The American Oil Chemists' Society defines the term "lipid" as a fatty acid and its derivatives and substances related both in structure and function to these compounds. In general, lipids are the substances that cannot dissolve in water. The egg yolk, the yellow portion of the egg, contains a variety of lipids that provide energy and nutrients to the developing embryo.
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Fat, a type of lipid, serves as a source of energy for your body. Each gram of fat contains 9 calories. Your body also needs fat to absorb fat-soluble vitamins like vitamin A, D, E and K, add structure to cell membranes and stimulate the production of hormones. The Institute of Medicine sets the accepted macronutrient distribution range for adults ages 19 and older as 20 to 35 percent of their total calorie intake. The range represents the amount of fat needed to provide essential nutrients while reducing the risk of chronic diseases. One egg yolk contains 4.51 g of total fat, or lipid, according to the National Nutrient Database.
All dietary fat consists of carbon and hydrogen atoms. The way these atoms are arranged determines the type of fat and how the fat affects the body. Fats classified as saturated fats contain carbon atoms completely covered in hydrogen atoms. These types of fat remain solid at room temperature. The American Heart Association warns that eating saturated fat causes blood cholesterol levels to increase. For this reason, the AHA recommends that you limit your saturated fat intake to less than 7 percent of your total daily calories, which for a 2,000-calorie diet equals about 14 g of saturated fat per day. Each egg yolk contains 1.64 g of saturated fat.
Monounsaturated and polyunsaturated fats remain liquid at room temperature. These types of fat, commonly found in plant, nut and seed oils, do not cause high blood cholesterol levels. The AHA recommends the majority of your total fat intake consist of unsaturated fats. Consuming unsaturated fats, especially in place of saturated fats, can help to lower your blood cholesterol levels. Each egg yolk contains 1.995 g of monounsaturated fat and 0.715 g of polyunsaturated fat.
Cholesterol, a type of lipid, plays a vital role in normal body function, and too much cholesterol in your blood contributes to the onset of heart disease. Egg yolk contains a significant amount of dietary cholesterol, 184 mg per egg. The Cleveland Clinic reports that to reduce your risk for high cholesterol and heart disease you should limit your intake of dietary cholesterol to less than 200 mg per day. Eating just one egg provides nearly your daily intake of cholesterol. The Harvard School of Public Health reports that most healthy people can eat one whole egg per day, the white and the yolk, without increasing their risk for heart disease.