HDL cholesterol (high density lipoprotein) is a lipoprotein of the chylomicron group whose functions are to transport lipids such as phospholipids, triglycerides or cholesterol. It differs from other lipoproteins such as LDL by its higher density, due to having a greater amount of proteins in its composition.

The main function of HDL is the transport of cholesterol from peripheral tissues to the liver, playing a role in lipid biodistribution. It is commonly known as "good cholesterol" due to its anti-atherogenic and anti-inflammatory properties, thanks to its uptake and return to the liver of cholesterol stored in the foam cells of atherosclerotic plaques. It thus reduces plaque size and its associated inflammation, reducing the risk of atherosclerosis, a precursor of myocardial infarction, transient ischemic attack and stroke.

It is well established that elevated HDL levels (>40 mg/dL) are associated with a lower risk of coronary heart disease and are thought to play a role in predicting cardiovascular risk.

HDL cholesterol levels can vary for different reasons. Genetic factor is among the factors influencing the levels of this lipoprotein. Compared to genetically determined abnormalities of HDL metabolism, low HDL-C levels occur much more frequently in patients with metabolic syndrome or diabetes mellitus. Low HDL-C levels are also associated with systemic inflammation, e.g., smoking, chronic inflammatory diseases or chronic kidney disease (Fig. 3) [45, 46]. In cases of extremely low HDL-C, a rare diagnosis, e.g., neoplasia or an increased risk of sepsis, may be considered.

Some established HDL cholesterol levels based on their relationship to cardiovascular disease risk are mentioned below:

Less than 40 mg/dL in men = low HDL (increased risk).
Less than 50 mg/dL in women = low HDL (higher risk)
40 to 59 mg/dL = Higher is better
60 mg/dL and above = high HDL (lower risk)

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