High-Density Lipoproteins

Why This Matters

High-density lipoproteins (HDL) have traditionally been called "good cholesterol," but this description oversimplifies their biology. HDL is not cholesterol; it is a complex lipoprotein that participates in cholesterol transport, lipid metabolism, and cellular homeostasis. Understanding HDL requires moving beyond HDL cholesterol levels and appreciating the diverse functions of the HDL particle itself.

What Is HDL?

HDL is the smallest and most dense of the major lipoproteins. Unlike chylomicrons, VLDL, IDL, and LDL, which primarily deliver lipids to tissues, HDL removes excess cholesterol from peripheral cells and transports it back to the liver.

The principal structural protein of HDL is apolipoprotein A-I (ApoA-I), which serves as the foundation for HDL formation and many of its biological functions.

HDL particles differ considerably in size, composition, and function, continuously changing as they circulate throughout the body.

Formation of HDL

HDL begins as a small, lipid-poor particle synthesized primarily by the liver and intestine.

These nascent HDL particles contain ApoA-I and readily accept free cholesterol from peripheral tissues through membrane transporters, particularly ATP-binding cassette transporter A1 (ABCA1). As cholesterol enters the particle, it is converted into cholesteryl esters by lecithin-cholesterol acyltransferase (LCAT), allowing HDL to mature into progressively larger particles.

This process enables HDL to continually collect cholesterol from cells throughout the body.

HDL Is a Dynamic Particle

HDL is not simply a cholesterol carrier. It functions as a dynamic participant in lipid metabolism.

As HDL circulates, it exchanges cholesterol, triglycerides, phospholipids, and apolipoproteins with other lipoproteins. It also interacts with numerous enzymes and transport proteins, including LCAT, cholesteryl ester transfer protein (CETP), hepatic lipase, endothelial lipase, and phospholipid transfer protein (PLTP).

These continuous interactions allow HDL to adapt its composition and function in response to changing metabolic conditions.

HDL and Reverse Cholesterol Transport

One of HDL's most important functions is reverse cholesterol transport, covered in the next chapter.

Peripheral cells transfer excess cholesterol to HDL, which transports it to the liver either directly or indirectly through exchange with other lipoproteins. The liver may then recycle this cholesterol, incorporate it into lipoproteins, convert it into bile acids, or excrete it into bile.

HDL Function Versus HDL Cholesterol

For many years, HDL cholesterol concentration was considered an inverse marker of cardiovascular risk. While low HDL cholesterol is associated with increased cardiovascular risk, raising HDL cholesterol alone has not consistently reduced cardiovascular events.

This is not a theoretical caveat — it has been tested directly, repeatedly, and the results were consistent enough to change the field. Torcetrapib, a drug that raised HDL cholesterol by roughly 60 percent, was stopped early after its own outcomes trial (ILLUMINATE) showed an increase in cardiovascular and total mortality rather than a reduction. Two later drugs in the same class, dalcetrapib and evacetrapib, raised HDL cholesterol substantially and produced no reduction in cardiovascular events despite, in evacetrapib's case, also lowering LDL cholesterol by more than a third. Niacin, one of the oldest HDL-raising therapies, failed similarly: when added to statin therapy in the AIM-HIGH and HPS2-THRIVE trials, it raised HDL cholesterol but did not further reduce cardiovascular events, and the larger of the two trials found more side effects in the niacin group. Taken together, these trials are why raising the HDL cholesterol number is no longer treated as a therapeutic goal in its own right, even though a low HDL cholesterol level still carries real prognostic meaning as a marker of risk.

This body of evidence has shifted attention from the quantity of cholesterol carried by HDL to the functional capacity of the HDL particle itself. Its ability to promote cholesterol efflux, participate in reverse cholesterol transport, and maintain normal lipid metabolism appears to matter more than the HDL cholesterol concentration alone — though it is worth being equally honest about the limits of this newer idea: cholesterol efflux capacity is a research and specialty-lab measurement, not a routine clinical test, and no efflux-targeted drug has yet been shown in a large outcomes trial to reduce cardiovascular events. It is a promising explanation for why the HDL number failed as a drug target, not yet a validated one.

Clinical Relevance

Low HDL cholesterol commonly accompanies insulin resistance, obesity, type 2 diabetes mellitus, metabolic syndrome, and hypertriglyceridemia.

In many of these conditions, HDL particles become enriched with triglycerides, undergo accelerated catabolism, and exhibit impaired function. Consequently, low HDL cholesterol often reflects underlying metabolic dysfunction rather than serving as an isolated therapeutic target — it is a symptom of the same insulin-resistant state that drives triglycerides up and shrinks LDL particles, not an independent problem to chase with an HDL-raising drug.

The primary clinical focus should therefore be on improving overall metabolic health rather than simply attempting to increase HDL cholesterol levels.

Clinical Pearls

• HDL is a lipoprotein, not a type of cholesterol.
• ApoA-I is the principal structural protein of HDL.
• HDL participates in reverse cholesterol transport.
• HDL continuously exchanges lipids and apolipoproteins with other lipoproteins.
• HDL function is often more important than HDL cholesterol concentration.

Common Misconceptions

Misconception: HDL is "good cholesterol."

Reality: HDL is a complex lipoprotein with multiple biological functions that extend far beyond carrying cholesterol.

Misconception: Raising HDL cholesterol always reduces cardiovascular risk.

Reality: Increasing HDL cholesterol alone has not consistently reduced cardiovascular events. HDL function appears to be more important than HDL cholesterol concentration.

Misconception: HDL works independently of other lipoproteins.

Reality: HDL continuously interacts with other lipoproteins, enzymes, and transport proteins as part of an integrated lipid transport system.

Clinical Bottom Line

HDL is a dynamic lipoprotein that plays a central role in maintaining cholesterol homeostasis. Its most important contribution is not simply carrying cholesterol, but facilitating reverse cholesterol transport and coordinating multiple aspects of lipid metabolism throughout the body.