why HDL-C is consider as a good cholesterol

 HDL

High-density lipoprotein (HDL) is one of the five major groups of lipoprotiens, VLDL, IDL, LDL and HDL,  which enable lipids like cholesterol and triglyceride to be transported within the water-based blood stream based. In healthy individuals, about thirty percent of blood cholesterol is carried by HDL.

HDL particles are able to remove cholesterol from within artery atheroma and transport it back to the liver for excretion or re-utilization, which is the main reason why the cholesterol carried within HDL particles (HDL-C) is sometimes called "good cholesterol.

 Reverse cholesterol transport 

The overall process by which HDL removes cholesterol from extra hepatic tissues and returns it to the liver is called reverse cholesterol transport.

Reverse cholesterol transport is a multi-step process resulting in the net movement of cholesterol from peripheral tissues back to the liver via the plasma.

Cholesterol from non-hepatic peripheral tissues is transferred to HDL by the ABCA (ATP binding cassette transporter). ApoA-1 acts as an acceptor, and the phospholipid component of HDL acts as a sink for the mobilised cholesterol.

The cholesterol is converted to cholesteryl ester by the enzyme LCAT (lecithin cholesterol acyltransferase). The cholesteryl esters can be transferred, with the help of the cholesterol-ester transfer protein (CETP) in exchange for triglycerides, to other lipoproteins  (such as LDL and VLDL), and these lipoproteins can be taken up by the liver  through its LDL receptors.

However, the receptor SR-B1(scavenger receptor class B1) present on the liver cells’plasma membranes mediates most of the liver’s uptake of cholesteryl esters from HDL in the absence of uptake of  apolipoproteins..

Once in the liver, the cholesteryl esters are converted to cholesterol and enter the general pool. Therefore, the liver can eliminate cholesterol from the body by secreting unesterified cholesterol into the bile or by converting cholesterol to bile acids. 

Uptake of HDL₂ is mediated by hepatic lipase, a special form of lipoprotein lipase found only in the liver. Hepatic lipase activity is increased by androgens and decreased by estrogens, which may account for higher concentrations of HDL₂ in women.

High and low HDL-C values correlation with cardiovascular health:

Those with higher levels of HDL-C seem to have fewer problems with cardiovascular diseases,  while those with low HDL-C cholesterol levels (less than 40 mg/dL or about 1 mmol/L) have increased rates for heart disease. While higher HDL levels are correlated with cardiovascular health, no incremental increase in HDL has been proven to improve health. In other words, while high HDL levels might correlate with better cardiovascular health, specifically increasing one's HDL might not increase cardiovascular health.  Additionally, those few individuals producing an abnormal, apparently more efficient, HDL Apo-A1 protein variant called Apo A-1 Milano, have low measured HDL-C levels yet very low rates of cardiovascular events even with high blood cholesterol values.