Low density lipoproteins -- LDL -- and high density lipoproteins -- HDL -- are types of lipid particles found circulating in your blood. LDL and HDL each have distinct characteristics that contribute to different aspects of lipid metabolism. LDL is commonly referred to as "bad cholesterol" because of its strong association with heart disease. HDL, on the other hand, is protective against heart disease and is commonly referred to as "good cholesterol."
Cholesterol
Cholesterol is a type of lipid that serves many functions in your body. According to "Advanced Nutrition and Human Metabolism," cholesterol is an important component of cell membranes and also serves as a precursor for molecules such as testosterone, estrogen, vitamin D and bile. The cholesterol molecule itself is neither "good" nor "bad." Differences in the way cholesterol is processed and transported through your body are the primary determinants of the health effects of cholesterol.
Lipoproteins
According to "Advanced Nutrition and Human Metabolism," lipoproteins are a family of compounds that transport cholesterol and lipids throughout your body. Lipoproteins, such as LDL and HDL, allow fat-soluble molecules to travel freely through the water-soluble blood. All lipoproteins are composed of phospholipids, cholesterol, triglyceride and protein. The relative proportions of these components differ for each type of lipoprotein. Lipoprotein composition will also determine its density. For example, LDL contains a high proportion of lipid, whereas HDL contains a higher proportion of protein.
Low-density Lipoprotein
Your liver synthesizes cholesterol that can be carried through the blood on LDL particles. The primary goal of LDL is to deliver cholesterol to various tissues throughout your body. According to "Advanced Nutrition and Human Metabolism," LDL particles can bind to LDL-receptors. The binding of LDL to its receptor allows cholesterol to be delivered to the cell.
High-density Lipoprotein
According to "Advanced Nutrition and Human Metabolism," HDL particles originate in your liver and intestine. They are involved in a process called reverse cholesterol transport. In this process, HDL removes cholesterol from tissues and transports it to the liver for recycling and disposal.
Lipoproteins and Heart Disease
The cholesterol carried in LDL and HDL is the same molecule, but LDL is said to carry "bad" cholesterol because the cholesterol is being delivered to tissues such as the coronary arteries. According to "Advanced Nutrition and Human Metabolism," cholesterol in the arteries may become oxidized and lead to the development of heart disease. HDL is said to carry "good" cholesterol because HDL can remove cholesterol from tissues such as the coronary arteries, helping prevent the development of heart disease. To decrease your risk of heart disease, the American Heart Association recommends maintaining LDL concentrations below 100 mg/dL, while maintaining HDL concentrations greater than 60 mg/dL.
References
- "Advanced Nutrition and Human Metabolism (Fifth Edition)"; S.S. Gropper, J.L. Smith, J.L. Groff; 2009
- American Heart Association: Cholesterol Guidelines
