Amino Acid Derivatives and Related Compounds

Compounds sold alongside amino acids that are not proteinogenic amino acids: metabolites (5-HTP from tryptophan, SAM-e from methionine), non-protein amino acids (beta-alanine, citrulline, ornithine, GABA, L-theanine), a D-enantiomer, and two that are not amino acids at all — creatine, a guanidino compound, and collagen, which is a protein made of amino acids. Grouping them by what they are is the only honest option; the nutritional split above does not apply to any of them.