The role of protein in food, as already mentioned, is not to provide body proteins directly, but to supply the amino acids from which the body can make its own proteins. Because body cells cannot store amino acids for future use, it follows that all the essential amino acids must be eaten as part of a balanced diet. To manufacture body proteins, then, all the needed amino acids must be available to the cells. Three important characteristics of dietary protein, therefore, are (1) that it should supply at least the nine essential amino acids, (2) that it should supply enough other amino acids to make nitrogen available for the synthesis of whatever nonessential amino acids the cell may need to make, and (3) that it should be accompanied by enough food energy (preferably from carbohydrate and fat) to prevent sacrifice of its own amino acids for energy.
This presents no problem to people who regularly eat complete proteins, such as those of meat, fish, poultry, cheese, eggs, milk, or many soybean products, as part of balanced meals.2 The proteins of these foods contain ample amounts of all the essential amino acids relative to our bodies’ need for them, and the rest of the diet provides protein-sparing energy and needed vitamins and minerals. An equally sound diet choice is to eat two or more incomplete protein foods from plants, each of which supplies the limiting amino acid in the other—also, of course, as part of a balanced diet. The quality of plant proteins (legumes, grains, and vegetables) having different limiting amino acids can therefore be balanced by combining different sources of plant proteins, either during a meal or over the course of a day, making sufficient amounts of all the essential amino acids available for protein synthesis. This strategy of using complementary proteins is shown in Figure 6-2. Note that by combining a grain (whole-wheat bread) that is low in lysine but high in methionine with a legume (peanut butter) that is low in methionine but high in lysine, all the essential amino acids are provided.
A person in good health can be expected to use dietary protein efficiently. However, malnutrition or infection can seriously impair digestion (by reducing enzyme secretion), absorption (by causing degeneration of the absorptive surface of the small intestine or losses from diarrhea), and the cells’ use of protein (by forcing amino acids to meet other needs). In addition, infections cause increased production of antibodies, which are made of protein. Thus, malnutrition or infection can greatly increase protein needs while making it hard to meet them.3
People usually eat many foods containing protein. Each food has its own characteristic amino-acid balance, and a mixture of foods almost invariably supplies plenty of each individual amino acid. However, when food energy intake is limited, this is not the case (as discussed in the section “Protein-Energy Malnutrition,” later in the chapter). Also, even when food energy intake is abundant, if the selection of foods available is severely limited (for example, when a single food such as potatoes or rice provides 90 percent of the calories), protein intake may not be adequate. The primary food source of protein must be taken into account because its quality is of great importance.
Researchers have studied many different individual foods as protein sources and have developed many different methods of evaluating the protein quality of foods. One evaluation category judges how easily the body can absorb the protein. In general, amino acids from animal and soy proteins are the most easily absorbed (90 to 100 percent). Amino acids from other legumes are next best (80 to 90 percent); the absorption rates for those from grains and other plant foods vary (70 to 90 percent).
When amino acids are wasted, their amine groups (which contain their nitrogen) cannot be stored. Therefore, the efficiency of a protein can be assessed experimentally by measuring the net loss of nitrogen from the body. The higher the amount of nitrogen retained, the higher the quality of the protein. This is the basis for determining
The biological value (BV) of proteins. A high-quality protein by this standard is egg-white protein, which has been designated the reference protein and given a score of 100. Other proteins are compared with it. The best guarantee of amino acid adequacy is to eat a variety of foods containing protein in the presence of adequate amounts of vitamins, minerals, and energy from carbohydrate and fat.