HDL Cholesterol

What is HDL-C?

HDL cholesterol is the cholesterol carried inside high-density lipoprotein particles, the smallest and densest lipoproteins in blood. Like LDL-C, it measures cargo rather than vehicles: it reports how much cholesterol the HDL fraction holds, not how many HDL particles there are or how well they work. That distinction has become central, because the evidence of the last decade links protection to what HDL particles do, not to how much cholesterol they happen to contain.

What is HDL-C made of, and where does it come from?

HDL particles are built around apolipoprotein A-I, often joined by apoA-II, enclosing a small core of cholesteryl esters and triglycerides. They are a heterogeneous family, ranging from small lipid-poor discs newly released by the liver and intestine to larger cholesterol-laden spheres. The laboratory measures the cholesterol left after apoB-containing particles have been precipitated or masked, so 'HDL-C' is in effect the cholesterol that is not in LDL, VLDL or chylomicrons.

What does HDL-C do in the body?

HDL's best-known task is reverse cholesterol transport: collecting surplus cholesterol from peripheral cells, including macrophages in the artery wall, and returning it to the liver for disposal in bile. HDL particles also carry antioxidant and anti-inflammatory enzymes such as paraoxonase-1 and support the function of the vessel lining. A high HDL-C, however, does not guarantee that any of this is happening. Mendelian randomisation by Voight and colleagues (Lancet 2012) found that genetic variants which raise HDL-C do not lower the risk of heart attack, and drugs that raised HDL-C substantially — niacin, CETP inhibitors — failed to cut events in proportion. HDL-C is therefore read as a marker of metabolic health, travelling with insulin sensitivity, low triglycerides, normal weight and not smoking, rather than as a lever that can be pulled on its own.

What does a high HDL-C mean?

A high HDL-C usually reflects a favourable metabolic state and the habits behind it — regular exercise, a healthy weight, not smoking — which are protective in their own right. It is not protection in itself, and the relationship with outcomes is not a straight line: in the Copenhagen cohort analysis by Madsen and colleagues (Eur Heart J 2017), people with extremely high HDL-C — above about 2.5 mmol/L (97 mg/dL) in men and 3.0 mmol/L (116 mg/dL) in women — had higher all-cause mortality than those with intermediate values, not lower. Very high values can also come from heavy alcohol use, certain medications, or genetic variants such as CETP deficiency that bring no cardiovascular benefit. A high HDL-C is best read as 'no added concern from this number', not as a reason to disregard LDL-C, apoB or blood pressure.

Common causes:

• Regular aerobic exercise, a healthy body weight and good insulin sensitivity — the ordinary, favourable reasons.
• Alcohol, which raises HDL-C in proportion to intake without conferring the protection the number would suggest.
• Oestrogen — higher levels in women than men, and a rise with oral oestrogen therapy.
• Inherited variants in CETP, hepatic lipase or SR-BI that slow HDL clearance; some are neutral for the heart, others are not.
• Medications such as fibrates, niacin and some anticonvulsants.

What does a low HDL-C mean?

A low HDL-C is one of the most consistent markers of raised cardiovascular risk in population studies, and it usually arrives with the rest of the insulin-resistance cluster: high triglycerides, abdominal fat, raised fasting glucose and higher blood pressure. The 2019 ESC/EAS guideline lists HDL-C below 1.0 mmol/L (40 mg/dL) in men and 1.2 mmol/L (45 mg/dL) in women as a marker of increased risk, and the 2018 AHA/ACC guideline counts a low HDL-C among the features of metabolic syndrome that enhance risk. The useful response targets the causes — weight, activity, smoking, triglycerides — because those change risk; medications that raise HDL-C on its own have not.

Common causes:

• Insulin resistance, abdominal obesity and type 2 diabetes, which raise triglycerides and speed the breakdown of HDL particles.
• Smoking, which lowers HDL-C and impairs its function.
• Physical inactivity and a diet high in refined carbohydrate and sugar.
• Medications including anabolic steroids and other androgens, some progestins, beta-blockers, thiazide diuretics and isotretinoin.
• Rare inherited disorders of apoA-I, ABCA1 (Tangier disease) or LCAT, in which HDL-C is extremely low from birth.

What moves HDL-C apart from disease?

Alcohol raises it

Alcohol raises HDL-C in a dose-dependent way, mostly by increasing apoA-I production, and the effect is visible in anyone who drinks regularly. The rise does not bring the protection normally associated with the number, so a good HDL-C in a regular drinker is less reassuring than the same value in a non-drinker.

Smoking lowers it

Smokers run an HDL-C several percent lower than non-smokers, and the value recovers within weeks of stopping. A rise in HDL-C after quitting is one of the earliest measurable benefits.

Sex hormones — menstrual cycle, pregnancy, menopause, contraception either direction

Oestrogen raises HDL-C and androgens lower it, which is why women run higher values than men from puberty onward. HDL-C moves modestly across the menstrual cycle, rises in pregnancy, drifts down after menopause, and shifts with oral contraceptives depending on the progestin they contain.

Recent acute illness, infection or surgery lowers it

The acute-phase response drops HDL-C sharply, sometimes by a third or more, and the value takes weeks to recover. An HDL-C drawn during or shortly after an illness understates the usual level.

Medications either direction

Anabolic steroids and testosterone, some progestins, beta-blockers, thiazides and isotretinoin lower HDL-C; fibrates, niacin, oral oestrogens and some anticonvulsants raise it. A change that coincides with a new or stopped prescription usually has that explanation.

Which phase of weight change the sample falls in either direction

HDL-C typically falls while weight is actively being lost and then rises above the starting value once weight has stabilised. A value taken mid-diet can look worse than before the diet began; the result after a few weeks of stable weight is the meaningful one.

Which other tests are read together with HDL-C?

Total Cholesterol read as a ratio

Total cholesterol minus HDL-C gives non-HDL cholesterol, which counts every atherogenic particle in one number; the total-to-HDL ratio is used in several risk calculators for the same reason.

Triglycerides read as a ratio

Triglycerides and HDL-C move in opposite directions through the same mechanism — insulin resistance — so a high TG/HDL-C ratio is a practical proxy for insulin resistance and small dense LDL. A low HDL-C almost always needs the triglyceride value next to it.

Apolipoprotein A1 same panel

Apolipoprotein A-I is the main structural protein of HDL and reflects particle number more closely than HDL-C does. The apoB/apoA-I ratio is an alternative summary of the balance between atherogenic and protective particles.

LDL Cholesterol same panel

LDL-C is the fraction that treatment decisions rest on. A reassuring HDL-C does not offset a high LDL-C, and guidelines set no HDL-C target precisely because raising it has not changed outcomes.

Fasting Insulin upstream

Fasting insulin, read with fasting glucose, estimates the insulin resistance that sits upstream of a low HDL-C and high triglycerides. Improving insulin sensitivity is the route by which HDL-C usually rises.

Gamma-Glutamyl Transferase can distort it

A raised GGT alongside a high HDL-C points toward alcohol as the explanation for the HDL-C, in which case the number carries no protective meaning.

Which foods affect HDL-C?

• Replacing refined carbohydrate with unsaturated fat — olive oil, nuts, avocado, oily fish — raises strong evidence
• A Mediterranean dietary pattern — raises moderate evidence
• Sugar-sweetened drinks and a high-refined-carbohydrate diet — lowers strong evidence
• Industrial trans fats — lowers strong evidence
• Alcohol — raises the number, but is not a means of improving cardiovascular risk — raises strong evidence

Which supplements are studied for HDL-C?

Niacin (nicotinic acid, high dose) strong evidence

Pharmacological doses of niacin raise HDL-C by around 15-25%, the largest effect of any agent, by slowing HDL clearance and reducing VLDL output. The large AIM-HIGH and HPS2-THRIVE trials nevertheless found no reduction in cardiovascular events when niacin was added to a statin, which is the clearest evidence that the HDL-C number is not itself the target.
Typical dose: no general dose given
Cautions: The HDL-raising doses are prescription-level and are not appropriate to self-select; ordinary multivitamin amounts of niacin do not affect HDL-C. Flushing, itching, raised glucose, raised uric acid and liver enzyme elevation are common; sustained-release products carry a higher liver risk. In HPS2-THRIVE, adding niacin increased serious adverse events — infections, bleeding, new diabetes — without cardiovascular benefit.

Omega-3 fatty acids (EPA + DHA) moderate evidence

Marine omega-3 fatty acids lower triglycerides and produce a small rise in HDL-C as a by-product; DHA raises it slightly more than EPA. The HDL effect is modest and secondary — the reason to consider omega-3 is triglycerides, not this number.
Typical dose: 1-2 g/day of combined EPA and DHA
Cautions: Higher intakes have been linked to a small increase in atrial fibrillation in recent trials. Can prolong bleeding time; people on anticoagulants or antiplatelet drugs should discuss it with their doctor first. Fishy aftertaste and loose stools are common and usually improve when taken with meals.

How does exercise affect HDL-C?

Aerobic exercise strong evidence

How much: At least 120 minutes a week, with session length mattering more than intensity; benefits appear from about 30 minutes per session and grow with duration.
What to expect: Aerobic training raises HDL-C by a small but consistent amount — about 0.065 mmol/L (2.5 mg/dL) on average in the meta-analysis by Kodama and colleagues (2007). The gain is modest in size, yet unlike drug-induced rises it travels with better HDL function and lower cardiovascular risk.

Resistance training moderate evidence

How much: Two or three sessions a week covering the major muscle groups.
What to expect: Resistance training on its own changes HDL-C little and inconsistently. Its contribution comes through insulin sensitivity, body composition and blood pressure, and through making a combined programme easier to sustain.

High-intensity interval training emerging evidence

How much: Two or three sessions a week of short hard intervals separated by recovery, in place of some steady-state sessions.
What to expect: Interval training raises HDL-C at least as much as continuous moderate exercise in short trials, in less total time. The evidence base is smaller and the trials shorter than for conventional aerobic training.

Which lifestyle factors affect HDL-C?

Stopping smoking strong evidence

Quitting raises HDL-C within weeks and improves how well HDL particles work, which the number alone does not show. Among the levers that move HDL-C, this is the one with the largest effect on outcomes.

Body weight, especially around the waist strong evidence

Sustained weight loss raises HDL-C by roughly 0.01 mmol/L per kilogram once weight has stabilised, and lowers triglycerides at the same time. Waist circumference tracks the visceral fat that drives a low HDL-C better than total body weight does.

Alcohol, read honestly strong evidence

Alcohol raises HDL-C, but the 2019 ESC/EAS guideline does not recommend drinking to do so, and genetic studies suggest the apparent heart benefit of moderate drinking is largely confounded. An HDL-C propped up by alcohol should not be counted as protection.

How should you prepare for a HDL-C test?

• Fasting is not needed for HDL-C, which changes very little after a meal.
• Avoid testing during or within a few weeks of an infection, operation or other acute illness, when the value reads falsely low.
• If weight is actively falling, wait until it has been stable for a few weeks; HDL-C dips during weight loss and recovers afterward.
• Note recent alcohol intake when reading the result. A value raised by drinking is not a protective value.
• Read HDL-C next to triglycerides rather than alone; the pair says far more about insulin resistance than either does by itself.

References

1. Madsen CM et al. Extreme high high-density lipoprotein cholesterol is paradoxically associated with high mortality in men and women: two prospective cohort studies. Eur Heart J 2017 — https://doi.org/10.1093/eurheartj/ehx163
2. Voight BF et al. Plasma HDL cholesterol and risk of myocardial infarction: a mendelian randomisation study. Lancet 2012 — https://doi.org/10.1016/S0140-6736(12)60312-2
3. Mach F et al. 2019 ESC/EAS Guidelines for the management of dyslipidaemias. Eur Heart J 2020 — https://doi.org/10.1093/eurheartj/ehz455
4. Grundy SM et al. 2018 AHA/ACC/AACVPR/AAPA/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA Guideline on the Management of Blood Cholesterol. Circulation 2019 — https://doi.org/10.1161/CIR.0000000000000625
5. Kodama S et al. Effect of aerobic exercise training on serum levels of high-density lipoprotein cholesterol: a meta-analysis. Arch Intern Med 2007 — https://doi.org/10.1001/archinte.167.10.999