Fats, Dietary Health Article

Definition

Dietary fats are one of the three forms of energy-giving nutrients, providing a rich source of energy.

Purpose

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 such other materials 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.

Precautions

Research shows that two types of dietary fats in particular, saturated fats and trans fats, raise blood cholesterol, which increases CHD risk.

Description

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 SFA 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 (MFA) 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 other factors play a role. MFAs lower blood cholesterol when they replace SFAs. Aside from olive oil, other high MFA foods include canola oil, peanut and peanut oil, such nuts as pecans and almonds, and avocados.
• Polyunsaturated fatty acids (PUFA), 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 such inflammatory disease 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 alinolenic 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 lowdensity 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, in which 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 levels 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).