Along with proteins and carbohydrates, lipids play an integral role in maintaining cell function and sustaining life. Lipids are a family of water-insoluble organic compounds that includes fats, oils, sterols, and triglycerides. Cholesterols and phospholipids are the building blocks for the cell wall and the plasma membrane, respectively. Triglycerides are made of fatty acids, which can be utilized for energy production.
Given that the human body is primarily aqueous and lipids are water-insoluble, lipids are transported via a unique lipoprotein family. This consists of a water-soluble outer coat with nonpolar lipid coat. Apolipoproteins are proteins that help in this transport system (Figure 2-1).
Apolipoprotein on lipid surface membrane.
Briefly, there are two major sources for lipids in the body: diet and hepatic production. The chylomicrons deliver lipids from dietary fat to the plasma, and the very low-density lipoproteins (VLDLs) transport lipids from the hepatic production of lipids to the body (Figure 2-2). Lipoprotein lipase breaks down the VLDL to low-density lipoprotein (LDL), which is internalized by the cells to provide fatty acid for cell function and energy (Figure 2-3). Cholesteryl ester transfer protein (CETP) permits exchange of triglycerides and cholesteryl esters from VLDL to high-density lipoprotein (HDL), allowing for reverse cholesterol transport via the HDL system (Figure 2-4).
Lipid subtypes. The circles indicate the relative size of these lipid particles. Abbreviations: CM: chylomicrons; FA, fatty acids; HDL: high-density lipoprotein; LDL: low-density lipoprotein; LPL: lipoprotein lipase; TG: triglycerides; VLDL; very low-density lipoprotein.
Lipids and their interactions. Abbreviations: FA: fatty acids; HDL: high-density lipoprotein; LDL: low-density lipoprotein; LPL: lipoprotein lipase; TG: triglycerides; VLDL: very low-density lipoprotein.
Hyperlipidemia is a condition in which there are abnormally elevated levels of lipids or lipoproteins in the blood. They can be divided into primary or secondary causes. Primary hyperlipidemia is typically due to a genetic cause, whereas secondary hyperlipidemia usually occurs as a result of a systemic disorder, such as diabetes mellitus. The types of familial hyperlipidemia are categorized by the Fredrickson classification (Table 2-1).
Table 2-1Frederickson Classification System ||Download (.pdf) Table 2-1 Frederickson Classification System
|Type ||Lipoprotein Abnormality ||Triglycerides ||Name |
|I ||↑ Chylomicron ||Markedly elevated ||Familial chylomicron syndrome |
|IIa ||↑ LDL ||Normal ||Familial hypercholesterolemia |
|IIb ||↑ LDL and VLDL ||Elevated || |
|III ||↑ Remnants ||Elevated ||Familial dysbetalipoproteinemia |
|IV ||↑ VLDL ||Elevated || |
|V ||↑ VLDL and chylomicron ||Markedly elevated ||Familial hypertriglyceridemia |