Understanding the Essentials of Blood Lipid Metabolism

Kori J. Kingsbury, RN, MSN, Greg Bondy, MD, FRCPC

In This Article

Lipid Terminology

Some of the complexity for the novice striving to understand lipid metabolism may be due to the terminology used in relation to this topic. For example, the terms "cholesterol" and "lipid" may be considered by some to be synonymous, and as such, are used without distinction in reference to dyslipidemic conditions. Although this may be appropriate when the need arises to provide simple and succinct information in lay contexts, it is problematic to uphold this nomenclature when attempting to embrace a greater understanding of the topic. Cholesterol is one of the major lipid particles in the body; the other is triglyceride (TG). Both of these particles serve important functions; however as insoluble molecules, they must be transported in the blood in complexes known as lipoproteins.[7] Plasma lipoproteins are composed of a core of TG and cholesterol ester, enveloped by a surface coat of phospholipid, unesterified ("free") cholesterol, and special proteins called apolipoproteins (or apoproteins). The term "lipoprotein" refers to this unique combination of "lipid" and "protein." Several lipoprotein complexes exist, and each is identified according to its density, lipid composition, and the apolipoproteins on the surface of the particle. The five main classifications of lipoproteins are chylomicrons, very low-density lipoprotein cholesterol (VLDL), intermediate-density lipoprotein cholesterol (IDL), low-density lipoprotein cholesterol (LDL), and high-density lipoprotein cholesterol (HDL). In the laboratory, total cholesterol, TG, and HDL are measured directly, from which a calculated estimation of VLDL and LDL cholesterol are derived. These measurements of VLDL and LDL are based on a fasting TG level and are valid only when the TG level is less than 400 mg/dL (5 mmol/L). This explains the requirement for a fasting blood sample (overnight fast of 12 hours) to determine blood lipid levels. Direct measurement of VLDL and LDL is also possible; however, due to their high cost and technical complexity, these are performed primarily in reference laboratories.