HDL and LDL are opposing chemicals that help control the transportation and deposition of cholesterol, triglycerides and other fats in your body. HDL, short for high-density lipoprotein, is the "good" cholesterol. It circulates in your bloodstream, protecting your arteries from the ill effects of its "bad" twin, LDL. HDL uses several weapons in the fight for your cardiovascular health.
Reverse Cholesterol Transport
Cholesterol traveling in your bloodstream as LDL can damage your arteries by accumulating in the walls of these blood vessels, beginning a disease process called atherosclerosis. The cholesterol deposits, called plaques, grow and absorb calcium. Your arteries stiffen and the growing plaques progressively obstruct blood flow, depriving your organs and tissues of needed oxygen. HDL is an excess cholesterol scavenger. As it circulates in your bloodstream, HDL absorbs excess cholesterol from your artery walls and tissues, and delivers it to your liver for breakdown and elimination. This process, called reverse cholesterol transport, is the only mechanism your body has to get rid of excess cholesterol. HDL's action as a reverse cholesterol transporter helps protect you from the development of atherosclerosis. A low HDL cholesterol level increases your risk of atherosclerosis and its complications, including coronary heart disease and stroke.
Anti-inflammatory Actions
Many chemical reactions occur in the walls of your arteries that may contribute to the development of atherosclerosis. Inflammation is an integral part of this process. When you get a scratch, a red inflamed area develops around the injury. Similar inflammation occurs inside your arteries as atherosclerosis develops. Immune system cells accumulate in areas where cholesterol accumulates in the arterial wall. HDL disrupts this movement of immune cells into a developing plaque. It also inhibits chemical reactions that perpetuate the inflammatory response. These anti-inflammatory HDL activities work against the development and progression of atherosclerosis.
Antioxidant Actions
When LDL deposits cholesterol in the walls of your arteries, enzymes break down the fat. This breakdown of fat releases chemicals called oxidants, which fuel the inflammation associated with atherosclerosis. In addition to its anti-inflammatory properties, HDL slows the formation of oxidants, short-circuiting the inflammatory cycle and preventing further oxidative damage to the affected arteries.
Promotion of Nitric Oxide Formation
Cells that line your arteries produce the chemical nitric oxide, which keeps the blood vessels relaxed. In arteries undergoing atherosclerotic changes, nitric oxide helps inhibit plaque growth by preventing clot-forming platelets from sticking to the injured blood vessel wall. HDL stimulates the formation of nitric oxide, tipping the balance against atherosclerotic progression. When HDL levels are low, less nitric oxide is produced, which may lead to further growth of atherosclerotic plaques.
References
- Baylor College of Medicine Lipids Online: Reverse Cholesterol Transport
- "Circulation Research"; Antiinflammatory Properties of HDL; Philip J. Barter, M.D., Ph.D., et al.; October 2004
- "Circulation Research"; New Insights Into the Regulation of HDL Metabolism and Reverse Cholesterol Transport; Gary F. Lewis, M.D., Daniel J. Rader, M.D.; June 2005
- Cardiovascular Physiology Concepts; Nitric Oxide; Richard E. Klabunde, Ph.D.; September 2008
- "Trends in Cardiovascular Medicine"; HDL Stimulation of Endothelial Nitric Oxide Synthase: A Novel Mechanism of HDL Action; Chieko Mineo, Philip W. Shaul; August 2003
