Proteins are composed of the elements carbon (C), oxygen (O), hydrogen (H), and nitrogen (n). They have a variety of uses in the body, including serving as a source of energy, as substrates (starter materials) for tissue growth and maintenance, and for certain biological functions, such as making structural proteins, transfer proteins, enzyme molecules, and hormone receptors. Proteins are also the major component in bone, muscle, and other tissues and fluids. When used for energy, protein supplies an average of 4 kcal/g.
Proteins are formed by the linking of different combinations of the twenty common amino acids found in food. Of these, ten are essential for the human in the synthesis of body proteins (eight are essential throughout a human's life, whereas two become essential during periods of rapid growth, such as during infancy).
Protein may be found in a variety of food sources. Proteins from animal sources (meat, poultry, milk, fish) are considered to be of high biological value because they contain all of the essential amino acids. Proteins from plant sources (wheat, corn, rice, and beans) are considered to be of low biological value because an individual plant source does not contain all of the essential amino acids. Therefore, combinations of plant sources must be used to provide these nutrients.
Protein deficiency is not common in the American diet because most Americans consume 1.5 to 2 times more protein than is required for the body to maintain adequate health. This excess intake of protein is not considered to be harmful for the average healthy individual. However, when protein intake is inadequate, but total caloric intake is sufficient, a condition known as kwashiorkor may occur. Symptoms of kwashiorkor include an enlarged stomach, loss of hair and hair color, and an enlarged liver. Conversely, if protein and caloric intake are both inadequate, a condition known as marasmus occurs. Marasmus presents with a stoppage of growth, extreme muscle loss, and weakness.
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