The classification and quality of a protein depends on the number and types of amino acids it contains. There are 20 amino acids, but only 10 are considered essential to humans (Table 6-2). Two additional amino acids are sometimes incorporated into proteins during translation: selenocyteine and pyrrolysine. Essential amino acids are necessary for normal growth and development and must be provided in the diet. Proteins containing all the essential amino acids are of high biologic value; these proteins are called complete proteins and are extremely bioavailable. The nonessential amino acids can be produced in the body from the essential amino acids, vitamins, and minerals.
Incomplete proteins are those that lack one or more of the essentialamino acids. Consequently, incomplete proteins cannot build tissue without the help of other proteins. The value of each is increased when it is eaten in combination with another incomplete protein, not necessarily at the same meal but during the same day. In this way, one incomplete protein food can provide the essential amino acids the other lacks. The combination may thereby pro-vide all the essential amino acids (Figure 6-1). When this occurs, the proteins are called complementary proteins (Table 6-3). Gelatin is the only protein from an animal source that is an incomplete protein.