Summary
HDL (high-density lipoprotein) cholesterol is widely known as 'good cholesterol' because HDL particles collect excess cholesterol from the arterial wall and return it to the liver for disposal — a process called reverse cholesterol transport. Low HDL is an independent risk factor for heart disease; higher HDL is generally protective. The total cholesterol:HDL ratio is one of the most powerful cardiovascular risk predictors available.
Epidemiological studies consistently show that each 0.03 mmol/L (1 mg/dL) increase in HDL cholesterol is associated with a 2–3% reduction in cardiovascular risk — independent of LDL levels. Low HDL is common in people with metabolic syndrome, type 2 diabetes, obesity, physical inactivity, and smoking.
The mechanism of HDL’s protection extends beyond simple cholesterol efflux: HDL particles have anti-inflammatory, antioxidant, anti-thrombotic, and endothelium-protective properties. However, HDL function (not just quantity) matters — some conditions can produce dysfunctional HDL that does not confer full cardiovascular protection.
The total cholesterol:HDL ratio (ideally < 4.0 in men and < 3.5 in women) is considered the most clinically useful single lipid ratio for cardiovascular risk assessment.
What It Is
HDL particles are the smallest and densest lipoprotein particles, synthesised by the liver and small intestine. The primary structural protein is apolipoprotein A-I (ApoA-I), which activates LCAT (lecithin-cholesterol acyltransferase) — the enzyme responsible for esterifying cholesterol within HDL.
HDL exerts cardiovascular protection through multiple mechanisms: (1) reverse cholesterol transport — removing cholesterol from peripheral macrophages and foam cells in arterial plaques; (2) anti-inflammatory effects — inhibiting LDL oxidation and endothelial inflammatory signalling; (3) anti-thrombotic effects — promoting fibrinolysis.
Reference ranges: HDL-C ≥ 1.0 mmol/L in men and ≥ 1.2 mmol/L in women is considered adequate; 2.0 mmol/L is generally very protective, though extreme elevation (> 3.5 mmol/L from genetic mutations) may paradoxically not be protective.
Functions
Reverse cholesterol transport
HDL particles collect cholesterol from arterial plaques and peripheral tissues and transport it to the liver for excretion — directly opposing atherosclerosis.
Anti-inflammatory protection
HDL inhibits LDL oxidation and suppresses endothelial inflammatory signalling — key early steps in atherosclerosis development.
Cardiovascular risk ratio component
The total:HDL ratio and HDL alone are powerful cardiovascular risk predictors — HDL is among the most important lipid protective factors.
Endothelial function support
HDL promotes nitric oxide production in endothelial cells, supporting vascular dilation and blood pressure regulation.
Reference Ranges
HDL Cholesterol
Measured in mmol/L| Status | Range (mmol/L) | Range (mg/dL) | What it means |
|---|---|---|---|
| Low | < 1.0 | < 39 | Low HDL — significant cardiovascular risk factor. Lifestyle intervention strongly recommended. |
| Borderline | 1.0–1.2 | 39–46 | Low-normal — still a mild risk factor. Aim to raise through lifestyle. |
| Optimal | 1.2–2.0 | 46–77 | Protective HDL range — associated with reduced cardiovascular risk. |
| High | > 2.0 | > 77 | Elevated HDL — generally very protective in the context of a healthy lifestyle. |
HDL thresholds differ by sex: risk factor is < 1.0 mmol/L in men and < 1.2 mmol/L in women (WHO). The total cholesterol:HDL ratio (ideal < 4.0) provides more cardiovascular risk information than either marker alone.
Symptoms of Imbalance
Low HDL causes no direct symptoms — its consequences are long-term cardiovascular events from progressive atherosclerosis.
- Low HDL is usually asymptomatic
- Over years: symptoms of coronary artery disease (angina, breathlessness) or stroke
- High HDL from a healthy lifestyle has no adverse symptoms
- Very high HDL from rare genetic mutations may paradoxically be associated with increased risk — investigate if unexplained
Causes of Imbalance
- Physical inactivity — one of the strongest modifiable determinants
- Smoking — lowers HDL significantly
- Obesity, particularly central/visceral adiposity
- Type 2 diabetes and insulin resistance
- Hypertriglyceridaemia — triglycerides and HDL move inversely
- High refined carbohydrate diet
- Anabolic steroid or androgenic medication use
- Regular aerobic exercise (the most effective lifestyle HDL raiser)
- Moderate alcohol consumption (though this is not a reason to drink)
- Oestrogen (pre-menopausal women have higher HDL than men)
- Familial hyperalphalipoproteinaemia (genetic)
- Cholesteryl ester transfer protein (CETP) deficiency
FAQs
Regular aerobic exercise is the most effective lifestyle intervention — 150 minutes of moderate-intensity or 75 minutes of vigorous exercise per week raises HDL by 5–10%. Quitting smoking raises HDL by 0.1–0.2 mmol/L. Weight loss, reducing refined carbohydrates and trans fats, and increasing mono- and polyunsaturated fats (olive oil, nuts, avocado, oily fish) all contribute. Moderate alcohol raises HDL, but the net health effects of alcohol make it an inadvisable strategy.
Not necessarily. Epidemiological studies show a U-shaped relationship between HDL and mortality at population extremes. Very high HDL from rare genetic mutations affecting CETP or SR-BI may paradoxically impair reverse cholesterol transport function. HDL driven to high levels by healthy lifestyle appears unambiguously protective. If your HDL is very high without an obvious lifestyle explanation, it may warrant investigation.
Oestrogen stimulates hepatic ApoA1 production and CETP activity, raising HDL production and functionality. Pre-menopausal women typically have HDL 0.3–0.4 mmol/L higher than age-matched men. After menopause, HDL falls and LDL rises — partially explaining the increase in cardiovascular risk that occurs in women after menopause. HRT (particularly oestrogen-based) maintains the pre-menopausal lipid profile.
No — low HDL is a risk factor, not a diagnosis of heart disease. It means your long-term cardiovascular risk is higher than it would be with adequate HDL, particularly in combination with elevated LDL, hypertension, smoking, or diabetes. The QRISK3 score integrates HDL with other risk factors to estimate your 10-year cardiovascular event risk — a much more informative number for clinical decision-making.
This is one of the most interesting findings in cardiology. CETP inhibitors (anacetrapib, torcetrapib) dramatically raise HDL but have largely failed to reduce cardiovascular events. Niacin, which raises HDL, also failed in large trials. The conclusion is that HDL quantity alone is not sufficient — HDL function (its ability to efflux cholesterol and protect the endothelium) is what matters. Dysfunctional HDL from inflammation or diabetes may be high in quantity but impaired in function. Healthy lifestyle achieves both high quantity and good function.
References
- Barter PJ, et al. HDL cholesterol, very low levels of LDL cholesterol, and cardiovascular events. N Engl J Med. 2007;357(13):1301–1310. View source
- Voight BF, et al. Plasma HDL cholesterol and risk of myocardial infarction: a Mendelian randomisation study. Lancet. 2012;380(9841):572–580. View source
- NICE. Cardiovascular disease: risk assessment and reduction, including lipid modification. CG181. Updated 2023. View source