Dietary fats are one of the three forms of energy-giving nutrients, providing a rich source of energy.
Fats or lipids carry unique properties; they are packed with calories (9 kcal per gram, more than double the amount in protein and carbohydrate), and because they are insoluble in water, they help with the absorption and transport of the fat-soluble vitamins A, D, E, and K. Dietary fat not required immediately for energy is stored in layers of fatty tissue under the skin and around internal organs, where it can be called upon for energy. Fats contain about 95% triglycerides, which are lipids consisting of three fatty acid chains.
Besides providing energy and essential fatty acids necessary for brain development and proper growth, dietary fats get considerable attention because of their association with coronary heart disease (CHD) and stroke, leading causes of death in the United States. Atherosclerosis, the underlying cause in both, involves changes in the innermost layer of the large arteries. Atherosclerosis often begins in childhood, and while many factors play roles, lesions called plaque form from fat deposits and other materials such as calcium and fibrin.
According to a U.S. Department of Agriculture (USDA) food consumption study, fat intake peaked to 70 lb (31.75 kg) per person between 1970 and 1993. Since then, levels have dropped slightly, to around 66 lb (29.9 kg); but one concern is the rising percentage provided by added fats and oils from fried food and snack foods. Current recommendations state that total fat intake should comprise less than 30% of total calories, and saturated fat less than 10%. For a 1500-calorie diet, this standard means less than 1.76 oz (50 g) total fat and 0.59 oz (17 g) saturated fat; for 2500-calorie diets, it is 2.92 oz (83 g) and 0.98 oz (28 g), respectively. People can refer to food labels, which list these amounts.
Research shows that two types of dietary fats in particular, saturated fats and trans fats, raise blood cholesterol, which increases CHD risk.
The major fats in foods are saturated, polyunsaturated, monounsaturated and trans fatty acids, which are discussed below.
- Saturated fatty acids (SFA) or saturated fats get their name because they hold the maximum number of hydrogens possible in their chains. In general, the longer and more saturated the chain, the harder the fat is at room temperature. Coconut oil is an exception. Foods contain a mixture of fatty acids, but SFAs are concentrated in certain animal and vegetable foods, such as beef, chicken, pork, dairy products, palm and coconut oil. Reducing dietary SFAs has the biggest effect on decreasing blood cholesterol.
- Monounsaturated fatty acids (MFAs) are getting good press in the fat world, with many experts advocating the Mediterranean eating style that is high in olive oil, fish, and vegetables. Mediterranean countries have low CHD incidence, but it is unknown how much of a role other factors play. MFAs lower blood cholesterol when they replace SFAs. Aside from olive oil, other high MFA foods include canola oil, peanuts and peanut oil, nuts such as pecans and almonds, and avocados.
- Polyunsaturated fatty acids (PUFAs), such as margarine and sunflower oil, were recommended during the 1980s for lowering blood cholesterol. However, research shows PUFAs can lead to toxic products that promote cardiovascular disease, so heavy intake is not recommended. High sources include vegetable seeds and their oils. Research on a type of PUFA called omega-3 fatty acid is demonstrating benefits in inflammatory disease such as rheumatoid arthritis and atherosclerosis. Seafood contains the most omega-3s, although tofu, soybean, canola oil and nuts all contain one type of omega-3. Two PUFAs, linoleic acid and a-linolenic acid, are called essential fatty acids because they cannot be synthesized in the body. If linoleic acid replaces other forms of fatty acid in the diet, then low-density lipoprotein (LDL) cholesterol decreases and high-density lipoprotein (HDL) cholesterol increases. If linoleic acid replaces saturated fat, total cholesterol decreases but HDL levels also decrease. Safflower oil has the most linoleic acid. A-linolenic acid comes mainly from fish oils. It results in decreased production of triglycerides but has little effect on total cholesterol levels.
- Trans fatty acids are the latest "bad guys," with recent research showing they raise blood cholesterol. Trans fats are high in processed food products, where manufacturers add hydrogen to liquid oils (hydrogenation) to increase stability. Major sources include stick margarine, commercial baked goods, and the frying fats used in most snack foods, restaurants, and fast-food chains. Food labels will list trans fats as hydrogenated or partially hydrogenated fats. Healthy people should eat no more than 10% of total calories as saturated fats and trans fats. For those with CHD, diabetes or high LDL cholesterol, this percentage drops to 7%.
- Other types of dietary fats exist, including lecithin and cholesterol. Cholesterol, an essential component in brain and nerve cells, is found in animal foods, but is also produced and stored in the liver. The role of dietary cholesterol in blood cholesterol and CHD is less clear than in the past. Meat, poultry, seafood, egg yolks and dairy products all contain cholesterol, while plant foods such as fruits, vegetables, grains, nuts, and seeds do not. Americans should not exceed the recommended level of 0.1 oz (300 mg) per day. The average American woman eats 0.07 oz (217 mg) cholesterol daily, the average man 0.12 oz (337 mg).
Treating high lipids
If a person's blood lipid levels are high, dietary treatment is often initiated to lower the low density lipoprotein fraction and total blood cholesterol levels. Current American Heart Association recommendations include reducing saturated fat to less than 7% of calories, limiting cholesterol to less than 200, losing weight if appropriate, and increasing soluble fiber to 0.35-0.88 oz (10-25 g) per day. High-density lipoprotein (HDL) levels, also referred to as the good cholesterol, is related more to genetic and other factors than to diet, although moderate alcohol consumption raises HDL.
Some people also have high plasma triglyceride (TG) levels, which accumulating evidence is showing may be an important heart disease risk factor. Dietary guidelines involve low simple sugars, low-saturated fat diets, restricted alcohol and weight loss if indicated. Drug therapy is often initiated.
A diet containing 30% or less fat is considered compatible with good health. Diets with dietary fat intakes below 15% can negatively affect blood lipids in some people. For people with high blood lipid levels, dietary fat management goals involve moving the total cholesterol and LDL levels to normal. However, because diet modification lowers serum cholesterol levels by about 10-15%, patients whose blood levels remain high often seek drug therapy.
Health care team roles
- Registered dietitians are often sought out by patients and health professionals to assess fat intake and instruct on diet modifications, and for those seeking weight loss through dietary fat restriction.
- Nursing and allied health professionals can provide health histories and check laboratory test results for potential treatment. The American Heart Association provides dietary guidelines and patient education materials. All health professionals can play important roles in educating the public about dietary fat.
Atherosclerosis— Sclerosis or hardening and thickening of the arterial wall, causing a loss of elasticity.
High-density lipoprotein (HDL)— A plasma lipoprotein containing mostly protein and less cholesterol and triglyceride; high levels of HDL lower CHD risk.
Low-density lipoprotein (LDL)— The major cholesterol carrier in the blood, with high levels associated with increased coronary heart disease risk.
Fatty acid— Fatty acids are a group of carbon chains that make up fat. The body requires some, called essential fatty acids, to form membranes and synthesize important compounds.
Lipids— A group of compounds that include fats, oils and related compounds found in foods and the human body.
Monounsaturated fatty acid (MUFA)— A fatty acid containing one double bond, which lowers plasma cholesterol when replacing SFAs and is believed to be safer than polyunsaturated fats.
Omega-3 fatty acids— Named because their first double bond occurs at the third carbon from the methyl end, the three omega-3s (EPA, LNA and DHA) are believed to benefit inflammatory diseases.
Polyunsaturated fatty acid (PUFA)— A fatty acid that is liquid at room temperature, lowers plasma cholesterol when replacing SFAs, but too much can promote cardiovascular disease
Saturated fatty acid (SFA)— A fatty acid that has no double bonds, is solid at room temperature, and raises blood cholesterol levels.
Mahan, L. K., Escot-Stump, S. Krause's Food, Nutrition, & Diet Therapy. Philadelphia: W. B. Saunders Company, 1996.
American Heart Association. National Center, 7272 Greenville Ave., Dallas, TX 75231. 1-800-AHA-USA1. 〈http://www.americanheart.org〉.