A single egg is an excellent source of protein. The egg falls in the Meat, Poultry, Fish, Dry Beans, Eggs, and Nuts Group in the Food Guide Pyramid, notes the American Council on Science & Health (ACSH). Protein is found throughout the entire egg, including the egg white.
Egg White Nutrition Basics
According to the Egg Nutrition Center (ENC), the average egg contains about 6.29 grams of protein, 3.6 of which come from the egg white. In addition to proteins, other nutrients found in the egg white are riboflavin, niacin, folate, vitamin B12, calcium, iron, copper, zinc and sodium. Of the 72 calories in an egg, the white comprises around 17.
Egg whites provide complete protein. This means that they contain each of the amino acids you need to include in your diet to support good health. Your body can utilize the protein found in egg whites to keep your muscles strong as healthy, to support tissue maintenance and to keep your immune system function. The protein in egg whites also allows your cells to produce enzymes -- proteins essential for healthy metabolism -- and hemoglobin, a protein that transports oxygen to your tissues.
Comparison to Egg Yolk
Most of the nutritional value is found within the egg yolk, which also contains all the egg's cholesterol. The ACSH notes that eggs are the "largest single source of cholesterol" among food types commonly eaten in America. However, the ACSH indicates that studies have indicated that the dietary cholesterol from eggs is far less hazardous than saturated fat. Objections to eating egg yolks center on the fact that eggs are usually eaten two at a time, which exceeds the amount of daily cholesterol. Also, eggs are also typically eaten in conjunction with fried meats, such as sausage and bacon, which are high in saturated fats.
Protein & Weight Loss
The ENC notes that eggs provide a considerable amount of protein, given that they're relatively low in calories. Protein consumption is one way that dieters feel full. The ENC cites at 2005 study done at Wayne State University in which overweight subjects ate either a breakfast of two eggs and toast or an equal-calorie breakfast consisting of a bagel and yogurt. The group that ate eggs felt fuller and got less hungry during the course of the day. Calorie counters who want all the protein of the egg but not all of its calories and fat would consume roughly 68 calories in a four-egg white breakfast—less than the sum total of a single 72-calorie egg.
Some recipes, such as those for egg custards, cooked sauces and homemade mayonnaise, require use of the whole egg to achieve the right texture and taste. However, if you're concerned about the cholesterol and fat contained in egg yolks, egg whites can be substituted for whole eggs (two egg whites for every egg yolk) for the purpose of making baked goods, casseroles and other foods that use eggs to bond ingredients or give food loft as it cooks. Egg whites should never be consumed raw due to safety concerns, notes the ENC. Cooking the whites will change the structure of the protein in the egg, but it won't take away how much protein is in the white itself.