Essential amino acids cannot be built from other amino acids:
Generally, the body can build semi-essential amino acids from other amino acids and vitamins. However, during growth spurts, periods of illness, or old age, the body may not be able to produce adequate amounts.
The semi-essential amino acids are:
The body can produce its own non-essential, proteinogenic amino acids, but in order to do so, it requires other amino acids, vitamins, and trace elements.
Therefore, it is also important to take in enough non-essential amino acids through a good diet: