Protein is one of the three main classes of food that provide energy to the body. The others are carbohydrates and fats. Proteins exist in every cell and are essential to plant and animal life. Plants build proteins from materials in the air and the soil. Human beings and animals obtain proteins from foods. Foods high in protein include cheese, eggs, fish, meat, and milk.
The structure of proteins.
All proteins contain carbon, hydrogen, nitrogen, and oxygen. Some proteins also contain iron, phosphorus, and sulfur. Proteins are large, complex molecules made up of smaller units called amino acids. The amino acids are linked together into long chains called polypeptides. The polypeptide chains fold into complex three-dimensional shapes. A protein consists of one or more polypeptide chains.
Twenty common amino acids are assembled into the thousands of different proteins required by the human body. To assemble the proteins it needs, the body must have a sufficient supply of all these amino acids. Nine amino acids, called essential amino acids, either cannot be produced by the body, or are produced, but in insufficient amounts. Therefore, they must be supplied by various foods. The remaining amino acids, called nonessential amino acids, can be manufactured by the body in sufficient amounts.
Proteins in the diet.
The best sources of proteins are cheese, eggs, fish, meat, and milk. The proteins in these foods are called complete proteins because they contain adequate amounts of all the essential amino acids. Cereal grains, legumes (plants of the pea family), nuts, and vegetables also supply proteins. These proteins are called incomplete proteins because they lack adequate amounts of one or more of the essential amino acids. However, a combination of two incomplete proteins can provide a complete amino acid mixture. To do so, each incomplete protein must have sufficient amounts of the essential amino acids of which the other incomplete protein contains small amounts. For example, a cereal grain, such as barley or corn, could be combined with a legume, such as peas or peanuts. The foods must be eaten together to provide the correct balance of amino acids.
In the United States, the Recommended Dietary Allowance of protein for adults is 0.8 gram per kilogram (0.013 ounce per pound) of body weight per day. Infants and children need extra protein, as do pregnant women and nursing mothers.
Insufficient protein in the diet may cause lack of energy, stunted growth, and lowered resistance to disease. A protein shortage also may lead to edema, a condition in which fluids accumulate in body tissues, causing the tissues to swell. In developing countries, many infants and children have a disease called kwashiorkor as a result of eating little or no food containing complete proteins. Severe cases of kwashiorkor may cause liver damage and eventual death.
How the body uses proteins.
Proteins make up a large part of each cell in the human body. Therefore, they are important in building, maintaining, and repairing tissues in the human body, especially bone, cartilage, and muscle.
In addition, every cell contains proteins called enzymes, which speed up chemical reactions. Without enzymes, the cells could not function. Certain proteins perform specific jobs. For example, the blood contains such proteins as albumin and hemoglobin. Albumin helps maintain the body’s fluid balance by keeping water in the blood. Hemoglobin carries oxygen from the lungs to body tissues. Antibodies are proteins in the blood that help protect the body from disease. Chemical substances called hormones, many of which are proteins, control such processes as growth, development, and reproduction.
The body obtains most of its energy from carbohydrates and fats. However, the body uses proteins for energy when carbohydrates and fats cannot meet its energy needs. Proteins produce about 1,800 calories of energy per pound (4 calories per gram), the same amount provided by carbohydrates.
After proteins have been eaten, hydrochloric acid, which most people have in their stomach, causes the protein molecules to coagulate (thicken and clump together). Enzymes in the stomach and in the intestines break down the coagulated proteins into individual amino acids. The amino acids are absorbed into the blood and travel throughout the body. Every cell assembles the amino acids into the proteins it needs. This process is controlled by DNA (deoxyribonucleic acid), a substance in the nucleus of each cell. For a detailed description of how a cell produces proteins, see Cell (Producing proteins) and Cell ( The genetic code).
Food containing proteins should be included in the daily diet because the body cannot store proteins for later use. The body converts excess proteins into carbohydrates and fats. If the body does not receive enough proteins from the food eaten, it uses proteins from the cells of liver and muscle tissues. However, continued use of such proteins by the body can permanently damage those tissues.
