Energy
The body's main priority is satisfying the need for energy. Protein supplies four calories per gram, as does carbohydrate. The body will first use carbohydrate and then fats for energy; if these sources are low, it will burn dietary protein. Should the diet be deficient in energy sources, we will break down tissue proteins to meet our needs. We do not store extra amino acids (as we do fat) other than in tissue proteins, so we will destroy body protein when fuel is needed, usually after our fat stores are depleted (see the upcoming section, Protein Requirements).

Building Important Substances

Enzymes are protein catalysts that stimulate biochemical reactions. There are literally thousands of different enzymes within a single cell that help join together or separate a wide variety of substances.

Hemoglobin, an iron-bearing protein that is the key component of the red blood cell, is the molecule that carries the oxygen to the tissues of the body.

Hormones have a dynamic effect on our metabolism. The primary protein hormones are insulin, which regulates our blood sugar levels, and thyroid hormone (really an amino acid, tyrosine with iodine), which controls our metabolic rate.

Antibodies are proteins formed as a response to the stimulus of something foreign (usually a protein) that enters the body. These foreign proteins (antigens) can be bacteria, viruses, fungi, pollens, or protein from a food. The body will produce a specific antibody to bind with the foreign antigen and inactivate it. Different invaders require different proteins that are specific for them, and thus the body must be in constant surveillance and preparedness for production of new antibodies. Proteins are a big part of our immunity.

The immune response is the basis for using immunizations to create immunity to disease through recognition and response to an antigen. Giving a small dose of the disease-causing antigen allows the body to produce the antibody. Our immune system has near-perfect memory, so once we produce an antibody, it will always be available. Should we be contaminated with a polio or tetanus germ after being immunized, we would rapidly produce the antibody to deactivate the germs and not be infected. Thus, we must always have our protein-making capacities available. This immune response, so important for maintaining health, can also cause problems, as when we react to tissue transplants or blood transfusions. Antibody responses to antigens can be the basis for some allergic reactions. We can also misguidedly make antibodies to our own tissues, a process called autoimmunity, which may lead to a number of serious and/or difficult-to-treat problems.

Fluid and Salt Balance
Proteins inside cells help keep the correct amount of water in the cells. Most proteins do not move in and out of cells, as can water, and large protein molecules attract water. The proteins in plasma help maintain the blood volume as well. When overall protein concentrations are low, we can get fluid imbalances. Proteins also help maintain the normal sodium and potassium balance, which is essential to life. Sodium is concentrated outside the cells, while potassium is mainly inside, a situation necessary for normal muscle and nerve cell function. Proteins push sodium out of the cell and potassium into it, thus aiding the heart, lungs, and nervous system to function optimally.

Acid-Alkaline Balance
Proteins can help normalize the acid-base balance by acting as buffers. The body constantly produces acids and bases from chemical reactions. Proteins help in the elimination of excess hydrogen ions, which are part of acids. In this way, the pH, the acid-base balance of the blood, is kept near constant at about 7.4.