HDL is the "Good" Lipoprotein That Transports Cholesterol from Peripheral Tissues to the Liver
High-density lipoprotein (HDL) is the "good" cholesterol that transports cholesterol away from peripheral cells to the liver for processing and elimination from the body. 1
Understanding Lipoprotein Functions
HDL: The Good Cholesterol
- HDL plays a critical role in reverse cholesterol transport, removing cholesterol from arterial deposits and peripheral tissues and carrying it to the liver 1, 2
- In the liver, this cholesterol is converted to useful metabolites and eventually cleared from the body via bile secretions 1
- HDL contains apolipoprotein A-I as its major protein component, with typically two or three ApoA-I molecules per particle 1
- Higher levels of HDL and larger HDL particle size are consistently associated with decreased risk of coronary artery disease 1
LDL: The Bad Cholesterol
- LDL is the main carrier of cholesterol in plasma, delivering cholesterol to peripheral tissues via LDL receptors 2
- Contains apolipoprotein B-100 as its main surface protein 2
- Small, dense LDL particles are more susceptible to oxidation and are more atherogenic than larger LDL particles 1, 2
- LDL is considered "bad" cholesterol because elevated levels contribute to atherosclerosis and cardiovascular disease risk 1
Cholesterol Transport System
Reverse Cholesterol Transport
- HDL is the key lipoprotein in reverse cholesterol transport, which is the process of moving excess cholesterol from peripheral tissues back to the liver 1, 2
- This process is protective against atherosclerosis as it removes cholesterol from arterial walls 2
- HDL functionality includes cholesterol efflux capacity, which is its ability to accept cholesterol from cells 2, 3
Other Lipoproteins in Cholesterol Transport
- Chylomicrons transport dietary lipids from the intestine 1
- Very low-density lipoprotein (VLDL) is secreted by the liver and metabolized in circulation into intermediate-density lipoprotein (IDL) and eventually LDL 1
- LDL delivers cholesterol to peripheral tissues for cellular functions 2
Clinical Significance
Cardiovascular Risk Assessment
- Low HDL-C levels are independently associated with higher cardiovascular disease risk 1
- HDL-C <1.0 mmol/L (40 mg/dL) in men and <1.2 mmol/L (45 mg/dL) in women may be regarded as markers of increased cardiovascular risk 1
- The ratio of total cholesterol to HDL-C is considered a better indicator of heart disease risk than LDL-C alone 2
Beyond Cholesterol Levels
- Recent research suggests that HDL functionality may be more important than HDL-C levels alone 4, 5
- HDL has multiple functions beyond cholesterol transport, including:
- Influence on immune function
- Modulation of vascular inflammation
- Effects on glucose metabolism
- Regulation of platelet function 1
Dysfunctional HDL
- HDL can become dysfunctional in inflammatory conditions, potentially transforming from protective to pro-atherogenic 5
- HDL isolated from patients with coronary artery disease often lacks endothelial anti-inflammatory properties 1
- Dysfunctional HDL may have lower paraoxonase 1 (PON1) enzyme activity, which normally protects against LDL oxidation 1
Common Pitfalls in Understanding Lipoproteins
- Focusing solely on HDL-C levels rather than HDL functionality can be misleading 4, 5
- Assuming that all HDL particles are equally protective; different HDL subfractions may have varying protective effects 6
- Overlooking that both genetic and environmental factors influence HDL function 1
- Not recognizing that HDL function can be impaired in conditions like diabetes, obesity, and dyslipidemia 3
In summary, HDL is correctly identified as the "good" lipoprotein responsible for transporting cholesterol from peripheral tissues back to the liver for elimination, while LDL is considered the "bad" lipoprotein that delivers cholesterol to peripheral tissues and can contribute to atherosclerosis when levels are elevated.