Why is Apolipoprotein B (ApoB) Low? Causes, Symptoms, and What to Do

A low apolipoprotein B (apoB) means you have fewer of the cholesterol-carrying particles that can build plaque in your arteries. In most people this is good news for heart health, not a problem. Typical reasons are cholesterol-lowering medicines doing their job, an overactive thyroid, poor nutrition or poor absorption of food, serious illness or advanced liver disease; inherited conditions are a rarer cause. A low apoB does not cause symptoms by itself.

What apoB is

Cholesterol and other fats travel in the blood inside particles called lipoproteins. Apolipoprotein B is the main protein of the particles that can cause plaque: LDL ("bad" cholesterol), VLDL, their remnants and lipoprotein(a). Each of these particles carries exactly one apoB molecule, so the apoB level is effectively a count of these particles. The form usually measured, apoB-100, is made by the liver.

European guidelines describe apoB as an accurate estimate of the total number of plaque-forming particles in all circumstances. Standard LDL cholesterol can underestimate the particle count in people with high triglycerides, diabetes, obesity or very low LDL cholesterol, which is why the guidelines recommend measuring apoB in such patients.

ApoB values

The goals come from the 2019 European Society of Cardiology / European Atherosclerosis Society (ESC/EAS) guidelines, where they are secondary targets after LDL cholesterol. Some laboratories report apoB in g/L instead of mg/dL (1 g/L equals 100 mg/dL). Reference ranges and units differ between laboratories, and there is no single agreed cut-off for "low", so compare your number with the range printed on your report.

Causes of low apoB

Reasons that are usually harmless

• Cholesterol-lowering treatment. Statins and other lipid-lowering medicines, such as niacin, lower apoB. For someone on treatment, a low value is usually the intended result.
• Naturally low cholesterol. Some people simply make or keep fewer LDL particles, sometimes because of inherited variants that are harmless.
• Other medicines. Oestrogen (after menopause) and thyroid hormone (thyroxine) have been linked with lower apoB.

Medical conditions (a clue to something else)

These secondary causes are far more common than the inherited ones.

• Overactive thyroid (hyperthyroidism), a recognised cause of unexpectedly low cholesterol and apoB.
• Malnutrition, weight loss or malabsorption. Not eating enough, or a gut that cannot absorb food well, is linked with low cholesterol and apoB. Undernutrition from heavy alcohol use counts here.
• Severe or chronic illness and recent surgery. Serious infections (including chronic hepatitis C), inflammatory states and some cancers can lower cholesterol and apoB.
• Advanced liver disease (cirrhosis). The liver makes apoB-100, so severe liver damage can reduce it.

Rare inherited causes

• Familial hypobetalipoproteinemia. Usually caused by a change in the APOB gene, it affects roughly 1 in 1,000 to 3,000 people. People with one altered copy usually have no symptoms and need no treatment, although some develop fat build-up in the liver (fatty liver). With two altered copies, problems absorbing fat and fat-soluble vitamins can occur.
• Abetalipoproteinemia. A very rare condition, usually noticed in infancy, in which apoB is absent from the blood. It causes fat malabsorption and, without treatment, severe vitamin E deficiency affecting the nerves and eyes.
• Loss-of-function variants in the PCSK9 gene lower LDL cholesterol and are described as having no adverse consequences.

Symptoms

A low apoB on its own does not cause symptoms. If symptoms are present, they come from the cause: for example weight loss with poor nutrition, fatty stools (steatorrhoea) and poor weight gain with fat malabsorption, or the signs of liver disease.

The same is true of a high apoB: it causes no symptoms, just as high cholesterol usually has none. Its importance is the raised risk of narrowed arteries, which may eventually show up as chest pain on exertion (angina) or a heart attack.

What usually happens next

If your apoB is low and you take a cholesterol-lowering medicine, often nothing further is needed. If it is unexpectedly low and you are not on such treatment, a clinician may:

• review a full lipid profile (total, LDL and HDL cholesterol, triglycerides);
• check liver enzymes (ALT, AST) and thyroid function (TSH), which are the standard first tests for unexpectedly low cholesterol;
• ask about diet, weight loss, alcohol, bowel symptoms and recent illness;
• consider genetic testing if a family pattern or signs of fat malabsorption suggest an inherited cause.

If these are normal, an inherited, usually harmless cause becomes the likely explanation.

Frequently asked questions

What does a low apolipoprotein B mean?

It means fewer plaque-forming particles in your blood. On its own this usually means lower cardiovascular risk. It is worth looking into only if it is unexpectedly low or comes with symptoms such as weight loss or digestive problems.

Is low apoB dangerous?

Usually not. The guidelines have not defined a level of LDL cholesterol below which harm begins, and people with the common inherited forms of low apoB usually have no symptoms. The rare exceptions are severe inherited forms and the illnesses listed above.

Do you need to fast for an apolipoprotein B test?

Usually not. European guidelines state that fasting is not required for apoB, because meals barely change it. Some clinics still ask for a fast of 4 to 6 hours, often because other tests are taken from the same sample, so follow your clinic's instructions.

Is apolipoprotein B fasting or non-fasting?

ApoB itself is reliable in a non-fasting sample. Any fasting request usually relates to the other tests ordered with it.

Can apoB, lipoprotein(a) and hs-CRP be measured from the same blood draw?

They are separate blood tests that are commonly ordered together for cardiovascular risk assessment, so they can usually be requested at the same visit. European guidelines suggest measuring lipoprotein(a) at least once in a lifetime.

What are the symptoms of high apolipoprotein B?

There are none from the number itself. High apoB raises the risk of plaque in the arteries; symptoms appear only once arteries are narrowed, for example chest pain on exertion.

When to see a doctor

If your apoB is low and you are not taking a cholesterol-lowering medicine, it is worth discussing with your doctor at your next appointment, especially if you have unintended weight loss, persistent diarrhoea or fatty stools, or a known liver or thyroid condition. If you are on treatment and your apoB is low, discuss it at your usual review rather than changing anything yourself. Seek urgent care for chest pain, fainting or severe breathlessness.

Sources

1. 2019 ESC/EAS Guidelines for the management of dyslipidaemias — European Heart Journal
2. Hypolipidemia — Merck Manual Professional Version
3. Familial hypobetalipoproteinemia — MedlinePlus Genetics, U.S. National Library of Medicine
4. Apo B test — Lab Tests Online UK
5. Apolipoprotein B — UCSF Health
6. Apolipoprotein B Particles and Cardiovascular Disease: A Narrative Review — JAMA Cardiology (PMC)
7. Cholesterol — MedlinePlus, U.S. National Library of Medicine

Decisions about your health should always be made in consultation with your healthcare provider.