All lipoproteins are made up of proteins, triglycerides, and cholesterol. Why are low-density lipoproteins (LDL) considered unhealthy, and people encouraged to lower LDL, while high-density lipoproteins are considered healthy and people are encouraged to increase HDL?

Answer 1

While LDL and HDL contain proteins, triglycerides, and cholesterol, they do not contain these molecules in the same proportions. LDL contains more lipids while HDL contains an equal proportion of lipids to protein. These composition differences affect the function of these lipoproteins.
LDL molecules carry triglycerides and cholesterol to the body's tissues. LDL can build up in the lining of the blood vessels and are more susceptible to reactions with oxygen (oxidation), which can cause them to induce inflammation in the walls of the blood vessels. It is thought that having a lower level of these LDL molecules will reduce the risk of cardiovascular disease.
HDL contains a higher proportion of protein relative to triglycerides and cholesterol. HDL functions to remove cholesterol and other lipids from the blood and tissues and return it to the liver for use in the digestive system. Having more of these HDL molecules will reduce the lipids and cholesterol in the blood, which may protect the cardiovascular system.