Proteins are made up of amino acids, needed for the building and repair of tissues in the body. Each food source of protein is assigned a biological value, which is an indication of how closely the protein's content of amino acids matches the human body's protein requirement. While animal source foods have a high biological value, plant source foods generally are lacking in one or more of the essential amino acids. The National Institutes of Health reports that even though certain foods lack certain amino acids, you can still take in those amino acids in other foods throughout the day. For example, what nuts lack in amino acids, you may make up throughout the day by eating beans or another source of protein.
While egg has a biological value of 100, meaning that it contains all of the essential amino acids, most legumes lack in one or more of the amino acids. According to a study published in the September 2004 issue of "Journal of Sports Science and Medicine," soy protein has a biological value of 74. While soy provides all of the amino acids, it does not provide the ideal quantities found in egg.
Some grains have an even lower biological value compared to legumes. Wheat gluten, a component of grains, has a biological value of 64. For vegans, who must obtain all of the essential amino acids from plant-source foods, it is recommended to consume both legumes and grains in order to form a complete protein. Legumes are lacking in certain amino acids, while grains are lacking in others, so they complement each other. MedlinePlus.com notes that these varying protein foods do not have to be eaten together to form complete proteins -- they may be eaten throughout the day.
Nuts and Seeds
Peanuts, cashews, almonds and sunflower seeds are a few examples of foods that have a lower biological value than eggs, the standard. Like grains, nuts and seeds are considered incomplete proteins, as they do not contain all of the essential amino acids.
Vegetables generally contain small amounts of protein, and the protein that they do contain is not a good source of all the essential amino acids. A 3-ounce serving of raw baby carrots, for example, contains only 1 gram protein, and this protein is not considered complete. To meet their protein requirement, vegans are well-advised to focus on legumes, especially soy, as vegetables are not a high source of total protein.