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Protein is the nutrient responsible for helping to build, repair and maintain most of the tissues in the body, including the muscles, internal organs, bones, skin and blood. There are estimated to be tens of thousands of different proteins in the human body. Proteins are present in every cell in the body.

Proteins are composed of much smaller substances called amino acids that join together in chains called peptides. Two amino acids joined together form a dipeptide. Three bonded amino acids form a tripeptide. When four or more amino acids are joined into a chain, they form a polypeptide. Proteins are complex molecules made up of strings of many joined amino acids (typically a few dozen to several hundred) that are folded over onto themselves. The shapes formed by these strings allow proteins to do their work.

The formation and breakdown of proteins and amino acids is a constant process. When the body digests protein in food, certain enzymes (e.g., protease enzymes) break the protein down into amino acids. In addition, proteins are constantly being both made and broken down within the body, a process known as protein turnover. Amino acids from foods join with amino acids produced within the body to form an amino acid pool. Any of the amino acids from this pool may be used in protein turnover or to create other compounds or energy the body can use.

Protein has numerous functions in the body. These functions include:

• Growth and maintenance. Proteins make up a portion of all cells in the body and help support the growth and repair of body tissue.
  
  
• Transportation. Proteins also act as transport cells, delivering nutrients and other substances to the cells throughout the body. For example, hemoglobin, the part of red blood cells that carries oxygen, is composed of protein.
  
  
• Fluid balance. Proteins are partially responsible for maintaining fluid balance within the body. Proteins also act as buffers to help maintain the pH balance of body fluids.
  
  
• Creation of necessary chemicals. Proteins play a vital part in creating other substances used by the body. These include:
  
  
• • Enzymes. About half of all dietary protein is used to make enzymes. Enzymes are used to stimulate chemical reactions in the body, such as the breaking down and building up of various substances. Enzyme activity in the body may include the digestion and absorption of food and the creation of new cells, tissues, chemicals and other substances.
    
    
  • Hormones. Some hormones used to regulate body processes are created from proteins.
    
    
  • Antibodies. Some antibodies used to protect the body against disease are created from proteins.

Proteins may be used for a variety of other functions in the body, including vision, blood clotting and to make neurotransmitters (substances that carry messages between nerves). If there is an abundance of available protein or if the body is not receiving enough fats and carbohydrates, proteins can also be broken down for energy. One gram of protein provides the body with 4 calories. If these calories are not needed, the protein can be converted into fat for later use.

Proteins are essential for many body processes. However, the amount of protein needed may vary from one person to another. Large bodies require more protein to maintain and repair body tissues. Growing bodies (e.g., children, pregnant women) and those recovering from injuries need more protein to help build new tissues. Women who are breastfeeding need more protein to produce milk.

The average adult can figure out about how much protein he or she needs every day by calculating the Dietary Reference Intake (DRI). An adult’s protein DRI is equal to 0.8 grams of protein per each kilogram (2.2 pounds) of body weight. The recommended dietary allowance of protein for most adult males is 56 grams per day. For women, it is 46 grams per day. During childhood, protein requirements may increase. Adolescents require 0.85 grams per kilogram of their weight each day, toddlers require 1.1 grams per kilogram, and infants 1.5 grams of protein per kilogram of their weight every day. Pregnant and breastfeeding women generally need an additional 25 grams of protein every day.

For most people, the protein DRI can be achieved by eating a 3-ounce serving of meat or meat substitute twice every day. The majority of Americans get more than enough protein. Even endurance athletes, who need more protein than non-athletes, typically get plenty of protein in their diet.