About 14 per cent of the human body is protein, so a growing child, or pregnant woman must have protein intake to increase the total amount of protein in the body, or foetus, as it grows. But why does an adult, whose body weight does not change, require protein in the diet? ‘Protein nutrition’ explains that proteins contain the element nitrogen in their constituent amino acids. Nitrogen balance is the difference between the intake of nitrogen-containing compounds in the diet and the excretion of nitrogen-containing compounds from the body. There is a requirement for dietary protein as the continual breakdown of tissue proteins in the body needs replacement by newly synthesized protein.