies from the small intestines. Plants do not manufacture, nor do they contain cholesterol; so a vegan diet contains no cholesterol, though this diet does contain fat. Furthermore, the body re-cycles cholesterol. The cholesterol that is made into bile acids is excreted into the intestine to help digest fats, but then is later reabsorbed from the small intestine (like consumed cholesterol) and taken back to the liver! How much cholesterol a person makes, how many cell membrane receptors a person has, and how much cholesterol is reabsorbed are all genetically determined.
Q. How does cholesterol move from one cell to another?
A. Cholesterol does not dissolve in blood; much like oil does not dissolve in water. Cholesterol is made inside a cell and then packaged inside other organic chemical compounds called lipoproteins (HDL=High Density Lipoprotein, LDL=Low Density Lipoprotein, VLDL=Very Low-Density Lipoprotein and IDL=Intermediate Density Lipoprotein). Lipoproteins then attach to other types of chemical compounds called Apolipoproteins (Apo-A, Apo-B, Apo C and Apo-E) which actually transport the cholesterol from the cell through the blood to other cells.
The two most important transport proteins are Apo-A and Apo-B. Apo-A carries HDL from peripheral cells and transports it back to the liver. Apo-B does the opposite and carries LDL from the liver and transports it to the body’s peripheral cells.
At the cell membrane, there are specific receptors where the lipoproteins attach to unload their cholesterol. When Apo-B attaches to a cell membrane receptor to unload its cholesterol, sometimes, an enzyme called PCSK-9 (Proprotein convertase subtilisin/kexin type 9) also attaches to the cell membrane receptor along with Apo-B. When the PCSK-9 enzyme attaches, it prevents the receptor from being recycled and the receptor is destroyed. The amount of lipoproteins, apolipoproteins, PCSK-9 and cell receptors are genetically determined.
Q. What is “good” and what is “bad” cholesterol?
A. HDL is considered “good” because it takes cholesterol out of the arteries and takes it back to the liver. LDL is considered “bad” because it takes cholesterol from the liver to the arteries.
Inside the artery wall, the “bad” lipoproteins (APO-B with LDL) attach and unload their cholesterol, if there is too much cholesterol or not enough receptors, not all of the cholesterol is absorbed by the cell. This left-over LDL-cholesterol that wasn’t able to move into the cell stays on the outside of the cell where the body tries to get rid of it by either using HDL to move it back to the liver or using a cell called a macrophage to “eat” it. When the macrophage consumes the left over cholesterol, it forms something called a foam cell. The foam cells stay at the lining of the artery and when they accumulate, they cause atherosclerosis (hardening of the arteries).
Atherosclerosis is a process that occurs in everyone and starts when we are teenagers. However, in some people, the atherosclerotic process is slow or normal; in others, it is fast – which leads to premature atherosclerosis. Atherosclerosis leads to fatty streaks and plaques in the lining of arteries. These plaques accumulate and either physically narrow the artery which decreases blood flow or ruptures to cause a clot which blocks blood flow. If this happens in a coronary (heart) artery, then there is no blood flow to heart muscle resulting in a heart attack; if this happens in a cerebral (brain) artery, then there is no blood flow to the brain resulting in a stroke. Atherosclerosis can occur in any artery and thus whatever is supplied by that artery will be affected – retinal (eye) arteries would cause blindness; leg arteries would cause peripheral artery disease (PAD) which can lead to amputations. Other factors contribute to atherosclerosis like smoking, high blood pressure, and diabetes. Since atherosclerosis occurs in everyone and starts at a young age; what is essential is to try to identify those who do or will have premature atherosclerosis and to try to slow that process down.
Q. How is cholesterol measured?
A. There are several blood tests that can be done to measure cholesterol. One calculates how much “bad” cholesterol there is using a formula; one directly measures how much “bad” cholesterol there is; one measures how much Apo-B there is and yet another measures how many lipoprotein particles there are. They all have advantages and disadvantages – if you really want to know, ask your provider to explain.
Q. What are desirable cholesterol levels?
A. At Triangle Family Care, in general, if you have no other medical problems, the current lipid goals are:
- Total cholesterol level less than 200mg/dl
- HDL cholesterol greater than 40mg/dl in men and greater than 50mg/dl in women
- LDL cholesterol less than 100mg/dl
- Triglycerides less than 150mg/dl
If you have two or more other primary risk factors (see below) or diabetes alone, the current lipid goals are:
- Same Total cholesterol, HDL and Triglyceride goals as above
- LDL cholesterol less than 70mg/dl
Sometimes we look at the bad to good cholesterol ratio (LDL:HDL). The lower the ratio is, the lower the risk. An ideal ratio is less than 2.0.
Ready for information on how to lower your “bad” cholesterol and improve your “good” cholesterol? Check out Dr. Stevens’ second installment in this series on cholesterol here on the TFC blog!