HDL cholesterol is a measurement of cholesterol carried inside high-density lipoprotein particles in your blood, and for decades it has been called “good cholesterol” because higher levels track with lower rates of heart disease in population studies. That reputation is not wrong, exactly, but it is incomplete in ways that matter. Raising HDL levels with drugs has repeatedly failed to prevent heart attacks, and genetic studies suggest the number on your lab report may be more of a bystander than a hero. The biology behind HDL turns out to be far richer and more complicated than a simple good-versus-bad framing allows.
What HDL Particles Actually Do
HDL particles are assembled when a protein called apolipoprotein A-I (apoA-I) picks up cholesterol and phospholipids from cells, a process that depends on a transporter protein called ABCA1. Liver cells are a major production site: they secrete apoA-I, which then loops back to interact with ABCA1 on the cell surface to form new HDL particles.1PubMed Central. ABCA1 and nascent HDL biogenesis These young particles start out disc-shaped. An enzyme called LCAT then converts the free cholesterol on their surface into a form that gets packed into the particle’s core, rounding it into a mature sphere.2PubMed. Role of apoA-I, ABCA1, LCAT, and SR-BI in the biogenesis of HDL
The headline function of mature HDL is reverse cholesterol transport: scavenging excess cholesterol from tissues, including the walls of arteries, and ferrying it back to the liver for disposal. Once cholesterol reaches the liver, it can be converted into bile acids or pumped directly into bile and eventually excreted.3PubMed Central. HDL and Reverse Cholesterol Transport: Basic Mechanisms and their Roles in Vascular Health and Disease In humans, there is also an indirect route: a transfer protein called CETP shifts cholesterol from HDL to LDL and VLDL particles, which the liver then clears through a different receptor. So HDL does not always deliver cholesterol to the liver personally; sometimes it hands it off to other lipoproteins for the final leg of the trip.4PubMed Central. HDL and Reverse Cholesterol Transport: Basic Mechanisms and their Roles in Vascular Health and Disease
The “Good Cholesterol” Reputation
The association between higher HDL cholesterol levels and fewer heart attacks is one of the most consistent findings in cardiovascular epidemiology. The Framingham Heart Study established this link in the late 1970s and early 1980s and then confirmed it held up over longer follow-up periods.5JAMA. Incidence of Coronary Heart Disease and Lipoprotein Cholesterol Levels: The Framingham Study In one analysis of Framingham participants aged 50 to 79, men with the lowest HDL levels (under 35 mg/dL) had roughly twice the overall death rate and nearly four times the coronary heart disease death rate compared to men with the highest levels (over 54 mg/dL), even after accounting for other risk factors like blood pressure, smoking, and diabetes.6PubMed. High density lipoprotein cholesterol and mortality. The Framingham Heart Study
These findings drove clinical guidelines to classify low HDL as a risk factor and high HDL as protective. For most people, an HDL level above 40 mg/dL for men and 50 mg/dL for women is considered acceptable by standard guidelines, and levels above 60 mg/dL have traditionally been seen as a bonus. That framing shaped a generation of public health messaging and gave HDL its “good cholesterol” nickname.
Why Raising HDL Has Not Worked
If higher HDL protects the heart, you would expect that raising HDL levels with medication would prevent heart attacks. That expectation has been tested repeatedly, and the results have been disappointing. The AIM-HIGH trial gave niacin (a B vitamin that reliably raises HDL) to patients already on statin therapy. Niacin bumped their HDL from about 35 to 42 mg/dL and lowered their triglycerides, but the trial was stopped early after three years because heart attack rates were virtually identical between the niacin and placebo groups.7PubMed. Niacin in Patients with Low HDL Cholesterol Levels Receiving Intensive Statin Therapy A second large niacin trial, HPS2-THRIVE, confirmed the failure and added concerns about side effects.8PubMed Central. Niacin Therapy, HDL Cholesterol, and Cardiovascular Disease: Is the HDL Hypothesis Defunct?
The story with CETP inhibitors is similar. These drugs block the protein that transfers cholesterol from HDL to LDL, so they dramatically raise HDL levels. Three different CETP inhibitors failed in large trials. A fourth, anacetrapib, did show a modest benefit in a very large, long trial, but the researchers concluded the benefit was largely explained by its ability to lower non-HDL cholesterol rather than by the HDL increase itself.9PubMed Central. Trials and Tribulations of CETP Inhibitors
Genetic evidence points in the same direction. Mendelian randomization studies use naturally occurring genetic variants that influence HDL levels as a kind of lifelong experiment. When researchers use newer statistical methods that account for these genetic variants also affecting other lipids, the relationship between HDL cholesterol and coronary heart disease risk disappears.10PubMed Central. Mendelian randomization to assess causal effects of blood lipids on coronary heart disease: lessons from the past and applications to the future A 2024 analysis using multivariable Mendelian randomization found no association between HDL cholesterol and cardiovascular disease after controlling for the effects on other lipids.11Metabolism. Nonlinear relationship between high-density lipoprotein cholesterol and cardiovascular disease: an observational and Mendelian randomization analysis Put plainly: genes that give you higher HDL numbers do not seem to give you a healthier heart, once you account for other things those genes also do.
It Is Not How Much HDL You Have but How Well It Works
The disconnect between HDL levels and clinical outcomes has pushed researchers toward measuring what HDL particles actually do rather than how many cholesterol molecules they carry. The measurement getting the most attention is cholesterol efflux capacity, which tests how effectively a person’s HDL pulls cholesterol out of cells in a lab dish. In a landmark study, people with better cholesterol efflux capacity had less artery-wall thickening and less coronary artery disease, and this held true even after adjusting for their HDL cholesterol level.12PubMed Central. Cholesterol Efflux Capacity, High-Density Lipoprotein Function, and Atherosclerosis
Follow-up work in large prospective studies confirmed that efflux capacity predicts future cardiovascular events. In one cohort, people in the top quarter of efflux capacity had roughly a 67% lower risk of cardiovascular events compared to those in the bottom quarter, even after accounting for HDL cholesterol concentration and other risk factors.13PubMed Central. HDL Cholesterol Efflux Capacity and Incident Cardiovascular Events Another prospective study found that higher efflux capacity was independently associated with fewer coronary heart disease events after adjusting for HDL cholesterol and apoA-I concentrations.14PubMed Central. Association of HDL cholesterol efflux capacity with incident coronary heart disease events: a prospective case-control study
The problem is that cholesterol efflux capacity is not a routine lab test. It requires live cell cultures and specialized handling, so it remains a research tool. HDL particle number, which can be measured by certain advanced lipid panels, correlates more closely with apoA-I levels than HDL cholesterol does, and some researchers argue it is a better marker of residual cardiovascular risk in patients already on statin therapy.15PubMed Central. High-density lipoprotein cholesterol, size, particle number, and residual vascular risk after potent statin therapy But guidelines have not yet adopted particle-based HDL metrics for routine clinical decision-making.
The U-Shaped Curve and Very High HDL
One of the more surprising findings in recent years is that extremely high HDL levels are not protective either. Two large prospective cohort studies found a U-shaped relationship between HDL cholesterol and death from all causes: both very low and very high concentrations were associated with increased mortality, and this pattern held for both men and women.16European Heart Journal. Extreme high high-density lipoprotein cholesterol is paradoxically associated with high mortality in men and women: two prospective cohort studies The reasons behind the right side of the U are not fully understood. Some researchers suspect that genetically or pathologically elevated HDL may carry dysfunctional particles that have lost their protective properties. Others point to the possibility that very high HDL reflects disrupted cholesterol metabolism rather than efficient cholesterol removal. Either way, the finding undercuts the simplistic idea that more HDL is always better.
HDL’s Other Jobs Beyond Cholesterol Taxi
Reverse cholesterol transport gets all the press, but HDL particles carry a cargo of dozens of proteins and lipids that do other useful things. Three of the best-studied roles are antioxidant protection, anti-inflammatory signaling, and vascular health.
HDL carries an enzyme called paraoxonase 1 (PON1), which is responsible for much of HDL’s antioxidant activity.17PubMed Central. Antioxidant and anti-inflammatory role of paraoxonase 1: implication in arteriosclerosis diseases PON1 can break down oxidized lipids that would otherwise damage artery walls and trigger inflammation. Interestingly, PON1’s anti-inflammatory effect depends on it being attached to HDL; when separated from the particle, it can actually become pro-inflammatory in some settings.18PubMed. The anti-inflammatory effect of paraoxonase 1 against oxidized lipids depends on its association with high density lipoproteins
On the vascular side, HDL promotes production of nitric oxide, the molecule that tells blood vessel walls to relax and dilate. It does this by supporting the enzyme that makes nitric oxide and by maintaining the membrane environment that enzyme needs to function properly.19PubMed. Endothelial and antithrombotic actions of HDL This vasodilatory effect is one reason HDL is considered more than just a cholesterol shuttle. It also has anti-clotting properties, which could independently reduce the risk of a vessel being blocked.
HDL Subclasses and the Particle Size Debate
Not all HDL particles are the same size or density. They are traditionally divided into larger, lighter particles (HDL2) and smaller, denser ones (HDL3), with further subdivisions possible depending on the measurement technique. The question of which subclass matters more for heart disease risk has been debated for decades without a clear resolution.
Early prospective data from Finland showed that both total HDL and the larger HDL2 subfraction had inverse associations with heart attack risk, while results for HDL3 were mixed.20PubMed. HDL, HDL2, and HDL3 subfractions, and the risk of acute myocardial infarction. A prospective population study in eastern Finnish men A large Canadian study found a similar pattern: HDL2 was the statistically significant predictor of ischemic heart disease risk in multivariate models, though the difference between the two subfractions was modest enough that the authors cautioned against dismissing HDL3 entirely.21PubMed. Associations of HDL2 and HDL3 subfractions with ischemic heart disease in men. Prospective results from the Québec Cardiovascular Study
A comprehensive review of the evidence found that measuring HDL subfractions by standard cholesterol-based methods does not consistently distinguish which subclass is protective. Case-control studies were all over the map: some found both subfractions lower in heart disease patients, some found only one or the other. No prospective study showed that subfraction measurements improved risk prediction beyond plain HDL cholesterol or the ratio of total-to-HDL cholesterol.22PubMed. High-density lipoprotein subclasses and their relationship to cardiovascular disease More detailed particle-sizing techniques may eventually prove useful, but for now, asking your doctor to measure HDL2 and HDL3 separately is unlikely to change your care.
How Smoking Sabotages HDL
Among modifiable risk factors, smoking may be one of the most damaging to HDL. Cigarette smoking is associated with lower HDL cholesterol levels, and the damage goes beyond just the number. Smoking alters the activity of enzymes that are central to HDL metabolism, including LCAT (needed for cholesterol packaging) and CETP (the transfer protein that moves cholesterol between lipoproteins). Smoking also exposes HDL particles to oxidative damage, which can turn them dysfunctional, stripping away the very properties that make HDL protective in the first place.23PubMed. Effects of cigarette smoking on HDL quantity and function: implications for atherosclerosis This dual hit, fewer particles plus damaged ones, may explain part of the excess cardiovascular risk smokers face. Quitting smoking is one of the most reliable ways to raise both HDL levels and HDL quality, with improvements detectable within weeks of cessation.
How HDL Changes Through Menopause
Women generally have higher HDL cholesterol than men, a gap that narrows after menopause. The explanation is largely hormonal. The estrogen estradiol promotes the formation of larger HDL particles and supports their cholesterol-removal capacity. As estradiol drops during the menopausal transition, HDL composition shifts: large particles decline, small particles increase, and HDL picks up more triglyceride.24PubMed Central. HDL (High-Density Lipoprotein) Subclasses, Lipid Content, and Function Trajectories Across the Menopause Transition: SWAN-HDL Study
What makes this subtle is that total HDL cholesterol may not change much during menopause, masking the shift happening inside the particles. The overall cholesterol efflux capacity may even go up slightly when measured in bulk, but when calculated per particle, it declines.25PubMed Central. HDL (High-Density Lipoprotein) Subclasses, Lipid Content, and Function Trajectories Across the Menopause Transition: SWAN-HDL Study Studies tracking hormone levels confirm that higher estradiol is associated with larger HDL particles, larger overall HDL size, and better per-particle efflux capacity, while rising FSH (the pituitary hormone that goes up when ovarian function declines) shows the opposite pattern.26The Journal of Clinical Endocrinology & Metabolism. Associations of Endogenous Hormones With HDL Novel Metrics Across the Menopause Transition: The SWAN HDL Study This means a postmenopausal woman whose HDL cholesterol number looks fine on a standard lipid panel may still have undergone meaningful changes in HDL quality. The decline in estradiol also weakens the activity of enzymes involved in HDL remodeling and promotes chronic low-grade inflammation that further degrades HDL function.27Atherosclerosis. Lipid metabolism in women: A review
HDL and the Brain
The brain makes its own lipoproteins, largely because peripheral HDL particles in the blood cannot cross the blood-brain barrier. Brain HDL is assembled from apolipoprotein E rather than apoA-I, and it handles cholesterol recycling among neurons, astrocytes, and other brain cells. Still, there is growing interest in whether HDL influences brain health through its effects on cerebral blood vessels.
A body of evidence in humans, animal models, and lab-grown tissue supports a role for HDL in maintaining cerebrovascular resilience, the ability of the brain’s blood supply to resist damage from inflammation, amyloid buildup, and small-vessel disease. Researchers have suggested that HDL-based therapies originally developed for heart disease could potentially be repurposed for preventing or treating the vascular component of Alzheimer’s disease.28PubMed Central. HDL from an Alzheimer’s disease perspective One hypothesis focuses specifically on small HDL particles, proposing that their ability to exchange lipids may affect neuronal membrane composition and synaptic function. In one study, higher concentrations of small HDL particles in cerebrospinal fluid were associated with better cognitive performance across several domains, independent of age, sex, education, and APOE ε4 carrier status.29PubMed Central. The small HDL particle hypothesis of Alzheimer’s disease This is still early-stage research, but it illustrates how HDL’s story extends well beyond the arteries.
HDL and Infection
One of HDL’s more exotic functions has nothing to do with cholesterol at all. Humans carry a specialized HDL particle called trypanosome lytic factor (TLF), which contains a protein called apolipoprotein L-1 (APOL1). This protein forms pores in the membranes of African trypanosome parasites, the organisms that cause sleeping sickness, killing them before they can establish infection. It is a primate-specific defense mechanism: most other mammals are vulnerable to these parasites, but humans and some other primates are naturally immune because of this HDL-borne weapon.30PubMed Central. Germline-targeted baboon apolipoprotein L-1 protects mice against African trypanosomes
TLF’s antimicrobial reach extends beyond trypanosomes. Research has shown it can also damage Leishmania parasites, which are related organisms that replicate inside immune cells. TLF accumulates in the compartment where Leishmania lives inside macrophages and kills the early-stage form of the parasite, though the more adapted intracellular form is resistant.31PLoS Pathogens. Trypanosome Lytic Factor, an Antimicrobial High-Density Lipoprotein, Ameliorates Leishmania Infection The killing depends on pH changes that occur inside immune cell compartments: when conditions acidify and then return to neutral, TLF lyses the parasites completely within a couple of hours.32PLOS Pathogens. Interplay of Trypanosome Lytic Factor and innate immune cells in the resolution of cutaneous Leishmania infection HDL, in other words, is part of the innate immune system, a role that likely preceded its metabolic functions in evolutionary terms.
When HDL Goes Bad
During acute illness, inflammation can transform HDL from protective to dysfunctional. When the body mounts an acute phase response to infection or injury, HDL loses phospholipids and its cholesterol efflux capacity drops, even if the HDL cholesterol number on a lab test stays the same.33PubMed Central. High-density lipoprotein and the acute phase response This “dysfunctional HDL” can lose its antioxidant enzymes, accumulate pro-inflammatory cargo, and even begin promoting oxidation rather than preventing it. The concept matters clinically because patients in sepsis, after surgery, or with chronic inflammatory diseases may have normal HDL cholesterol readings that hide profoundly impaired HDL function.
HDL also appears to influence blood sugar regulation. Lab studies show that HDL particles can protect the insulin-producing beta cells of the pancreas against stress-induced death and enhance their ability to secrete insulin in response to glucose. HDL may also promote glucose uptake in skeletal muscle, fat tissue, and the liver through both insulin-dependent and insulin-independent pathways.34Cardiovascular Research. High-density lipoprotein, beta cells, and diabetes Epidemiological data showing that low HDL is associated with higher diabetes risk are consistent with these findings, though it remains unclear whether HDL is a cause, a consequence, or merely a fellow traveler of metabolic dysfunction.
ApoA-I Infusion Therapies
If the problem with HDL-raising drugs was that they increased the number without improving function, one logical next step is to deliver functional HDL components directly. That is the idea behind apoA-I infusion therapies, which involve injecting the main HDL protein into the bloodstream. The hope is that these infusions can pull cholesterol out of vulnerable arterial plaques, reduce their lipid content, and stabilize them against rupture, potentially lowering the risk of heart attacks in the days and weeks after an acute coronary event.35PubMed Central. ApoA-I Infusion Therapies Following Acute Coronary Syndrome: Past, Present, and Future Early trials showed some promise for plaque regression, but the approach has not yet delivered a large, definitive reduction in clinical events. Several formulations are still in development or testing, and the concept remains one of the more active areas of HDL-focused research. Whether infusing functional HDL will succeed where simply raising HDL numbers failed is an open question, and the answer will say a lot about how well we truly understand what makes HDL useful in the first place.