The Lipids

Lipids share the common property of insolubility in water and solubility in the fat solvents. Fats, fatty acids, fatty oils, waxes, sterols, and esters of fatty acids all belong to this group, the lipids. Chemically, lipids are composed of carbon, hydrogen, and oxygen. Dietary lipids are a source of energy, and they can be converted to other essential tissue constituents, or converted into stored energy as reserve fat in adipose tissue.

While high-fat diets in industrial nations are often linked to higher rates of degenerative disease and obesity, a small amount of dietary fat is required for optimal health. This fact is overlooked by many. Fat serves as a high-energy food, facilitates the digestion and metabolism of other nutrients and forms structural and functional components of cell membranes. Fats play a key role in the transport and absorption of the fat-soluble vitamins (A, D, E, K, and the carotenoids and beta-carotene).

When fat is eliminated from the diet, one’s body will synthesize some fatty acids from protein and carbohydrates t meet its needs. However, one fatty acid, linoleic acid, is essential and cannot be synthesized. It must be obtained from food or supplements. Linoleic acid is an important building block for the prostaglandins, hormone-like substances. It is believed that omega-3 fatty acids might also be essential to normal development of vision and brain function.

Lipid Classification

Triglycerides

Triglycerides are the primary form of fat and comprise the bulk of fat in foods, stored fat in the body, and are a primary form of fat in the blood. Triglycerides are the lipids that provide calories or energy to the body. Triglycerides exist in many shapes and sizes, but they all exhibit a similar structure which is glycerol molecule with three fatty acids attached. The physical characteristics of a given fat are determined by the degree of saturation of its fatty acids.  Triglyceride levels greater than 100 have been found to increase the risk for a new heart attack by 50 percent. Triglyceride levels can be reduced with exercise, low-saturated-fat diets, and foods and supplements rich in omega-3 fatty acids, such as flaxseed and fish oil.

Saturation

Fatty acids are composed of chains of carbon atoms, usually sixteen to eighteen in number but ranging between two and twenty. If carbon a carbon atom is bound to its maximum number of hydrogens, it is said to be saturated. If two joined carbon atoms can bind on additional hydrogens, the fat is said to be monounsaturated. If more than one hydrogen atom can be accepted, the fat is polyunsaturated. Linoleic acid in safflower oil is a polyunsaturated fatty acid. Oleic acid in canoloa and olive oils is a monounsaturated fatty acid.

Unsaturated fats are more fluid at room temperature than saturated fats. For example, safflower oil, high in polyunsaturated fatty acids, is liquid at room temperature, while lard, high in saturated fats, is solid. This is because the saturation with hydrogen atoms gives the fatty acids a regular shape which fits closely together like stacked spoons. Unsaturated fatty acids are kinked and do not lie in a compact fashion. Therefore, the saturated fats are more dense and solid; unsaturated fats are less dense and fluid.

Hydrogenation

Hydrogenation describes the process of adding hydrogen atoms at the double bonds of unsaturated fatty acids. Hydrogenation is used to increase the shelf life of fats and to change liquid unsaturated fats to a firm, semisolid form. Hydrogenation solidifies liquid corn oil to margarine. Hydrogenation converts some unsaturated fatty acids from their natural cis configuration to the trans configuration. These trans fatty acids are under investigation for their potential cancer causing properties and ability to impair prostaglandin synthesis.

Cholesterol

Cholesterol is a white crystalline substance found in egg yolk, all animal fats, bile, gallstones, nervous tissue, and blood. Cholesterol is present in almost all body tissues but is concentrated in the liver, blood, and brain. Cholesterol, the best known type of sterol (an alcohol), is only synthesized in animals. Ergosterol and sitosterol are the most common sterols produced by plants.

Linoleic Acid

Linoleic and linolenic acid are essential polyunsaturated fatty acids that can be obtained only from dietary sources. Quality sources include vegetable oils, nuts, and seeds. Arachidonic acid can be synthesized within the body when adequate linoleic acid is available. Omega 3 fatty acids are polyunsaturated fatty acids plentiful in fish oils and believed to be necessary for the proper development of the brain and vision.

Dietary Requirements

Americans are eating less fat than ten years ago according to the third National Health and Nutrition Examination Survey (NHANES). Yet, Americans still have a long way to go to reach the recommended goal of less than 30 percent fat calories and less than 10 percent saturated fat calories. The NHANES reported Americans average 34 percent of their calories (82 grams) from fat and about 12 percent (29 grams) of it from saturated fat.

No Recommended Daily Allowance (RDA) has been established for fat. Fats are one of the macronutrients, but as little as 10 percent of calories with sufficient linoleic acid will meet basic requirements. Some nutritionists suggest the essential fatty acid, linoleic acid, should provide 1 to 2 percent of the total calories in the daily diet.

The National Academy of Sciences suggests an RDA of 15 to 25 grams of dietary fat to ensure an adequate intake of essential fatty acids and to provide a carrier for fat-soluble vitamins. The fat-soluble vitamins and essential fatty acids can be obtained through fortified foods, such as vitamin D from nonfat milk. However, the elimination of all fats from the diet is unwarranted and practically impossible because they coexist with proteins and carbohydrates in many foods. The general guideline is to limit your fat to no more than 30 percent and no less than 15 percent of calories and not exceed 10 percent of calories from saturated fats.