The protein content of egg whites

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The protein content of egg whites
An egg contains about 3g of protein, depending on size. (brown egg and white image by John Keith from Fotolia.com)

According to Audrey Ensminger in "Foods & Nutrition Encyclopedia", egg-white protein is highly digestible and has the highest biological protein rating of any food, scoring 94 out of 100. This rating indicates that the proteins in egg whites contain all the amino acids needed for growth and health.

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Kinds of proteins

Egg proteins in general are called albumen. The most abundant protein in albumen is ovalbumin, which accounts for 54 per cent of total egg protein. Other constituents include conalbumin at 13 per cent, ovomucoid at 11 percent, lysozyme at 3.5 per cent, globulin at about 8 per cent, and ovomucin at 1.5 percent. Trace proteins also occur, such as flavoprotein, ovoglycoprotein, ovomacroglobulin, ovoinhibitor and avidin -- each of which contribute between 0.8 and 0.05 per cent of the total.

Kinds of amino acids

Proteins consist of chains of amino acids. The proteins in egg whites are a good source of the essential amino acids needed in the human diet. In "Analytical Techniques in Material Conservation", Barbara Stuart lists the percentages of amino acids found in egg whites: glycine, 3.6; alanine, 2.7; valine, 7.2; leucine, 10.3; isoleucine, 6.2; proline, 4.5; phenylalanine, 5.2; tyrosine, 1.4; serine, 5.8; threonine, 3.7; cystine, 1.9; methionine, 1.2; arginine, 6.8; histidine, 2.4; lysine, 8.0; aspartic acid, 10.5; and glutamic acid, 13.9.

Dietary availability

Egg whites need to be cooked before they become fully digestible. Lead researcher Pieter Evenepoel reported a study on the digestibility of cooked versus uncooked egg whites in "The Journal of Nutrition" in 1998. Results showed that the intestinal digestibility of cooked egg protein was 90 per cent, plus or minus 0.08 per cent. That of uncooked egg white was 51.3 per cent, plus or minus 9.8 per cent. Another advantage of cooking egg whites is to protect against contamination by Salmonella bacteria.

Possible risks

Many athletes, particularly bodybuilders, value eggs as a source of relatively pure protein that provides a complete range of essential amino acids without unwanted fat. However, people who eat large amounts of egg white should avoid eating them raw. Avidin, one of the proteins in albumen, binds to dietary biotin and prevents the body from absorbing it. Cooking avidin keeps it from binding to biotin. According to the authors of "Exercise & Sport Nutrition: Principles, Promises, Science, & Recommendations", people who take in inadequate biotin may experience a decline in aerobic ability because of problems in the production of glucose and fatty acids. Other symptoms include flaking skin, lack of appetite, weight loss and muscle pain.

Egg protein allergies

Egg allergy is the second most prevalent food allergy, especially among children. In particular, it is the protein in eggs that give them their allergy-inducing quality. The egg proteins most likely to cause an allergic reaction include ovomucoid, ovalbumin, ovotransferrin and lysozyme.

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