Some of the amino acids can readily be synthesised in the body, from more or less common metabolic intermediates. These are known as non-essential, or dispensible, amino acids, since there is no need for them to be provided in the diet. As long as there is enough total protein in the diet, these amino acids can be synthesised.
If people are fed a mxiture of amino acids lacking one (or more) of the non-essential amino acids, they can still maintain N equilibrium.
Other amino acids that are required for protein synthesis cannot be synthesised in the body, but must be provided in the diet.
If people are fed a mixture of amino acids lacking one of the essential amino acids, they cannot maintain N equilibrium, but show negative N balance.
This program permits you to simulate experiments in which your subject is fed a mixture of amino acids - this may be either a complete mixture, or lacking one amino acid. Again you feed each mixture for a week and see the daily N balance.
Although it is possible for a healthy subject to maintain N equilbrium when any of the non-essential amino acids is excluded from the diet, there are conditions under which the capacity to synthesise some of the non-essential amino acids is less than the need for them. Such conditions include rapid growth in infants, and recovery from traumatic protein loss in adults