Amino acids are classified by the metabolic products formed from their carbon skeletons. Glucogenic amino acids produce intermediates that can support glucose generation, whereas ketogenic amino acids produce acetyl CoA or acetoacetyl CoA and can support ketone-body generation. Leucine and lysine are ketogenic only. Phenylalanine, isoleucine, tryptophan, and tyrosine are both ketogenic and glucogenic; the other 14 amino acids are glucogenic in this classification.