Apolipoprotein B

What is ApoB?

Apolipoprotein B (ApoB) is the protein scaffold of every lipoprotein particle that can cause atherosclerosis — VLDL, IDL, LDL and lipoprotein(a). Each of those particles carries exactly one ApoB molecule for its entire life, and the molecule never moves to another particle, so measuring ApoB in blood counts the particles themselves. This is the key difference from LDL cholesterol, which weighs the cholesterol cargo inside the particles: two people with identical LDL-C can differ substantially in ApoB, because their particles carry different amounts of cholesterol each.

What is ApoB made of, and where does it come from?

The liver makes ApoB-100, a single chain of 4,536 amino acids and one of the largest proteins in the body, and wraps it around each VLDL particle it secretes; as VLDL sheds triglyceride it becomes IDL and then LDL, keeping the same ApoB throughout. The intestine makes a shortened version, ApoB-48, for chylomicrons after meals. Standard clinical assays measure both, but more than 90% of circulating ApoB sits on LDL particles, so the result is effectively an LDL particle count. Results are reported in g/L or mg/dL (1 g/L = 100 mg/dL), standardised against a WHO-IFCC reference material.

What does ApoB do in the body?

ApoB does two jobs that together explain why particle number drives heart disease. On the surface of each particle it is the ligand the liver's LDL receptor recognises, so it governs how quickly particles are cleared from blood. It is also the region that binds to proteoglycans in the artery wall, which is what traps a particle after it crosses the endothelium — the 'response to retention' that starts a plaque. Particles enter the wall roughly in proportion to how many are circulating, not to how much cholesterol each carries, which is why ApoB tracks risk more closely than LDL-C when the two disagree. Mendelian randomisation studies summarised by Ference and colleagues in 2017 show that genetic variants change heart disease risk in proportion to their effect on ApoB particle number, whatever they do to cholesterol or triglyceride.

What does a high ApoB mean?

A high ApoB means more atherogenic particles are circulating, and therefore more are entering and being retained in artery walls; the risk it carries accumulates over years in the same way as LDL-C. ApoB matters most when it is high while LDL-C looks acceptable — a pattern common in insulin resistance, type 2 diabetes, high triglycerides and abdominal obesity, where each particle carries less cholesterol so LDL-C understates the count. Sniderman and colleagues' 2019 review found that in such discordant cases ApoB predicts events better than LDL-C or non-HDL-C. The 2019 ESC/EAS dyslipidaemia guideline accepts ApoB as an alternative primary measurement for this reason and sets risk-based goals rather than a single universal cut-off; what counts as high therefore depends on overall cardiovascular risk, decided with a clinician.

Common causes:

• Insulin resistance, metabolic syndrome and type 2 diabetes, in which the liver overproduces VLDL and the resulting LDL particles are small, numerous and cholesterol-poor — high ApoB with unremarkable LDL-C.
• A diet high in saturated and trans fats, which reduces LDL receptor activity and slows particle clearance.
• Familial hypercholesterolaemia and related inherited defects in the LDL receptor, ApoB itself, or PCSK9 — suspected when ApoB is very high from a young age or heart disease runs early in the family.
• Untreated hypothyroidism, nephrotic syndrome, cholestatic liver disease, and medications such as corticosteroids, some antiretrovirals and certain immunosuppressants.
• Weight gain and heavy alcohol intake, both of which raise VLDL output.

What does a low ApoB mean?

A low ApoB is almost always good news and rarely needs explaining: it means few atherogenic particles, and no floor has been found below which lower ApoB stops reducing risk. People born with low ApoB because of loss-of-function variants in PCSK9 or truncating variants in the APOB gene have markedly less coronary disease over a lifetime. The exceptions are uncommon: very low values that were not produced by treatment can accompany hyperthyroidism, malabsorption, malnutrition, advanced liver disease or an acute illness, and the rare inherited condition familial hypobetalipoproteinaemia can cause fatty liver despite its cardiovascular advantage.

Common causes:

• Lipid-lowering treatment — statins, ezetimibe or PCSK9 inhibitors — working as intended.
• Inherited low ApoB, which is favourable and needs no action in itself.
• Hyperthyroidism, malabsorption, malnutrition or advanced liver disease.
• Acute illness, major surgery or the weeks after a heart attack, when all lipoproteins fall temporarily.

What moves ApoB apart from disease?

Fasting — not required either direction

ApoB is one of the most meal-stable lipid measures: the intestinal ApoB-48 that rises after eating is a tiny fraction of total ApoB, so a non-fasting value differs from a fasting one by only a few percent. The 2016 EAS/EFLM consensus statement concluded that fasting is not routinely needed for lipid profiles, and ApoB is the component least affected by that choice.

Recent acute illness, infection, surgery or heart attack lowers it

Inflammation suppresses lipoprotein production, so ApoB falls during an acute illness and for several weeks afterwards and can read deceptively low. A value drawn in that window does not represent the usual level.

Pregnancy raises it

All lipoproteins rise physiologically through pregnancy, with ApoB typically up by a third or more by the third trimester and returning to baseline over the months after delivery. Results from this period are not comparable with pre-pregnancy values.

Oestrogen and other hormones either direction

Oral oestrogen — in combined contraceptives and menopausal hormone therapy — modestly lowers ApoB and LDL-C while raising triglycerides; transdermal oestrogen has much less effect. ApoB rises after the menopause as oestrogen falls, and it rises with androgen and anabolic steroid use.

Thyroid status either direction

An underactive thyroid slows LDL receptor activity and raises ApoB; an overactive one lowers it. Thyroid function is usually checked before a newly raised ApoB is treated as a lipid problem.

Units and assay standardisation either direction

ApoB is reported in g/L in most of the world and mg/dL in the United States, a hundredfold difference in the printed number. Between-laboratory agreement has improved with WHO-IFCC standardisation but is not perfect, so results from different laboratories are best compared with that caveat in mind.

Which other tests are read together with ApoB?

LDL Cholesterol read as a ratio

LDL-C weighs the cholesterol inside the particles that ApoB counts. When the two agree, either serves; when they disagree — high triglycerides, diabetes, obesity — the ApoB count is the better guide to risk, because particles, not cholesterol mass, enter the artery wall.

Triglycerides can distort it

High triglycerides are the main reason ApoB and LDL-C part company: triglyceride-rich VLDL and small dense LDL each carry one ApoB but little cholesterol, so the particle count climbs while LDL-C stays flat.

Apolipoprotein A1 read as a ratio

ApoA1 is the main protein of HDL, and the ApoB/ApoA1 ratio sets atherogenic particles against protective ones in a single number; it was the strongest lipid predictor of heart attack in the INTERHEART study.

Total Cholesterol same panel

Total cholesterol minus HDL-C gives non-HDL cholesterol, the cholesterol in all ApoB particles combined. It is the closest substitute for ApoB when ApoB itself is not available, though it still measures cargo rather than particles.

VLDL Cholesterol same panel

VLDL particles are the precursors of LDL and each carries an ApoB. A high VLDL-C alongside high ApoB points to overproduction by the liver, which is the metabolic-syndrome pattern.

Thyroid Stimulating Hormone can distort it

An underactive thyroid is a reversible cause of raised ApoB and is usually excluded before lipid-lowering treatment starts.

Which foods affect ApoB?

• Replacing saturated fat with unsaturated fat — olive oil, nuts and oily fish in place of butter, fatty meat and palm oil — lowers strong evidence
• Soluble fibre — oats and barley beta-glucan, legumes, psyllium — lowers strong evidence
• Cutting refined carbohydrate, sugar-sweetened drinks and alcohol — the levers that reduce VLDL overproduction when ApoB is high but LDL-C is not — lowers moderate evidence
• Foods fortified with plant sterols or stanols — lowers strong evidence
• Industrial trans fats — raises strong evidence

Which supplements are studied for ApoB?

Plant sterols / stanols strong evidence

Compete with cholesterol for absorption in the gut, so the liver draws down circulating LDL to compensate. Around 2 g a day lowers LDL-C by roughly 8-10% and ApoB by somewhat less, since the effect is on cholesterol per particle as well as particle number.
Typical dose: 2 g/day, taken with meals
Cautions: Not suitable for people with sitosterolaemia, a rare inherited disorder of sterol absorption. Slightly reduces absorption of carotenoids; eating plenty of vegetables offsets this.

Psyllium (soluble fibre) moderate evidence

Binds bile acids in the gut, forcing the liver to make more from circulating cholesterol and to upregulate LDL receptors. Trials show reductions in ApoB of a few percent, in line with the modest LDL-C effect.
Typical dose: 5-10 g/day
Cautions: Increase the dose gradually and take with plenty of water; abrupt introduction commonly causes bloating. Separate from other medications by at least two hours, as it can reduce their absorption.

Omega-3 fatty acids (EPA/DHA) emerging evidence

Reduce the liver's triglyceride output and so lower triglyceride-rich VLDL. The effect on ApoB is small and inconsistent at over-the-counter doses, and DHA-containing products can raise LDL-C slightly. The cardiovascular benefit seen in trials came from high-dose prescription EPA, which is not the same product as fish-oil capsules.
Typical dose: no general dose given
Cautions: High doses increase the risk of atrial fibrillation in randomised trials and can prolong bleeding time, especially alongside anticoagulants. Oxidation and content vary between products; the dose stated on the label often overstates the EPA and DHA actually present.

How does exercise affect ApoB?

Regular aerobic exercise moderate evidence

How much: At least 150 minutes a week at moderate intensity, or 75 minutes vigorous.
What to expect: Aerobic training lowers ApoB modestly, mainly by reducing triglyceride-rich VLDL particles and improving insulin sensitivity — so it helps most in the high-ApoB, normal-LDL-C pattern of metabolic syndrome. The direct effect on cholesterol-rich LDL is small.

Resistance training moderate evidence

How much: Two or three sessions a week covering the major muscle groups.
What to expect: Small additional reduction in ApoB, and it preserves lean mass during weight loss — which is when ApoB improves most.

Which lifestyle factors affect ApoB?

Weight loss where relevant strong evidence

Losing 5-10% of body weight lowers ApoB more reliably than it lowers LDL-C, because it reduces the liver's VLDL output and shrinks the excess of small dense particles. It is the most effective lifestyle lever for the discordant high-ApoB pattern.

Stopping smoking strong evidence

Smoking barely moves ApoB but multiplies the damage each particle does by oxidising it and injuring the artery lining. On cardiovascular risk it outweighs almost any dietary change.

Knowing your family history strong evidence

A parent or sibling with heart disease before 55 (men) or 65 (women), or a persistently very high ApoB, points toward familial hypercholesterolaemia — common, treatable, and usually recognised decades later than it could be.

How should you prepare for a ApoB test?

• Fasting is not needed; ApoB is the lipid measure least affected by a recent meal.
• ApoB is not on every standard lipid panel and often has to be requested separately; it is most informative when triglycerides are high, in diabetes or obesity, or when LDL-C is borderline.
• Check the units — g/L and mg/dL differ a hundredfold — and compare results from the same laboratory over time.
• Values drawn during or in the weeks after an acute illness, surgery or heart attack read low and do not represent the usual level.
• Reading ApoB next to LDL-C on the same sample is the point of measuring it: agreement is reassuring, disagreement is the finding.

References

1. Sniderman AD et al. Apolipoprotein B particles and cardiovascular disease: a narrative review. JAMA Cardiol 2019 — https://doi.org/10.1001/jamacardio.2019.3780
2. Mach F et al. 2019 ESC/EAS Guidelines for the management of dyslipidaemias. Eur Heart J 2020 — https://doi.org/10.1093/eurheartj/ehz455
3. Ference BA et al. Low-density lipoproteins cause atherosclerotic cardiovascular disease — EAS Consensus Panel. Eur Heart J 2017 — https://doi.org/10.1093/eurheartj/ehx144
4. Marston NA et al. Association of apolipoprotein B-containing lipoproteins and risk of myocardial infarction in individuals with and without atherosclerosis. JAMA Cardiol 2022 — https://doi.org/10.1001/jamacardio.2021.5083
5. Nordestgaard BG et al. Fasting is not routinely required for determination of a lipid profile — EAS/EFLM joint consensus statement. Eur Heart J 2016 — https://doi.org/10.1093/eurheartj/ehw152