High-Density Lipoprotein (HDL) – Good Cholesterol

What is High-Density Lipoprotein (HDL)?

High-density lipoprotein (HDL) is a class of lipoproteins in the blood commonly referred to as good cholesterol. Lipoproteins are transport molecules that carry fats (lipids) and cholesterol through the bloodstream, as these water-insoluble substances cannot travel through blood plasma on their own. HDL is characterized by a high protein content and a relatively low fat content, which gives it its characteristic high density.

In contrast to low-density lipoprotein (LDL), which transports cholesterol to body cells and is considered a risk factor for cardiovascular disease when elevated, HDL serves the opposite function: it collects excess cholesterol from tissues and the walls of blood vessels and carries it back to the liver, where it is broken down or excreted.

Function and Mechanism of Action

HDL performs several important functions in the human body that contribute to the protection of blood vessels:

• Reverse cholesterol transport: HDL picks up excess cholesterol from tissue cells and arterial walls and transports it to the liver, where it is converted into bile acids and eliminated through the intestine.
• Anti-inflammatory effects: HDL has anti-inflammatory properties and can inhibit inflammatory reactions in the vessel walls.
• Antioxidant effects: HDL protects LDL particles from oxidation, a process that contributes to the development of atherosclerosis.
• Endothelial protection: HDL supports the function of the endothelium, the inner lining of blood vessels, thereby promoting vascular health.

Normal Values and Health Significance

The HDL cholesterol level is measured through a simple blood test. The following reference values are recommended by the European Society of Cardiology (ESC):

• Increased risk: HDL below 40 mg/dL in men or below 45 mg/dL in women is considered a risk factor for cardiovascular disease.
• Normal range: Values between 40 and 60 mg/dL are considered within the normal range.
• Protective effect: HDL values above 60 mg/dL are considered cardioprotective and may reduce the risk of heart disease.

However, very high HDL values (above 100 mg/dL) are viewed with increasing caution in recent studies, as they are not necessarily associated with additional protection and may in rare cases even be linked to an elevated risk.

Causes of Low HDL Levels

Low HDL levels can be caused by a variety of factors:

• Physical inactivity: Lack of exercise is one of the most common reasons for low HDL levels.
• Unhealthy diet: A diet high in trans fatty acids and simple carbohydrates lowers HDL levels.
• Smoking: Cigarette smoking has been shown to reduce HDL cholesterol levels.
• Overweight and obesity: Excess body weight, especially abdominal fat, correlates with low HDL.
• Type 2 diabetes and insulin resistance: Disorders of glucose metabolism affect lipid metabolism and frequently lead to low HDL.
• Genetic factors: Certain inherited lipid metabolism disorders can lead to chronically low HDL levels.
• Certain medications: Some drugs such as beta-blockers or anabolic steroids can lower HDL levels.

Diagnosis and Measurement

The HDL value is determined as part of a lipid panel (also known as a lipid profile or blood fat analysis). Blood is typically drawn after a fasting period of 8 to 12 hours. In addition to HDL, total cholesterol, LDL cholesterol, and triglycerides are also measured. From these values, the cholesterol ratio (total cholesterol divided by HDL) can be calculated, which is an important indicator of cardiovascular risk.

Measures to Increase HDL Levels

Various lifestyle measures and medical interventions can positively influence HDL levels:

Lifestyle and Diet

• Regular physical activity: Aerobic exercise such as running, cycling, or swimming can significantly increase HDL levels.
• Mediterranean diet: A diet rich in unsaturated fatty acids (olive oil, nuts, avocado) and omega-3 fatty acids (fish) promotes higher HDL levels.
• Quitting smoking: Stopping smoking leads to a measurable increase in HDL levels within a few weeks.
• Weight reduction: Even moderate weight loss can noticeably improve HDL levels.
• Moderate alcohol consumption: Moderate alcohol intake has been associated with slightly elevated HDL in some studies; however, abstaining from alcohol or very low consumption is generally recommended.

Medical Treatment

Directly raising HDL levels through medication has proven challenging in clinical studies. Some substances such as niacin (nicotinic acid) can increase HDL levels, but have lost clinical relevance due to side effects and uncertain benefits. Statins, primarily used to lower LDL cholesterol, can also produce a moderate increase in HDL. The primary treatment strategy remains lowering LDL cholesterol combined with comprehensive lifestyle optimization.

HDL and Cardiovascular Risk

Numerous epidemiological studies have shown that low HDL levels are an independent risk factor for coronary artery disease, heart attack, and stroke. HDL is therefore considered an important component of cardiovascular risk assessment. More recent research, however, increasingly differentiates between the quantity (HDL level) and the quality or functionality of HDL particles, as not every HDL particle is equally cardioprotective.

References

1. European Society of Cardiology (ESC) / European Atherosclerosis Society (EAS) - Guidelines for the Management of Dyslipidaemias (2019). European Heart Journal.
2. Rader, D.J. & Tall, A.R. - The not-so-simple HDL story: Is it time to revise the HDL cholesterol hypothesis? Nature Medicine, 18(9), 1344-1346 (2012).
3. World Health Organization (WHO) - Cardiovascular diseases: Key facts. WHO Global Report (2023). https://www.who.int/news-room/fact-sheets/detail/cardiovascular-diseases