Which of the following is NOT an essential amino acid?

Insulin is not classified as an essential amino acid. Essential amino acids are those that the body cannot synthesize on its own and must be obtained through diet. There are nine essential amino acids, including lysine, isoleucine, and methionine. These amino acids play critical roles in protein synthesis, tissue repair, and nutrient absorption, among other functions.

Insulin, on the other hand, is a peptide hormone produced by the pancreas to regulate blood sugar levels. It is made up of amino acids but is not an amino acid itself. Therefore, this distinction is key in identifying which items in the list are essential amino acids and which are not.