Supplementation of the diet with valine or isoleucine protects the fruit bat from neurological impairment associated with experimentally induced vitamin B12 deficiency. Valine and isoleucine are precursors in the vitamin B12 dependent propionic acid pathway and it is proposed that they exert their protective effect through stimulation of this pathway. This suggests that the previously observed protective effect of methionine may be mediated also through this pathway, since methionine may be metabolised via the transsulfuration pathway to propionyl CoA. The results of this study refocus attention on the propionic acid pathway, and specifically on the methylmalonyl CoA mutase reaction, as central to the biochemical basis of the vitamin B12 neuropathy.