Lipids are a wide-ranging group of organic compounds found in all living organisms, including humans,
plants, and animals. Lipids are the body's reserve supply of energy. Unlike other organic compounds, lipids are soluble in alcohol, ether, and other organic substances but not in water.
Lipid comes from the Greek word lipos, meaning fat. Cells make lipids in the human body and, along with carbohydrates and proteins, are components of all life. Among the major classes of lipids in humans are acids, glycerol-derived lipids (including fats and oils), and steroids. The two major lipids found in the blood are cholesterol and triglycerides.
Cholesterol is a lipid that is essential for repairing cell membranes, manufacturing vitamin D on the skin's surface, and creating hormones, especially testosterone and estrogen. To circulate in the bloodstream, cholesterol must attach to proteins. The combination of cholesterol and protein is called lipoprotein.
The two major lipoprotein groups are high-density lipoprotein (HDL), commonly referred to as "good" cholesterol, and low-density lipoprotein (LDL), also known as "bad" cholesterol. HDL helps prevent fat buildup throughout the body by carrying cholesterol from the arteries to the liver, where it is disposed of. LDL carries most of the cholesterol in the body, so an excess of LDL can clog the arteries with cholesterol buildup.
High levels of LDL are 100 milligrams or more per deciliter (mg/dL) of blood for people with heart or vascular disease or diabetes, 160 mg/dL for people with two risk factors, and 190 mg/dL or more for people with no risk factors. A high LDL level is a primary cause of coronary heart disease (CHD) and stroke. This is because when LDL accumulates in the body, it forms a plaque that sticks to the walls of arteries, slowing or restricting blood flow and oxygen delivery to the heart and other vital organs. This causes atherosclerosis, commonly referred to as hardening of the arteries. The buildup of plaque usually occurs over a few years and without cholesterol tests the patient may not know about the problem until angina (chest pains) or an acute myocardial infarction (heart attack) occurs.
Among the key risk factors for high LDL are age, gender, smoking, diabetes, and a family history of the disorder. About 25% of people with high LDL can control the disorder with a diet low in saturated fats and cholesterol, weight control, and regular exercise. About 75% of people with high LDL require lipid-lowering medications in addition to the weight, diet, and exercise guidelines. First-line drugs recommended by the National Cholesterol Education Program to treat high LDL are bile acid sequestrants such as cholestyramine (Questran) and colestipol (Colestid), niacin (either over-the-counter or time-released prescription drugs such as Niaspan, Slo-Niacin, and Nicobid), and HMG-CoA reductase inhibitors, including fluvastatin (Lescol), pravastatin (Pravachol), cervistatin (Baycol), lovastatin (Mevacor), simvastatin (Zocor), and atorvastatin (Lipitor). The second-line drug choice are fibric acid derivatives such as gemfibrozil, clofibrate, and fenofibrate (Tricor.) Estrogen replacement therapy should also be considered as complementary therapy in post-menopausal women.
Levels of HDL between 30 and 75 mg/dL are associated with decreased risk of CHD and stroke. But HDL levels under 30 mg/dL are associated with a greater risk for CHD and stroke.
Triglycerides are another form of fat that comes from foods and is carried through the bloodstream to the tissues. High levels of triglycerides in the blood can mean that there is too much fat in the diet. Hypertriglyceridemia (high levels of triglycerides) is associated with coronary heart disease, especially since elevated triglycerides levels are usually associated with unhealthy low levels of HDL, which is necessary for good health.
High triglyceride levels (more than 150 mg/dL) can be caused by excessive intake of alcohol or high-calorie foods. Other risk factors include a family history of high triglycerides, obesity, hypertension (high blood pressure), and diabetes. Treatment generally includes controlling other disorders such as diabetes and high blood pressure, proper diet and regular exercise, and fibric acid derivatives such as gemfibrozil, clofibrate, or fenofibrate.
Lipoprotein(a) is a cholesterol-carrying molecule similar in structure to LDL and is believed to carry a protein that interferes with the body's ability to dissolve blood clots. Elevated levels may contribute to heart attacks. Apolipoprotein A-1 is a molecule associated with healthy hearts and may lower the risk of heart disease due to high HDL. Apolipoprotein B is associated with high LDL and may be more effective in predicting heart disease in women. Remnant lipoproteins are byproducts of chylomicrons, lipid particles common in the blood during fat digestion and assimilation, and/or very low density lipoproteins. Initial research suggest they may be a risk factor for CHD.
Lipids manufactured by cells in the body form part of the protoplasmic structure of cells. Lipids act as a reserve source of energy. When broken down to be used as energy, lipids are converted to an energy-rich compound called adenosine triphosphate by a process known as fatty acid oxidation or beta oxidation.
Role in human health
Lipids are important to the human body since they helps produce hormones, and builds cell membranes and other needed tissue. Lipids, both lipoproteins and triglycerides, are made and stored in the body and are used as energy sources. Lipids also play a major role in cardiovascular health.
Common diseases and disorders
- Angina, which is chest pain that occurs when the heart does not get enough oxygen. When angina is not caused by stress or physical exertion and becomes frequent and more severe, it is called unstable angina, and may indicate an impending heart attack.
- Atherosclerosis, also called hardening of the arteries, a condition in which fatty deposits called plaque build up inside the arteries, restricting blood flow.
- Coronary heart disease, in which the arteries narrow, restricting the flow of blood and oxygen to the heart. Lack of sufficient oxygen to the heart can lead to angina or a heart attack. Most cases of CHD are due to atherosclerosis.
- Stroke, a group of brain disorders involving loss of brain functions that occur when the blood supply to any part of the brain is interrupted. Strokes are most commonly caused by atherosclerosis.
Hypercholesterolemia—An excess of cholesterol in the blood.
Protoplasmic —Relating to protoplasm, a colorless jellylike substance that is the main constituent of all human cells and tissue.
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American Heart Association. National Center, 7272 Greenville Ave., Dallas, TX 75231. (800) 242-8721. #x003C;http://www.americanheart.org>.
National Cholesterol Education Program. National Heart, Lung and Blood Institute, P.O. Box 30105, Bethesda, MD 20824. (301) 592-8573. <http://www.nhlbi.nih.gov>.
Ken R. Wells