Conditionally essential amino acids include threonine, methionine, leucine, isoleucine, phenylalanine, valine, lysine, tryptophan and histidine. 1. Conditionally essential amino acids, which can be synthesized by the human body but usually do not satisfy normal requirements, are also known as semi-essential amino acids or conditionally essential amino acids. 2. Human protein consists of more than 20 amino acids. Most of them can be synthesized in the human body, but there are 9 kinds of amino acids that the human body can not synthesize or the synthesis rate can not meet the needs of the body, must be supplied by the diet, these 9 kinds of amino acids are called essential amino acids, including threonine, methionine, leucine, isoleucine, phenylalanine, valine, lysine, tryptophan, and histidine, and other amino acids are called non-essential amino acids. 3. Tyrosine can be converted from phenylalanine, while cystine can be converted from methionine. If tyrosine and cystine are consumed in sufficient quantities, the essential amino acids phenylalanine and methionine can be saved, hence tyrosine and cystine were once called semi-essential amino acids.