This 2016 study reviews vitamin B12 status, how it is measured, sequelae of deficiency and appropriate sources, within a high-risk population of vegans and vegetarians. The reviewers confirmed research relating to the varying forms of B12 and their physiologic functions.
References:
Rizzo, G., Laganà, A. S., Rapisarda, A. M., La Ferrera, G. M., Buscema, M., Rossetti, P., Nigro, A., Muscia, V., Valenti, G., Sapia, F., Sarpietro, G., Zigarelli, M., Vitale, S. G. Vitamin B12 among Vegetarians: Status, Assessment and Supplementation. Nutrients. 2016: 8(12), 767. doi:10.3390/nu8120767 https://pubmed.ncbi.nlm.nih.gov/27916823/