Within each cell of the body, proteins are continually being made and broken down, a process known as protein turnover. Amino acids must be continuously available to build the proteins of new tissues. The new tissues may be in an embryo, in the muscles of an athlete in training, in a growing child, in the scar tissue that heals wounds, or in new hair and nails.
Less obvious is the protein that helps replace worn-out cells and internal cell structures. For example, the millions of cells that line the intestinal tract live for three to five days; they are constantly being shed and must be replaced. The cells of the skin die and rub off, and new ones grow from underneath.
When proteins break down, their component amino acids are liberated within the cells or released into the bloodstream. Some of these amino acids are promptly recycled into other proteins. By reusing amino acids to build proteins, the body conserves and recycles a valuable commodity. Other amino acids are stripped of their nitrogen and used for energy. Each day, about a quarter of the body’s available amino acids are irretrievably broken down and used for energy. For this reason, amino acids from food are needed each day to support the new growth and maintenance of cells.
Researchers use nitrogen balance studies to estimate protein requirements. In healthy adults, protein synthesis balances with protein degradation, and nitrogen intake from protein in food balances with nitrogen excretion in the urine, feces, and sweat.
When nitrogen intake equals nitrogen output, a person is in nitrogen equilibrium, or zero nitrogen balance. If the body synthesizes more than it degrades and adds protein, nitrogen status becomes positive. Nitrogen status is positive in growing infants, children, and adolescents; pregnant women; and people recovering from protein deficiency or illness; their nitrogen intake exceeds their nitrogen output.
They are retaining protein in new tissues as they add blood, bone, skin, and muscle to their bodies. If the body degrades more than it synthesizes and loses protein, nitrogen status becomes negative. Nitrogen status is negative in people who are starving or suffering other severe stresses such as burns, injuries, infections, and fever; their nitrogen output exceeds their nitrogen intake. During these times, the body loses nitrogen as it breaks down muscle and other body proteins for energy.
The process by which proteins are continually being made and broken down is known as protein turnover.
The body needs dietary amino acids to grow new cells and to replace worn-out ones.
When nitrogen intake equals nitrogen output, a person is in nitrogen equilibrium, or zero nitrogen balance.