Proteins are complex molecules constructed of about 22 amino acids-9 of which are essential in the diet and the rest of which we make in our liver. Generally, protein foods of animal origin are”complete” because they contain sufficient amounts of all the essential amino acids, whereas plant proteins are “incomplete” because they lack one or more of these amino acids. The diagram at right shows that milk is a high-quality protein food since it fills the protein pattern. Rice, on the other hand, is lacking a sufficient amount of lysine, making it incomplete. However, when these two foods are combined, the protein pattern of rice can be improved because the additional lysine in milk complements the missing lysine in rice. In the same way, some plant proteins can complement other plant proteins, as in beans and rice.
The ideal level of protein intake allows the body to be in a state of nitrogen equilibrium – a condition nutrition scientists determine by measuring the amount of nitrogen waste resulting from use of body protein. In theory, consumption of protein in the diet in amounts equal to those used, or “lost,” in the body would create this equilibrium, and would enable researchers to determine the exact protein requirements of certain groups of people. Protein intake that is too high would result in “positive nitrogen balance” – more consumed than excreted – whereas protein intake that is too low would result in “negative nitrogen balance” – more excreted than consumed.
Studies of nitrogen balance are not completely precise. Measurements are subject to error, and the calorie intake of subjects must be neither too high nor too low for true equilibrium to be established. Nonetheless, nutrition experts find these studies useful in determining protein needs.
A recommendation of 0.8 grams of protein per kilogram of body weight has been established, based on the mixed proteins that make up the typical American diet, and allows a generous amount (30percent) above the proven minimum need to cover individual differences. This amount allows 56 grams of protein per day for a man of average size, and 44 grams per day for a woman. This requirement is easily met by two 3-ounce servings of lean meat, fish, or poultry, a cup of milk, and an egg (although this may not be the ideal diet for some people, who would have to look elsewhere for their protein).