Is there High-Density Lipoprotein (HDL) present in the bloodstream?

HDL Cholesterol in the Bloodstream

Yes, High-Density Lipoprotein (HDL) is present in the bloodstream and serves as a key component of the lipid profile with protective cardiovascular effects. 1, 2

HDL Structure and Function

• HDL is one of the major lipoprotein classes in the bloodstream, containing both lipids and proteins, with apolipoprotein A1 (apoA1) being its major protein component 1, 3
• HDL particles vary in size from very small preβ-1 HDL to large α-1 HDL, with different subclasses having distinct functional properties 3
• HDL is often referred to as "good cholesterol" because it mediates reverse cholesterol transport, the process of extracting cholesterol from peripheral tissues (including arterial walls) and transporting it to the liver for excretion 4, 5

HDL's Protective Effects

• HDL has multiple protective functions beyond cholesterol transport, including:
  • Antioxidant properties that protect against LDL oxidation 6
  • Anti-inflammatory effects on vascular tissues 7
  • Modulation of vasomotor function and thrombosis 7
• Low concentrations of HDL cholesterol are independently associated with higher cardiovascular disease risk 1

Normal HDL Levels and Risk Assessment

• The European Society of Cardiology classifies HDL cholesterol levels as:
  • Low (increased risk): <40 mg/dL in men, <45 mg/dL in women 1, 2
  • Desirable: 40-59 mg/dL 2
  • Optimal (protective): ≥60 mg/dL 2
• HDL cholesterol is included in cardiovascular risk assessment tools such as the SCORE charts 1
• Low HDL combined with elevated triglycerides creates a particularly atherogenic lipid profile common in patients with type 2 diabetes, abdominal obesity, and insulin resistance 1

HDL in Relation to Other Lipoproteins

• While LDL is the dominant atherogenic lipoprotein promoting atherosclerosis, HDL plays a protective role against atherosclerotic cardiovascular disease 2
• The ratio of total cholesterol to HDL or LDL to HDL provides additional risk stratification beyond individual lipid measurements 2
• Non-HDL cholesterol (total cholesterol minus HDL cholesterol) is calculated to assess all atherogenic lipoproteins and may be a better predictor of cardiovascular risk than LDL alone in some patients 1, 8

Clinical Implications

• Despite HDL's protective association, there is insufficient evidence to establish specific HDL cholesterol values as therapeutic targets 1
• While low HDL is an established risk factor, pharmacologic interventions specifically targeting HDL elevation have not consistently shown outcome benefits 2
• The best approach for patients with low HDL-C is optimizing other risk factors including LDL-C, plasma glucose, body mass index, blood pressure, and smoking cessation 3

Emerging Concepts

• Recent research suggests that HDL functionality (particularly cholesterol efflux capacity) may be more important than HDL cholesterol mass in determining cardiovascular protection 4, 6
• HDL particles can become dysfunctional in patients with coronary artery disease due to oxidative processes and proteome remodeling 6
• HDL contains approximately 110 proteins involved in lipid metabolism and immune function, highlighting its complex role beyond simple cholesterol transport 3