Because fats do not dissolve in water or blood they need another way to travel around the body to wherever they may be needed. So the liver packages lipids together with proteins to make special transport vehicles called lipoproteins that carry fats through the bloodstream. According to the American Heart Association, HDL and LDL are two kinds of transport vehicles for carrying lipids such as triglycerides, phospholipids and sterols in the lymph and blood.
LDL Function
Low-density lipoproteins (LDL) carry triglycerides, cholesterol and phospholipids to all the cells of all tissues. The LDL contain few triglycerides but are loaded with cholesterol. The cells remove what they need from the LDL and use it to build membranes, make hormones or other compounds or perhaps store it for later use.
HDL Function
Fatty acids, cholesterol and phospholipids released to the blood by fat cells are transported by high-density lipoproteins (HDL). In their book "Understanding Nutrition," Eleanor Whitney and Sharon Rolfes explain that HDL are rich in protein and perform the vital job of carrying cholesterol and other lipids from the cells back to the liver for disposal or recycling.
Implications
The difference between HDL and LDL has health implications for the heart and blood vessels. LDL carries cholesterol to the cells and is linked to heart disease because too much cholesterol circulating in LDL can build up in the arteries. Eventually a clot may form and block a narrowed artery causing a heart attack or stroke, according to the American Heart Association.
HDL also carries cholesterol but high levels of HDL represent cholesterol returning from the rest of the body back to the liver to be broken down and excreted. High HDL has a protective effect for the heart.
Recommended Levels
A desirable blood lipid profile includes a total cholesterol level of less than 200 mg per deciliter, LDL optimally less than 100 mg per deciliter and HDL greater than 40 mg per deciliter, according to the National Cholesterol Education Program clinical guidelines.
Considerations
HDL and LDL are not different kinds of cholesterol but rather they are different specialized transport vehicles and their concentration levels in the blood reflect the proportions of lipids and proteins within them.
Factors that improve the LDL to HDL ratio include weight control, replacing saturated fat with monounsaturated or polyunsaturated fat in the diet and increasing soluble fibers and antioxidants. Physical activity also helps and if alcohol is consumed it should be a moderate amount.
References
- American Heart Association: LDL and HDL Cholesterol--What's Bad and What's Good?
- "Understanding Nutrition, Ninth Edition"; Eleanor Noss Whitney and Sharon Rady Rolfes; 2002
- National Cholesterol Education Program: Adult Treatment Panel III
- "Krause's Food, Nutrition, & Diet Therapy, 10th Edition"; Mahan and Escott-Stump; 2000
