Classifying amino acids as gluco(glyco)genic, ketogenic, or both

Eleven popular and recent biochemistry textbooks were examined and compared concerning the classification of amino acids as being gluco(glyco)genic, ketogenic, or both. The main differences concerned mostly two amino acids, threonine, which in some cases is considered only gluco(glyco)genic and in others both gluco(glyco)-ketogenic, and lysine, which is considered chiefly ketogenic, but in two cases is reported as being gluco(glyco)-ketogenic. Some more mistakes, discrepancies and curiosities among different textbooks are also reported. Biochemistry textbook authors and teachers should be aware of this situation to avoid any confusion, especially among students.