Triglycerides
Triglycerides
Definition
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.
Purpose
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.
Triglyceride levels
| Normal | Less than 150 mg/dL |
| Borderline-high | 150-199 mg/dL |
| High | 200-499 mg/dL |
| Very high | 500 mg/dL or above |
| SOURCE: National Heart, Lung and Blood Institute, National Institutes of Health, U.S. Department of Health and Human Services |
(Illustration by GGS Information Services/Thomson Gale.)
Description
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
KEY TERMS
Adipose tissue — A type of connective tissue that contains stored cellular fat.
Malnutrition —Poor nutrition because of an insufficient or poorly balanced diet or faulty digestion or utilization of foods
Malabsorption —Poor absorption of nutrients by the intestinal tract
Pancreas —A long, irregularly-shaped gland near the stomach that secretes a digestive fluid into the intestine through one or more ducts and that secretes the insulin, glucagen, and somatostatin into the bloodstream.
Polycystic ovary syndrome —A condition in which cysts in the ovary interfere with normal ovulation and menstruation
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:
- Lifestyle factors
- Weight gain
- Lack of exercise
- Smoking
- Skipping meals
- Eating large portions of food at one time
- Dietary factors
- Excessive intake of alcohol, saturated and trans fats, sugar, starch, and calories
- Medical conditions
- Medicines, including birth control pills, steroids, and diuretics
- Illnesses, including poorly controlled diabetes, insulin resistance (a precursor to diabetes), polycystic ovary syndrome (PCOS), hypothyroidism, kidney disease, and liver disease
- Age
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:
- Lose weight
- Get regular exercise
- Eat less sugar and sugar-containing foods
- Eat smaller meals and snacks throughout the day, rather than consuming two or three large meals
- Drink less alcohol (even small amounts of alcohol has been shown to elevate triglycerides)
- Limit fat in the diet to less than 35% of daily calories
- Avoid deep-fried foods
- Substitute monounsaturated and polyunsaturated fats, such as those found in canola or olive oils, for saturated fats
- Use a prescription medicine, as directed by the health care provider, to decrease the production of triglycerides by the liver
- Instead of eating meats high in saturated fats, consume fish high in omega-3 fatty acids, such as salmon, lake trout, herring, sardines, albacore tuna, or mackerel (about 10 to 15 grams of fish oil a day is recommended - 15 grams of fish oil can be obtained from an 8-ounce serving offish
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.
Complications
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.
Parental concerns
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.
Resources
BOOKS
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
ORGANIZATIONS
American Heart Association National Center, 7272 Greenville Avenue, Dallas, TX 7523. Telephone: 800-242-8721. Website: [www.americanheart.org]
Judith L. Sims
Triglycerides
Triglycerides
Fats exist in foods—and are usually stored in the body—as triglycerides. Recent research relating levels of triglycerides in the blood stream to heart attacks in human presents a sometime confusing picture but a mounting level evidence suggests that, along with other indicators, triglyceride levels can be used to predict heart attack risk, especially in women and diabetics.
Although the exact mechanisms are not fully known, elevated triglycerides allow increased blood clot formation and may slow the natural deterioration of clots once formed.
Fat molecules are generally made up of four parts: a molecule of glycerol and three molecules of fatty acids. Each fatty acid consists of a hydrocarbon chain with a carboxyl group at one end. The glycerol molecule has three hydroxyl groups, each able to interact with the carboxyl group of a fatty acid. Removal of a water molecule at each of the three positions forms a triglyceride. The three fatty acids in a single fat molecule may be all alike or they may be different. They may contain as few as four carbonatomsor as many as24. Because fatty acids are synthesized from fragments containing two carbon atoms, the number of carbon atoms in the chain is almost always an even number. In animal fats, 16-carbon, for example, palmitic acid and 18-carbon, for example, stearic acid fatty acids are the most common.
Some fatty acids comprising a given triglyceride have one or more double bonds between their carbon atoms. They are then said to be unsaturated because they can hold more hydrogen atoms than they do. Mono-unsaturated fats have a single double bond in their fatty acids while polyunsaturated fats, such as trilinolein, have two or more. Additionally, there are trans-fats, which are only partially hydrogenated having fewer double bonds in a trans (as opposed to the usual cis) chemical configuration, and also omega-3 fats, which have at least one double bond, three carbon atoms in from the end of the fatty acid molecule. Linolenic acid is an example and fish oils are generally a rich source of omega-3 fatty acids.
Double bonds are rigid and those in natural fats introduce a kink into the molecule. This prevents the fatty acids from packing close together and as a result, unsaturated fats have a lower melting point than saturated fats. Because most of them are liquid at room temperature, they are called oils. Corn oil, canola oil, cottonseed oil, peanut oil, and olive oil are common examples. As this list suggests, plant fats tend to be unsaturated while fats from such animals as cattle tend to be saturated.
Ingested fats provide the precursors from which fat as well as cholesterol and various phospholipids are created (synthesized). In humans, fat provides the concentrated form of energy. The energy content of fat (9 kcal/gram) is more than twice as great as carbohydrates and proteins (4 kcal/gram).
Humans can synthesize fat from carbohydrates. However, there are two essential fatty acids that cannot be synthesized this way and must be incorporated into the diet. These are linoleic acid (an omega-6 fat, with the endmost double bond six carbons from the methyl end) and alpha-linolenic acid (an omega-3 fat, with the end-most double bond three carbons from the methyl end). Many studies have examined the relationship between fat in the diet and cardiovascular disease. There is still no consensus, but the evidence seems to indicate that a diet high in fat is harmful and that mono- and polyunsaturated fats are less harmful than saturated fats,
KEY TERMS
Polyunsaturated fat —A fat missing two or more hydrogen atoms from the maximum number of hydrogen atoms that can be bonded to each carbon in the carbon chain of the compound. These fats can remain liquid at room temperatures.
Saturated fats —Fats containing the maximum number of hydrogen atoms that can be bonded to each carbon in the carbon chain of the compound.
Triglycerides —A molecule containing three fatty acids chemically bonded to a glycol molecule.
with the exception of trans unsaturated fats which, according to some, are more harmful than saturated fats. It is also been suggested that ingestion of omega-3 unsaturated fats may be protective for the human body.
See also Biochemistry; Compound, chemical; Heart diseases; Heat.
Resources
BOOKS
Campbell, N., J. Reece, and L. Mitchell. Biology. 6th ed. Menlo Park: Benjamin Cummings, Inc. 2004.
PERIODICALS
Austin, M.A., B. McKnight, K.L. Edwards, et al. “Cardiovascular Disease Mortality in Familial Forms of Hypertriglyceridemia: A 20-year Prospective Study.” Circulation. (June 2000): 2777-82.
Avins, A.L., J.M. Neuhaus. “Do Triglycerides Provide Meaningful Information about Heart Disease Risk?” Archives Internal Medicine. (July 2000): 1937-44.
Cullen P. “Evidence that Triglycerides are an Independent Coronary Heart Disease Risk Factor.” American Journal of Cardiology. (November 2000): 943-949.
Matsubara, M., S. Maruoka, S. Katayose. “Decreased Plasma Adiponectin Concentrations in Women with Dyslipidemia.” Journal of Clinical Endocrinologic Metabolism. 87, no. 6. (2002): 2764-2769.
Judyth Sassoon
Triglycerides
Triglycerides
Fats exist in foods—and are usually stored in the body—as Triglycerides. Recent research relating levels of triglycerides in the blood stream to heart attacks in human presents a sometime confusing picture but a mounting level evidence suggests that, along with other indicators, triglyceride levels can be used to predict heart attack risk, especially in women and diabetics.
Although the exact mechanisms are not fully known, elevated triglycerides allow increased blood clot formation and may slow the natural deterioration of clots once formed.
Fat molecules are generally made up of four parts: a molecule of glycerol and three molecules of fatty acids . Each fatty acid consists of a hydrocarbon chain with a carboxyl group at one end. The glycerol molecule has three hydroxyl groups, each able to interact with the carboxyl group of a fatty acid. Removal of a water molecule at each of the three positions forms a triglyceride. The three fatty acids in a single fat molecule may be all alike or they may be different. They may contain as few as four carbon atoms or as many as 24. Because fatty acids are synthesized from fragments containing two carbon atoms, the number of carbon atoms in the chain is almost always an even number. In animal fats, 16-carbon, for example, palmitic acid and 18-carbon, for example, stearic acid fatty acids are the most common.
Some fatty acids comprising a given triglyceride have one or more double bonds between their carbon atoms. They are then said to be unsaturated because they can hold more hydrogen atoms than they do. Mono-unsaturated fats have a single double bond in their fatty acids while polyunsaturated fats, such as trilinolein, have two or more. Additionally, there are trans-fats, which are only partially hydrogenated having fewer double bonds in a trans (as opposed to the usual cis) chemical configuration, and also omega-3 fats, which have at least one double bond, three carbon atoms in from the end of the fatty acid molecule. Linolenic acid is an example and fish oils are generally a rich source of omega-3 fatty acids.
Double bonds are rigid and those in natural fats introduce a kink into the molecule. This prevents the fatty acids from packing close together and as a result, unsaturated fats have a lower melting point than saturated fats. Because most of them are liquid at room temperature , they are called oils. Corn oil, canola oil, cottonseed oil, peanut oil, and olive oil are common examples. As this list suggests, plant fats tend to be unsaturated while fats from such animals as cattle tend to be saturated.
Ingested fats provide the precursors from which fat as well as cholesterol and various phospholipids are created (synthesized). In humans, fat provides the concentrated form of energy . The energy content of fat (9 kcal/gram) is more than twice as great as carbohydrates and proteins (4 kcal/gram).
Humans can synthesize fat from carbohydrates. However, there are two essential fatty acids that cannot be synthesized this way and must be incorporated into the diet. These are linoleic acid (an omega-6 fat, with the endmost double bond 6 carbons from the methyl end) and alphalinolenic acid (an omega-3 fat, with the endmost double bond 3 carbons from the methyl end). Many studies have examined the relationship between fat in the diet and cardiovascular disease . There is still no consensus, but the evidence seems to indicate that a diet high in fat is harmful and that mono- and poly-unsaturated fats are less harmful than saturated fats, with the exception of trans unsaturated fats which, according to some, are more harmful than saturated fats. It is also been suggested that ingestion of omega-3 unsaturated fats may be protective for the human body.
See also Biochemistry; Compound, chemical; Heart diseases; Heat.
Resources
books
Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed. Menlo Park: Benjamin Cummings, Inc. 2000.
periodicals
Austin, M.A., B. McKnight, K.L. Edwards, et al. "Cardiovascular Disease Mortality in Familial Forms of Hypertriglyceridemia: A 20-year Prospective Study." Circulation (June 2000): 2777-82.
Avins, A.L. and J.M. Neuhaus. "Do Triglycerides Provide Meaningful Information about Heart Disease Risk?" Archives Internal Medicine. (July 2000): 1937-44.
Cullen, P. "Evidence that Triglycerides are an Independent Coronary Heart Disease Risk Factor." Am J Cardiol. (November 2000): 943-949.
Matsubara, M., S. Maruoka, and S. Katayose. "Decreased Plasma Adiponectin Concentrations in Women with Dyslipidemia." J. Clin. Endocrinol. Metab. 87, (no. 6) (2002): 2764-2769.
Judyth Sassoon
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Polyunsaturated fat
—A fat missing two or more hydrogen atoms from the maximum number of hydrogen atoms that can be bonded to each carbon in the carbon chain of the compound. These fats can remain liquid at room temperatures.
- Saturated fats
—Fats containing the maximum number of hydrogen atoms that can be bonded to each carbon in the carbon chain of the compound.
- Triglycerides
—A molecule containing three fatty acids chemically bonded to a glycol molecule.
Triglycerides
Triglycerides
Triglycerides are the most common storage form of fat in many organisms. They are neutral lipid molecules created via the esterification of three fatty acids to a single glycerol molecule. Triglycerides are an efficient storage medium because their highly hydrophobic nature allows them to be stored as part of droplets (in which they have little or no contact with water molecules).
Triglycerides vary in molecular composition according to the identities of the fatty acids used in their synthesis . Fatty acids that have been esterified to the glycerol moiety of the triglyceride may be unsaturated (containing double bonds) or saturated (containing no double bonds). The number of double bonds in the fatty acids affects the melting temperature of the triglyceride. Saturated fats have higher melting points and are often solids at room temperature. Unsaturated fats have lower melting points and are often liquids at room temperature. Although all triglycerides are correctly identified as fats, triglycerides with melting points below room temperature are also known as oils. Animal triglycerides generally have more saturated fatty acyl groups than plant triglycerides. Beef triglycerides (lard) have a saturated to unsaturated fatty acyl group ratio of 50:50, whereas the ratio for olive oil is about 20:80. Hydrogenation (removal of the double bonds in the fatty acids) will convert an oil, such as vegetable oil, from a liquid to a solid.
In animals, triglycerides are either ingested as part of the diet or synthesized in the liver. Triglycerides are transported in blood as part of lipoprotein particles. Dietary triglycerides are transported as part of lipoprotein particles called chylomicrons. Triglycerides synthesized in the liver are transported as part of lipoprotein particles called very low density lipoproteins or VLDLs. Triglycerides are then removed from lipoprotein particles as they move through the circulatory system. Tissues either utilize this transported triglyceride immediately, or it is stored in adipose tissue .
Triglyceride is stored within cells that make up adipose tissue (fat). Triglycerides are the most abundant form of stored potential fuel in the human body. A typical 70-kilogram (154-pound) man will have approximately 15 kilograms (33 pounds) of stored triglyceride, representing several months of stored fuel. When physiologic conditions necessitate the use of triglycerides stored in adipose tissue, a hormone or neurotransmitter signals their release. Exercise or stress triggers the release of the neurotransmitter norepinephrine from nerve terminals in the adipose tissue, thereby stimulating triglyceride release. Fasting also initiates the release of triglycerides. Insulin and glucagon, two hormones produced by the pancreas, control this release of triglycerides. During fasting, blood glucagon levels increase and insulin levels decrease. The combination of increased glucagon and decreased insulin levels in the blood is the hormonal signal that triggers the release of triglycerides from the adipose tissue. However, triglycerides do not exit adipose tissue intact. Hormonal signaling activates an enzyme called a lipase that hydrolyzes a triglyceride molecule into a glycerol molecule and three fatty acids. The glycerol and fatty acids are then transported within the circulatory system to tissues that will utilize them as fuel. Fatty acids are transported in the blood bound to the serum protein albumin, as their hydrophobic natures would otherwise make them insoluble in the blood.
see also Fats and Fatty Acids; Lipids.
Robert Noiva
Bibliography
Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.
Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.
Triglycerides
TRIGLYCERIDES
"Triglycerides" is the chemical name for fat. Chemically, triglycerides have a three-carbon backbone (glycerol) to which are attached fatty acids, which are strings of carbon and hydrogen atoms, most of which will eventually be oxidized to carbon dioxide and water, producing energy in the process. When not being actively oxidized or metabolized, triglycerides are stored in adipose, or fatty tissue, for oxidation at a later time. High levels of triglycerides in the blood have been associated with increased risk for heart attacks and strokes. Triglycerides may be increased by heredity, abdominal obesity, resistance to insulin, diabetes, and certain medications. They may be decreased by weight loss, control of glucose in diabetes, decreased simple sugar intake in the diet, and increased activity.
Donald A. Smith
(see also: Atherosclerosis; Blood Lipids; Cholesterol Test; Genetics and Health; HDL Cholesterol; Hyperlipidemia; LDL Cholesterol; Nutrition; VLDL Cholesterol )