What is protein?

Structure and Functions

Proteins are composed of twenty-two different amino acids in various combinations joined together in a chain. The human body can synthesize fourteen amino acids, known as inessential amino acids, whereas the other eight, known as essential amino acids, must be provided by the diet. Proteins from dietary sources are considered complete or incomplete based on their amino acid content. Complete proteins provide all eight essential amino acids, whereas incomplete proteins have low amounts of, or are missing, at least one essential amino acid. Only fish, meat, poultry, eggs, cheese, and other foods from animal sources contain complete proteins. Peanuts, soy, nuts, seeds, green peas, legumes, and some grains provide the richest plant proteins. Omnivores typically eat a sufficient variety of foods to consume adequate essential amino acids. Vegetarians ensure sufficient amino acid intake by consuming a variety of plant proteins.

Every cell in the body contains protein. Proteins are essential for cell structure and function and comprise major components of skin, tissues, muscles, and internal organs. Dietary protein, one of three types of nutrients used for energy, is required for the building, maintenance, and repair of body tissue. Growth and development during childhood, adolescence, and pregnancy depend on proteins. Proteins participate in all cellular functions, such as acting as enzyme catalysts and transport and storage molecules. They also provide mechanical support and immune protection, generate movement, transmit nerve impulses, and control growth and differentiation. Important classes of proteins include enzymes, hormones, and antibodies.

Disorders and Diseases

Protein deficiency is relatively rare in developed countries, although poverty can prevent adequate protein consumption. Protein deficiency can result from stringent weight loss diets or in older adults. Severe protein deficiency is fatal. In developing countries, protein deficiency causes the disease known as kwashiorkor, which includes symptoms such as apathy, diarrhea, inactivity, failure to grow, fatty liver, and edema. Since the body cannot store protein, too much protein can also be harmful, with a high-protein diet potentially leading to high cholesterol or other diseases and possible effects on kidney function. Excess protein may also result in immune system hyperactivity, liver dysfunction, bone loss, and obesity. Proteins contribute to food allergies, with many people allergic to proteins in milk (casein), wheat (gluten), or proteins found in peanut, shellfish, or other seafoods.

Perspective and Prospects

A nutritionally balanced diet provides adequate protein. The average diet in the United States includes nearly twice the amount of protein required to maintain a healthy body. Although it was once believed that vegetarians needed to combine proteins by consuming all amino acids in the same meal, it is now realized that benefits of protein combining can be achieved over a longer period of time. In May 2013, the New York University Fertility Center published a study showing that women undergoing fertility treatment increased their chances of conception by increasing the amount of protein in their diets.

Bibliography

"Can High-Protein, Low-Card Diet Boost Fertility Treatment?" MedlinePlus. May 6, 2013.

"Dietary Proteins." MedlinePlus. July 18, 2013.

Duyff, Roberta Larson. American Dietetic Association Complete Food and Nutrition Guide. Hoboken, N.J.: John Wiley & Sons, 2006.

Marshall, Keri. User’s Guide to Protein and Amino Acids. Laguna Beach, Calif.: Basic Health Publications, 2005.

"Protein in diet." MedlinePlus. May 5, 2011.

Shils, Maurice E., Moshe Shike, A. Catharine Ross, and Benjamin Caballero. Modern Nutrition in Health and Disease. 10th ed. Philadelphia: Lippincott Williams & Wilkins, 2005.