Eggs are a staple of the American diet — and with good reason. These small natural wonders provide you with a broad variety of nutrients. The nutritional composition of egg yolks and whites, however, differs significantly. The yolk is the primary source of nourishment for a developing chick and contains most of the nutrients in an egg. The protein-rich egg white, or albumen, protects the yolk and holds it in place.
When you eat an egg, you're consuming high-quality protein that contains all of the building blocks your body needs to manufacture new proteins. Both parts of an egg contain protein, with approximately 60 percent in the white and 40 percent in the yolk. The amount of protein in an egg increases with its size and weight. A large egg provides you with approximately 3.6 grams of protein in the white and 2.7 grams in the yolk. A small egg contains 2.7 grams of protein in the white and 2.1 grams in the yolk.
Approximately 99 percent of the fat in an egg is in the yolk. The 5 grams of fat present in a large egg yolk includes approximately 210 milligrams of cholesterol, 1.6 grams of saturated fat and 2.6 grams of unsaturated fat. You need to limit the amount of saturated fats and cholesterol you consume because these fats promote increased levels of total cholesterol and LDL, or "bad" cholesterol. High blood levels of these fats increase your risk of developing heart disease. Eaten in moderation, unsaturated fats are more healthful because their metabolic effects counter those of the bad fats.
The undesirable fat in an egg yolk has a positive aspect in that it carries healthful doses of the fat-soluble vitamins. Egg yolks are one of the few foods that naturally contain vitamin D, with 0.6 microgram in a large egg. Vitamin D, along with calcium, helps you build and maintain strong bones. Other fat-soluble vitamins in egg yolks include vitamins A and E, and a small amount of vitamin K. Vitamins A and E are potent antioxidants, which protect your tissues from chemical damage. You need vitamin K to produce the proteins that help your blood clot.
Most of the water-soluble, B-complex vitamins in eggs are located in the yolk, including B-6, B-12, folate, thiamine and riboflavin. Small amounts of these vitamins are also present in egg whites. The B-complex vitamins aid in many important functions in your body, including the production of new red blood cells and the conversion of carbohydrates, fats and proteins into energy. Notably, neither the yolk nor the white of an egg contains vitamin C.
The minerals in eggs are concentrated in the yolks, with a few exceptions. Egg yolks provide you with significant amounts of iron, zinc, calcium, phosphorus and selenium. Egg whites, however, contain most of the sodium, potassium and magnesium in eggs. These minerals are also present in egg yolks, but in lower concentrations than found in egg whites. Your body needs iron to make red blood cells and other oxygen-carrying chemicals. Calcium, phosphorus and magnesium are important for maintaining strong bones. Zinc and selenium support your immune system, and sodium and potassium help maintain your body's salt and water balance.
- USDA Agricultural Research Service: What's in the Foods You Eat Search Tool
- Egg Science and Technology: William J. Stadelman, Ph.D., Owen J. Cotterill, Ph.D.; 1995
- MadSci Network; What's the Function of the Egg White?; Joe Regenstein, Ph.D.; October 2000
- Egg Nutrition Center: Nutrition Facts, Large Egg
- American Heart Association: Fat
- Chicken shaped eggs holder with three eggs image by velora from Fotolia.com
This article reflects the views of the writer and does not necessarily reflect the views of Jillian Michaels or JillianMichaels.com.