When you hear HDL or read in your lipid profile, we immediately assume that the higher the HDL is “good”. However, there are enough evidence suggest that HDL measurement alone may not be a good indicator of you are at low risk of atherosclerotic cardiovascular disease. Before going into this topic we’ll look into physiology of HDL.
HDL is a type of lipoprotein, and its role is to transfer lipids such as cholesterol, phospholipids, triglycerides and so on. Reason why we need lipoprotein is because lipids are fat and cannot transfer through bloodstream since blood is mainly water. The body creates something called lipoprotein, which is half lipid and half protein as the name suggests. The inside half, which is lipid side will be attracting lipids, and protein side, which will be outside which allows the lipoprotein and lipids inside to transport through blood.
HDL, LDL, VLDL are categorized based on the density of lipoprotein. Since they are different in density, they travel differently in the bloodstream. High density lipoprotein or HDL will be traveling rather closer to the middle of the bloodstream whereas low density lipoprotein (LDL) and very low-density lipoprotein (VLDL) will be traveling closer to the blood vessels.
Why HDL a “good” Cholesterol?
In late 70’s to early 80’s, study has observed risk of atherosclerotic cardiovascular disease in 5 cities. Outcome was higher the HDL cholesterol was better than lower HDL cholesterol.
This is where HDL was labeled “good” cholesterol and LDL to be “bad” cholesterol.
Although it is good to have higher HDL cholesterol and lower LDL cholesterol, however, having higher HDL cholesterol does not seem to be a reliable indicator for preventing atherosclerotic cardiovascular disease.
HDL Metabolism
When HDL is formed, they go through complex metabolism in the blood. HDLs are acted upon by lipases, which is a digestive enzyme to breakdown fat. Lipoprotein lipase is important for triglycerides and energy metabolism. Some of the lipoprotein lipases are hepatic lipases, which is created in liver and endothelial lipase, which is created in vascular cells. They both feed off of phospholipids in HDL. This results in changes in composition of HDL particle which leads to various size and protein structure in HDL.
Another important protein to HDL is cholesteryl ester transfer protein (CETP). Their role is to transfer lipids between VLDL and HDL. When people lack CETP, HDL cholesterol level seems increases even higher than 100mg/dL. Since the ideal metric of HDL cholesterol is 60mg/dL or above for both men and women, this sounded good to have CETP inhibited either by genetically or pharmacologically since the HDL cholesterol would be higher, especially for those who have lower HDL-C.
Once this was found, pharmaceutical company created CETP inhibitor medication called torcetrapib. The CETP inhibitor raised HDL cholesterol, however shown not only no benefit but an adverse effect. Which means although it is important to have high HDL, however, it doesn’t necessarily decrease the risk of cardiovascular disease.
From the finding, most important thing that was learned was HDL cholesterol itself is not directly protect against atherosclerotic cardiovascular disease.
HDL is Important
Although HDL may not be the good indicator of atherosclerotic cardiovascular disease, however, it serves important function as transporter of cholesterol to liver, adrenal, ovary and testes. This role can be done by LDL, however, because of its density, HDL seem to be safer transporter for it doesn't float towards blood vessels compared with LDL and VLDL.
Despite its importance, there are still a lot of mystery surrounding HDL. When looking at your lipid profile or consult with your physician regarding your lipid profile, look at the whole panel including:
Total Cholesterol
LDL Cholesterol
HDL Cholesterol
VLDL Cholesterol
Non-HDL Cholesterol
Triglycerides
If any measurement is not optimal, find a way to improve them rather than saying "HDL is high so I'm ok".
Kota Shimada
Comments