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Triglycerides are a form of fat, consisting of three molecules (‘‘tri’’) of a fatty acid combined with one molecule of the alcohol glycerol. Triglycerides serve as the backbone of many types of lipids (fats). Triglycerides are produced by the liver as well as are ingested as part of the diet. Fats in foods are digested and changed to triglycerides.
Triglycerides have several purposes in physiology. Triglycerides travel through the circulatory system and are either utilized immediately or are stored in adipose tissue, thereby serving as the most abundant form of stored energy in the body. Triglycerides can serve as this important storage medium because of their hydrophobicity, which allows them to be stored as droplets, without contact with water molecules. Often a typical human body may contain several months of fuel stored in the form of triglycerides. When physiological conditions dictate the need to use the triglycerides, hormones or a neurotransmitter signal their release. This release may be in response to exercise, stress, or fasting. An enzyme called lipase breaks down the triglyceride molecule into a glycerol molecule and three fatty acids before release from the adipose tissue. These breakdown products are transported within the circulatory system to the tissues that need them for energy.
In addition to serving as a source of energy, triglycerides carry the fat-soluble vitamins (including vitamin K, an important nutrient in normal blood coagulation). Triglycerides also provide thermal insulation and contribute to the structure of membranes by the formation of a lipid bilayer.
Triglycerides combine with a blood protein to form chemicals referred to as high-density and low-density lipoproteins. These lipoproteins contain cholesterol, another substance related to fats.
(Illustration by GGS Information Services/Thomson Gale.)
It is not yet clear whether high triglyceride levels act as a predictor of the risk for heart disease and heart attacks, especially in persons with normal levels of cholesterol. Some health care professionals feel that elevated triglycerides are a marker for other risk factors that do impact the risk of heart disease, that is, high levels of triglycerides are usually associated with low levels of high density lipoproteins, usually referred to as the ‘‘good’’ cholesterol.
However, there are some indications that high triglycerides may serve as a predictor for heart disease, especially in women. In a study involving postmeno-pausal women (aged 48 to 76 years old) conducted by a research group from the Center for Clinical and Basic Research in Ballerup, Denmark, it was found that women who had an enlarged waist and elevated levels of triglycerides had almost a five-fold increased risk of fatal cardiovascular events compared to women without those traits. The women at risk deposited fat centrally in their intra-abdominal compartment, rather than in their hips, thighs, and buttocks.
The mechanism of how triglycerides might affect heart health is not fully known, but it appears that elevated levels of triglycerides may allow increased blood clot formation and may slow the natural breakdown of clots after they have formed. However, high levels of triglycerides may mean an increased risk of diabetes, and very high levels of triglycerides may increase the risk of inflammation of the pancreas, resulting in pancreatitis.
Triglyceride levels are evaluated through blood testing.A fatty meal that is high in triglycerides will cause a short term increase in blood triglyceride levels. Therefore, before testing, a person should refrain from eating food for eight to ten hours before the test and not drink alcohol for 24 hours before the test. Some medications may interfere with test results, and the health care provider may request that the person cease taking the medications before testing. For example, oral
contraceptives, estrogen, and cholestyramine (a drug used to treat high cholesterol levels) may increase blood triglyceride levels, while vitamin C (ascorbic acid) asparaginase (an enzyme used in the treatment of cancer) and various drugs to treat high levels of blood lipids may decrease blood triglyceride levels. Triglyceride levels can also be affected by the menstrual cycle, time of day, and recent exercise. A person should have two or three tests, one week apart, for the most accurate results.
The normal range of blood triglyceride levels depends on age and gender, with women naturally having higher levels, especially when pregnant. As people age and gain weight, triglyceride levels usually increase. According to the guidelines promulgated by the National Cholesterol Education Program, a division of the National Heart, Lung, and Blood Institute, a normal fasting level for adultsisless than 150 milligrams per deciliter (mg/dl), with levels below 101 considered desirable. Levels of 150 - 199 mg/dl are considered borderline high, levels of 200 - 499 mg/dl are considered high, with levels greater than 500 mg/dl considered very high. Such high levels may indicate liver disease (cirrhosis), underactive thyroid activity, uncontrolled diabetes, pancreatitis, kidney disease, or a diet too low in protein and too high in carbohydrates.
Extremely low levels of triglycerides (less than 10 mg/dl) may indicate malnutrition, malabsorption, a diet too low in fat, or an overactive thyroid.
High triglyceride levels may be due to several causes, including:
Hereditary may also play a role in elevated levels of triglycerides. Familial hypertriglyceridemia is a common inherited disorder in which the level of triglycerides in a person’s blood is higher than normal. This disorder is an autosomal dominant disorder, that is, if one parent has an abnormal gene and the other parent a normal gene, there is a 50% chance each child will inherit the abnormal gene and therefore the dominant trait. Some people with this condition also have high levels of very low density lipoprotein (VLDL), the ‘‘bad’’ cholesterol. Obesity, hyperglycemia (high blood glucose levels), and high levels of insulin are often associated with this condition and may result in even higher triglyceride levels.
Familial hypertriglyceridemia is not usually detected until puberty or early adulthood. Symptoms include a mild-to-moderate increase in blood triglyceride levels and premature coronary artery disease. Persons with this condition are also at increased risk for pancreatitis.
Familial hypertriglyceridemia occurs in about 1 in 500 individuals in the United States. Risk factors are a family history of hypertriglyceridemia or a family history of heart disease before the age of 50. If triglyceride levels cannot be not controlled by dietary and lifestyle changes, medication may be needed. Nicotinic acid and gemfibrozil have been shown to effectively reduce triglycerides in persons with familial hypertriglyceridemia. Screening family members for elevated levels of triglycerides may help to detect the disease early.
A nutritionist or dietitian may be consulted to help develop a dietary plan to help control triglyceride levels. In general, to lower or prevent high levels of triglycerides, a person should:
Other good food choices include fruits (but not fruit juices, which are high in sugar), vegetables, whole grain breads and cereals, lean protein sources, such as lean meats, poultry without skin, eggs, egg substitute or egg white, cooked dried beans, lentils, peas, nuts, and low-fat soy products, fat-free or 1% milk products, nuts such as almonds, walnuts, and peanuts), avocados, and sugar-free products.
One approach to successfully changing the diet to reduce blood triglyceride levels is to make changes in stages. For example, individuals could cut fat intake to 30% for one month (current American levels are approximately 40%) and then return to their health care provider to see if there has been an improvement in their triglyceride levels. If the level of decreases was not satisfactory, the individuals could further restrict their fat intake to 25% and again be evaluated after one month. If no improvement is noted, the fat intake should be lowered to 20% for two months. At this level of fat intake, it is likely that most calories are being obtained from complex carbohydrates, and a reduction in triglyceride levels should be seen.
Other risk factors for coronary heart disease can increase the hazards from high levels of triglycerides. Therefore a person with high levels should in addition to making dietary changes should also control high blood pressure and avoid cigarette smoking. Dietary management is important even when drugs are used to control triglyceride levels.
If a child is suspected to have familial hypertrigly-ceridemia, the child should be tested for elevated levels of triglycerides. If the disorder is present, appropriate steps should be taken to help the child lower his or her triglyceride levels.
Sprecher, Dennis. What You Should Know about Triglycerides: The Missing Link in Heart Disease. New York, NY: Harper Torch Publishers, 2000.
Welson, Linda T. (Ed.) Triglycerides and Cholesterol Research Hauppauge, NY: Nova Science Publishers, Inc., 2006
American Heart Association National Center, 7272 Greenville Avenue, Dallas, TX 7523. Telephone: 800-242-8721. Website: [www.americanheart.org]
Judith L. Sims