LDL refers to low density lipoprotein and it is composed of more lipids than proteins and hence it is called low density. Cholesterol is a type of lipid that is transported by LDLs. LDLs are also referred to as the bad cholesterol because high circulating levels of LDLs are a risk factor for heart disease. Although, in high amounts LDLs are bad they do have a specific function in the body.
LDL Function
LDLs are one of five different lipid protein complexes that transport cholesterol, triglycerides and other lipids, including fat-soluble vitamins in the bloodstream. Proteins coat the outside of the LDL so that the lipids are enclosed and can be moved in the aqueous blood. Cells that need cholesterol make a protein and attach it to their membranes. This protein receptor attaches to a circulating LDL and moves it into the cell where the cholesterol is broken off and utilized.
Recommended Blood Levels
The American Heart Association has set the optimal level of LDLs at or below 100 mg per deciliter. However, a significant amount of patients with heart disease have levels of LDLs below 100 mg/dL, according to an article in the March 2009 issue of "The American Journal of Managed Care." This would suggest that LDL levels are not always the best predictors of heart disease.
LDLs and Atherosclerosis
Atherosclerosis is a chronic disease process by which plaque develops in arterial walls causing them to thicken leading to impaired blood flow and clot formation. Atherosclerosis precedes the development of cardiovascular disease and often the only symptom of this disease is a heart attack. The mechanism by which LDLs contribute to the disease process of atherosclerosis is by selectively adhering to the walls of blood vessels. This stimulates an inflammatory response that leads to plaque formation and blood vessel injury. When the plaques get large enough that they block blood flow or rupture and lodge in a smaller vessel a heart attack or stroke ensues.
Heart Disease Risk
Although reducing the level of LDLs has been the primary objective of strategies for reducing heart disease, it is becoming increasingly evident that low levels of high density lipoproteins and high levels of triglycerides are independent risk factors and may be better indicators of disease risk. A review in the September 2009 issue of "Vascular Health and Risk Management" reports that the ratios of LDL to HDL and total cholesterol to HDL are better predictors of heart disease risk than just LDL levels.
Decreasing LDLs
The first steps in lowering LDL are to follow a cholesterol-lowering diet, decrease body fat, especially around the abdomen, and increase physical activity. The diet recommended by The National, Heart, Lung and Blood Institute is the Therapeutic Lifestyle Change diet, and the main requirement is an intake of less than 7 percent of calories from saturated fat. If a person has more than one risk factor for heart disease they may also be prescribed a cholesterol-lowering drug. The five major drug classes that reduce bad cholesterol levels and also sometimes increase HDLs or the good cholesterol are statins, fibrates, bile sequestering agents, niacin and drugs like Ezetimibe that reduce cholesterol absorption in the intestine.
References
- The American Heart Association: Good vs. Bad Cholesterol
- National, Heart, Lung and Blood Institute: High Blood Cholesterol
- PubMed Central: Lipoprotein Ratios: Physiological Significance and Clinical Usefulness in Cardiovascular Prevention
- The American Journal of Managed Care: Beyond LDL Cholesterol: The Role of Elevated Triglycerides and Low HDL Cholesterol in Residual CVD Risk Remaining After Statin Therapy
