HDL – HDL, or high-density lipoprotein, is commonly referred to as the good cholesterol. HDL Levels in the body are supposed to be fairly high. This is because HDL helps excess cholesterol return to the liver for excretion through the gastrointestinal system. HDL is known as good cholesterol because it aids in the removal of cholesterol that can block arteries and reduce blood flow. It is also thought that HDL helps to protect against oxidation. Fat cells may release glycerol, fatty acids, cholesterol and phospholipids to the blood.
The liver makes HDL to carry the cholesterol from the cells back to the liver for recycling or disposal. VLDL – (Very low density lipoprotein) The liver cells use the fatty acids that are arriving in the blood to make cholesterol, other fatty acids, and other compounds. At the same time the liver cells may be making lipids from carbohydrates, proteins or alcohol. Ultimately the lipids made in the liver and those collected from the chylomicron remnants are packaged with VLDL and shipped to other parts of the body.
As the VLDL transfer through the body cells remove triglycerides causing the VLDL to shrink. As this happens the proportion of the lipids shifts and the lipoprotein becomes denser. The remaining lipoprotein eventually becomes LDL which is explained below. LDL – (Low Density lipoproteins) The LDL circulate through the body making contents available to cells of all tissues – muscles, fat stores and the mammary glands.
The cells take triglycerides, cholesterol and phospholipids to build new membranes, make hormones or other compounds. Special LDL receptors on the liver cells play a crucial role in the control of blood cholesterol concentration by removing LDL from circulation. The blood cholesterol linked to heart disease is LDL. High LDL is associated with high risk of heart attack and so it is also known as bad cholesterol.
References Books Whitney EN, Rolfes SR (2005), Understanding Nutrition, California: Thomson Wadsworth