High Density Lipoprotein (HDL) Explained
Let’s take a minute to talk to you about HDL or High-Density Lipoprotein. You may have heard it called HDL cholesterol. This is one of the biomarkers on your typical lipid profile. It’s important to understand because it has direct consequences on your health, especially cardiovascular disease.
Your body manages, stores and metabolizes fat through molecules collectively referred to as lipoproteins. Technically, there are 5 categories: chylomicrons, very low density lipoproteins (VLDL), intermediate density lipoproteins (ILDL), low density lipoproteins (LDL) and high density lipoproteins (HDL). I’m only going to touch on HDL in this lecture.
As the name lipoprotein suggests, they contain both lipids (i.e. fat) and proteins. This gives them both structure and function. Lipids normally don’t like water, that is to say they are hydrophobic, but attaching them to proteins makes them less hydrophobic. This allows us to move them around our body more effectively for various purposes. We are, after all, made up of about 60% water. So the purpose of lipoproteins is transportation of the things they contain.
Generally speaking, lipoproteins contain varying amounts of 5 different molecular structures: Protein, triglycerides, phospholipids, cholesterol and cholesterol esters. HDL contains approximately 33% protein, 30% cholesterol, and 30% phospholipids. It has the highest density (hence the name) and the highest protein content of any of the lipoproteins.
The purpose of HDL is to transport it’s contents back to the liver for metabolism. It is produced in the liver, circulates in the bloodstream, where it picks up cholesterol and phospholipids. This is a term called “reverse” transport, because they are bringing them back to the source, which is the liver. Once in the liver, they can be broken down for energy if required, or excreted with bile into the gastrointestinal tract. HDL also plays a role in delivering cholesterol to the adrenal glands, testes and ovaries for synthesis of different steroid molecules.
The reason that HDL plays an important role in cardiovascular health is because it can help stop or reverse atherosclerosis and plaque that build up in your arteries. A specific type of immune cell called macrophages have a tendency to gobble up cholesterol and triglycerides when they are in excess in the blood stream. This is one reason why doctors recommend low fat diets for certain people. These lipid-laden macrophages can end up getting deposited on damaged arterial walls, leading to plaque build up, and atherosclerosis. Atherosclerosis increases your risk of heart attack and stroke. The way that HDL helps is the same as what I mentioned before; it picks up the cholesterol, phospholipids, and triglycerides and reverse transports them back to the liver.
HDL also has additional properties that include antioxidation, protection against thrombosis, maintenance of endothelial function, and maintenance of low blood viscosity through a permissive action on red cell deformability. The mechanism of these actions are not as well understood at this time.
Many studies have linked low HDL to an increased risk of heart attack and stroke. There are many things that can cause you to have low HDL. Smoking, obesity, being sedentary and low physical activity have all been associated with a low HDL. Some individuals or families have genetic dyslipidemias that, for one reason or another, do not allow them to produce HDL or to utilize it appropriately.
There are many ways in which you can attempt to improve your lipid profile overall, including increasing your HDL. These include quit smoking, losing weight and exercising more. Diet’s high in mono- and polyunsaturated fats and low in trans and saturated fats have been shown to improve HDL. Drinking alcohol in moderation has also been shown to have a positive impact on HDL cholesterol. There are also pharmaceutical options that you can talk to your doctor about.
The target HDL values for men and women is more than 60 mg/dL. The average for men is 40 to 50 mg/dL and for women it is 50 to 60 mg/dL. Low is considered to be less than 40 mg/dL in men and less than 50 mg/dL in women. (This may vary slightly from one set of guidelines to the next).