In human physiology, high-density lipoprotein (HDL) is a fraction of total serum cholesterol concerned with the retrieval and transport of lipid to the liver and other needy tissues. Since the conventional route of cholesterol distribution is from the liver to tissues, this process is often referred to as reverse cholesterol transport. HDL has gained a positive reputation and is referred to as “good cholesterol” since its presence is associated with a reduction in cardiovascular disease and mortality.
HDL particles are usually created by the liver and are small collections of several proteins, enzymes, and some phospholipid. Since cholesterol does not readily mix with the watery environment of blood, these components interact to create an efficient transport medium that will not stick to the sides of vessels. The majority of HDL particles are released into the blood as discs with minimal lipid content. As they fill up with excess cholesterol from cells like macrophages, they transform into spheres. When loaded, they are then transferred to the liver and other tissues that require cholesterol as a substrate for products like steroidal hormones and bile.
Protein Contents of HDL
The major protein associated with HDL is apoA I. This is a 243 amino acid peptide arranged in helices that supply hydrophobic and hydrophilic faces. Proteins like apoA II, C, E, D, M, and A IV support these faces to present a secure container while on-board enzymes like lecithin-cholesterol acyl transferase (LCAT) esterify the cholesterol into even more hydrophobic species.
Normally, about 30% of total serum cholesterol is in the form of HDL. Serum HDL ranges widely with low levels (under 40-50 mg/dL) associated with higher risk of cardiovascular disease and higher levels (above 60 mg/dL) contributing a protective effect.
Protective Functions of HDL
HDL appears to decrease the incidence of athlerosclerotic plaques associated with heart disease through several mechanisms. HDL stimulates cells to transfer the cholesterol they consume to it, collects free cholesterol deposits, and has an antioxidant effect to neutralize oxidized cholesterol before it damages delicate membranes. Its small size allows it to slip between the cells lining arteries and penetrate deep into the tissues where cholesterol is often pooled by cells of the immune system (macrophages).
Raising HDL is a Good Thing
Serum HDL levels may be influenced positively or negatively by medications and lifestyle choices. Some choices appear to affect levels more than others. For instance, sedentary activity and smoking appear to drastically lower HDL levels while exercise and weight loss raise it substantially. Increasing HDL levels by even 6% appears to impact the risk of cardiovascular disease.
- Diet (high carbohydrate, low fat)
- Sedentary lifestyle
- Beta-blocker use
- Weight loss
- Moderate alcohol use
- Nutritional and herbal supplementation
- Dietary omega-3 fatty acids
- Pharmacological therapy