2.16: Proteins are an essential dietary component.

The atoms present in the proteins we eat—especially the nitrogen atoms—are essential to the growth, repair, and replacement that take place in our bodies. As we digest protein, breaking it down into its amino acids, our bodies use these amino acids for various building projects. Proteins also store energy in their bonds and, like carbohydrates and lipids, they can be used to fuel living processes.

The amount of protein we need depends on the extent of the building projects under way. Most individuals need 40–80 grams of protein per day. Bodybuilders, however, may need 150 grams or more a day to achieve the extensive muscle growth stimulated by their training; similarly, the protein needs of pregnant or nursing women are very high.

Contrary to the impression you might get from food labels, all proteins are not created equal. Every different protein has a different composition of amino acids. And while our bodies can manufacture certain amino acids as they are needed, many other amino acids must come from our diet. Those that we must get from our diet—about half of the 20 amino acids—are called “essential amino acids.” For this reason, we shouldn’t just speak of needing “x grams of protein per day.” We need to consume all of the essential amino acids every day.

Many foods, containing “complete proteins,” have all of the essential amino acids. Animal products such as milk, eggs, fish, chicken, and beef tend to provide complete proteins. Vegetables, fruits, and grains often contain “incomplete proteins,” which do not have all the essential amino acids. If you consume only one type of incomplete protein in your diet, you may be deficient in one or more of the essential amino acids. But two incomplete proteins that are “complementary proteins,” when eaten together, can provide all the essential amino acids.

Traditional dishes in many cultures often include such pairings. Examples are corn and beans in Mexico and rice and lentils in India (FIGURE 2-38).