What Proteins Are Made Of
Proteins are composed of amino acids, which are linked in chains. Nonessential amino acids can be synthesized in the body. The nine essential amino acids cannot be synthesized in the body or be made in amounts sufficient to meet physiological need. Since all body cells contain protein, routine maintenance and repair of body tissue requires a continual supply of amino acids to synthesize proteins. Growth of new tissue requires additional protein.
The Functions of Body Proteins
The major role of dietary protein is to supply amino acids for the synthesis of proteins needed in the body, although dietary protein can also serve as an energy source. Proteins act as enzymes, as well as perform many other functions in the body. They help regulate water and acid-base balance. Antibodies and some hormones are made of proteins. In the cell membrane, protein “pumps” enable the cell to take up and retain specific compounds while excluding others.
How the Body Handles Proteins
Food proteins are broken down into amino acids by the digestive system, and then reassembled into needed proteins within the body cells. Protein digestion begins in the stomach and continues in the small intestine, where the resulting amino acids are absorbed and then released into the bloodstream. The body cells must have all the essential acids required in order to make a specific protein. If any are missing, the process halts and the amino acid strand is dismantled so that the amino acids can be utilized by other cells.
Protein Quality of Foods
A complete protein supplies all the essential amino acids; a high-quality protein not only supplies them but also provides them in the appropriate proportions. Animal protein sources are generally of higher quality than vegetable protein sources, but diets composed of plant foods provide plenty of protein, as long as a variety of nutrient-dense and complementary protein foods are chosen.
Protein and Health
The RDA for protein for the healthy adult is 0.8 grams of protein per kilogram of appropriate body weight. Most Americans consume higher intakes of protein than recommended.
Kwashiorkor is a disease in which protein is lacking. It typically occurs in children after weaning, with the severest symptoms observed after the age of 2. Protein deficiency also can occur simply because calories are inadequate (marasmus). The deficiencies of protein and calories, which often go hand in hand, are together called protein-energy malnutrition (PEM) and are a worldwide malnutrition problem.
Misconceived notions abound regarding protein in the diet; the most obvious of these is that more is better. However, the higher a person’s intake of animal protein sources such as meat, the more likely it is that fruits, vegetables, and grains will be crowded out of the diet, making it inadequate in other nutrients.
Many health organizations now recommend a diet that emphasizes vegetables, fruits, legumes, and whole grains in order to protect against cancer, heart disease, stroke, diabetes, and obesity.

In USDA’s MyPyramid, the foods that supply protein in abundance are found in the Milk Group and the
Meat and Beans Group. The Vegetable Group and Grains Group also contribute some protein.
The Vegetarian Diet
The chapter addresses vegetarians and discusses the nutrient considerations and benefits of planning plant-based menus.
Food Allergy—Nothing to Sneeze At
The prevalence of food allergy is greatest in the first few years of life and is believed to be the result of the child’s immature immune system The immature digestive system may allow more intact allergen proteins to enter the bloodstream. Early exposure to certain foods can also play a role in development of food allergy in children.
Wonder Bean: The Benefits of Soy
Recent findings show that substances such as phytoestrogens and isoflavones, found in soybeans, can lower cholesterol and help prevent disease. Numerous studies attest to the role soy foods may play in reducing risk for certain forms of cancer, heart disease, and osteoporosis, in controlling diabetes, and in easing a woman’s transition through menopause.