Egg white or albumen comprises about 60 percent of the total egg weight, of which about 10 percent -- about 4.5g for a large egg -- is protein. Since avian eggs must develop into chicks without any additional nutrient input, the composition of the protein in the egg contains all 20 proteogenic amino acids required for protein synthesis. Egg white is considered a nutritionally complete and high quality protein for humans; it contains appropriate amounts of the essential amino acids recommended by the Food and Nutrition Board of the National Institutes of Medicine, and has a high Protein Digestibility Corrected Amino Acid Score.
Essential Amino Acids
The essential amino acids -- methionine, lysine, tryptophan, phenylalanine, threonine, histidine, and the branched chain amino acids -- cannot be synthesized by humans, and must be obtained through the diet. Insufficient dietary intake of the essential amino acids can affect protein synthesis, a process necessary for proper metabolism. Some of the essentials have additional, non-protein functions in the body, which can also suffer from low intake. For example, tryptophan and histidine are used to make the neurotransmitters seratonin and histamine, respectively; and methionine and lysine are the building blocks for carnitine, a cofactor for fatty acid catabolism. According to the USDA Nutrient Database, a large egg white contains about 1.7g of essential amino acids.
Branched Chain Amino Acids
The branched chain amino acids, or BCAAs -- leucine, valine, and isoleucine -- are nutritionally essential, and serve important roles in stabilizing the structures of the proteins in which they are incorporated. BCAAs are also unique in that they are metabolized to alpha-keto acids in the muscles, where they are used in addition to glucose and glycogen as a source of fuel. A large egg white contains about 0.8g of BCAAs.
Glutamine
Glutamine is has several roles in metabolism aside from protein production; it is used as a source of nitrogen for the construction of DNA, non-essential amino acids, neurotransmitters, and some polysaccharides. Glutamine is also used as a shuttle to remove excess nitrogen from the body, such as when amino acids are metabolized for fuel in the liver and muscles. Glutamine content is difficult to measure in foods; an egg white likely has less than 0.5g.
Non-essential Amino Acids
Egg whites also contain about 2.1g of the non-essential amino acids arginine, alanine, glycine, proline, aspartic acid, asparagine, glutamic acid, serine, cysteine, and tyrosine. They are "non-essential" by virtue of a person's ability to synthesize enough of them to satisfy our metabolic needs, however, dietary non-essential amino acids are still important building blocks for cellular proteins, and as additional sources of chemical energy. Several have additional metabolic functions. Arginine is used as a source of nitric oxide, an important vasodilating signal. Cysteine, glycine and glutamic acid are the source of glutathione, a ubiquitous antioxidant and detoxification compound in the body. Tyrosine is the precursor to the neurotransmitters norepinephrine and dopamine, as well as thyroid hormone thyroxine.
References
- "Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients)", Food and Nutrition Board, National Institutes of Medicine, 2005
- "Journal of Agricultural and Food Chemistry", Advances in the Value of Eggs and Egg Components for Human Health, J. Kovac-Nolan, M. Phillips, Y. Mine, September, 2005
- USDA National Nutrient Database for Standard Reference for Egg White, Raw
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