Protein is an essential macronutrient that acts to build, maintain, regulate, and repair body tissues. More specifically, essential functions of protein in the body include:
- serving as enzymes to catalyze reactions in processes such as metabolism, DNA replication, and digestion [Shils 2006]
- serving as certain hormones and antibodies
- regulating essential body processes including distribution of water in the body, nutrient transport, and muscle contractions
- helping maintain the acid-base balance
- providing a source of energy - about 4 calories per gram
- helping keep skin, hair, and nails healthy.
The Dietary Reference Intake guidelines recommend that protein should make up 5% to 20% of the calories consumed each day by children 1-3 years old, and 10-30% of total energy intake for 4-18 year olds.
Protein is a chemical compound that contains the same atoms as carbohydrate and fat - carbon (C), hydrogen (H) and oxygen (O) - but protein also contains nitrogen (N) atoms. These C, H, O and N atoms are arranged into amino acids, which are linked into chains to form proteins. The sizes and structures of proteins vary widely in complexity, which enables proteins to play a variety of functions in the cell. For example, the structure of the amino acid lysine is shown below.
H2N – (CH2) – COOH
Amino acids are identified as either essential or nonessential. Our bodies can make nonessential amino acids, but not the essential amino acids. These 8 essential amino acids must be supplied by our diets.
||Essential in certain cases
Amino acid requirements actually vary by the age and health of the individual; in children, the ability to synthesize several of the nonessential amino acids is not fully developed.
Amino acids are linked together to form proteins. They connect by means of a condensation reaction. When a molecule of water is split out from 2 amino acids, a bond forms to create a dipeptide. Most proteins are polypeptides, 100 to 300 amino acids in length.
Role of protein in food
The role of protein in food is not to provide body proteins directly, but to supply the amino acids from which the body can make its own proteins. To make body protein, a cell must have all of the needed amino acids available simultaneously. Dietary protein must supply the essential amino acids and enough nitrogen for the synthesis of others.
Complete and incomplete proteins
Proteins can also be classified as complete or incomplete. A complete protein is one that contains all the essential amino acids; an incomplete protein lacks one or more essential amino acids.
||Proteins that contain all the essential amino acids
||Animal products such as meat, eggs and milk and milk products
||Proteins that usually lack one ormolu essential amino acids
||Plant foods such as grains, legumes and nuts/seeds
Incomplete protein sources can be combined to create complete proteins. For children following a vegetarian diet, it is important to eat a variety of plant foods in order to get enough protein. For example, combining a grain and a legume (rice and beans), a grain and a nut/seed (whole wheat bun with sesame seeds) or a legume and a nut/seed (humus, which is chickpeas and sesame paste) create nutritious complete protein combinations that provide good amino acid balance when mixed in proper quantities. If the vegetarian diet permits eggs, milk or cheese, which are complete proteins, the inclusion of such foods with grains or legumes will yield extra amino acid synergies.
A sound understanding of proteins, including their critical role in the body and the ways in which dietary protein supports growth, development and maintenance of body tissues, can help to put the value of this macronutrient into perspective.
A healthy diet includes a balance of nutritious sources of proteins. A diet that is low or deficient in this critical nutrient is truly not "balanced", and is in need of correction to best support overall health.
American Academy of Pediatrics Committee on Nutrition. Kleinman RE, ed. Pediatric Nutrition Handbook. 6th Edition. © 2009 American Academy of Pediatrics.
Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals, Macronutrients
Food and Nutrition Board, Institute of Medicine, National Academies
U.S. Department of Health and Human Services and U.S. Department of Agriculture. Dietary Guidelines for Americans, 2010.
Shils ME et al., eds. Modern Nutrition in health and Disease. Philadelphia, PA: Lippincott Williams & Wilkins, 2006.