What are apolipoproteins?
Lipids, including cholesterol, are essential for the proper functioning of the body: for example, energy storage, hormone production, and in the formation of bile salts. However, dyslipidemias (imbalance of blood lipids) accelerate the development of cardiovascular diseases, such as coronary heart disease or cerebrovascular disease.
Cholesterol is not soluble in water. In order to be able to circulate in the blood, it needs to bind to transporter proteins called apolipoproteins. Apolipoproteins have a hydrophilic part that binds to water and a hydrophobic part that binds to cholesterol and other lipid molecules. Cholesterol and apolipoproteins together form lipoprotein particles that can be measured in the blood. Depending on the density of the lipoprotein particles, they can be classified into the following:
Chylomicrons: very large particles that transport fat from the diet
HDL: High Density Lipoprotein
IDL: Intermediate Density Lipoprotein
LDL: Low Density Lipoprotein
VLDL: Very Low Density Lipoprotein
Different lipoprotein particles contain different types of apolipoproteins. HDL mainly contains apolipoprotein A1 while apolipoprotein B is the main class of apolipoproteins found in chylomicrons, IDL, LDL and VLDL.
Different lipoprotein particles also have different effects on cardiovascular risk. Particles such as LDL transport cholesterol from the liver to peripheral tissue such as blood vessels and cause cholesterol build up which increases cardiovascular risk. On the other hand, particles such as HDL transport excess cholesterol from peripheral tissue to the liver to be excreted with bile and thereby decrease cardiovascular risk. Therefore, LDL is often called “the bad cholesterol”, while HDL is called “the good cholesterol”. Apolipoprotein A1 contained in HDL particles also has an anti-inflammatory effect which provides further protection against cardiovascular disease.
In short, it is useful to analyse apolipoprotein levels in the blood to:
Assess risk for cardiovascular disease
Track the effect of life-style changes
Why is this analysis important?
Apolipoproteins are measured to estimate cardiovascular risk. Unfavorable lipoprotein levels in the blood rarely give any symptoms, but if left untreated, it can lead to serious consequences such as heart attack or stroke. Early detection is therefore key to prevent these cardiovascular events from taking place.
The levels of apolipoproteins in the blood often follow the levels of cholesterol. Apolipoprotein A1 tends to mirror the levels of HDL cholesterol and apolipoprotein B tends to mirror the levels of LDL cholesterol. However, there are several advantages to measuring apolipoproteins over conventional lipid panels.
Apolipoprotein levels do not fluctuate depending on food intake, so there is no risk of obtaining an inaccurate value due to inadequate fasting.
In people with high triglyceride values, it is often not possible to obtain a LDL value because LDL is calculated from triglyceride and total cholesterol, making it difficult to correctly estimate the risk for future cardiovascular disease. Measurement of apolipoprotein B is not affected by triglyceride or cholesterol values.
LDL cholesterol is not the only type of “bad” cholesterol in the blood. IDL and VLDL are also detrimental to cardiovascular health, but are not measured in a conventional lipid panel. Apolipoprotein B molecule is present in all the harmful lipoprotein particles including LDL, IDL and VLDL and can therefore better reflect the risk for cardiovascular disease.
Calculation of ApoB:ApoA1 ratio gives a good estimate of the balance between “good” and “bad” lipoprotein particles in the blood. Several large studies have shown that the ApoB:ApoA1 ratio performs as well, and often better, than traditional lipids as an indicator of risk.
The reference range for apolipoprotein levels in the blood can vary depending on the laboratory and technique used. Doctors usually also take into account a number of factors when evaluating apolipoprotein values.
High Apolipoprotein B levels and/or low Apolipoprotein A1 levels in the blood may be associated with:
Lack of physical activity
High dietary intake of saturated fats and sugarsFamily history of dyslipidemia
Tobacco and/or alcohol use
Medication such as oral contraceptive (female), steroids, androgens, beta-blockers
Conditions such as diabetes, hypothyroidism, polycystic ovary disease (female), liver disease, kidney disease
Extremely low values of apolipoprotein B may be associated with:
Rare genetic disorder
Intake of lipid-lowering medication such as statins
Life-style, medication, current diagnosis and family history can all affect apolipoprotein levels. Please let our doctors know this information by filling in your health profile.
Walldius G et al. High apolipoprotein B, low apolipoprotein A-I, and improvement in the prediction of fatal myocardial infarction (AMORIS study): a prospective study. Lancet. 2001 Dec 15;358(9298):2026-33.
McQueen MJ et al. Lipids, lipoproteins, and apolipoproteins as risk markers of myocardial infarction in 52 countries (the INTERHEART study): a case-control study. Lancet. 2008 Jul 19;372(9634):224-33.