The content, binding, and forms of vitamin B12 in milk

The vitamin B12 (cobalamin, Cbl) content of 19 human milks ranged from 0.33 to 3.20 ng/ml, mean 0.97 ng/ml. The milk content of 10 mothers taking 5 to 100 micrograms of supplemental cyanocobalamin daily was not significantly different from that of unsupplemented mothers. The Cbl native to milk was bound to an R type binder. The R binder was also the dominant, by far, unsaturated Cbl binder, but transcobalamin II was found in every milk. The amounts of transcobalamin II were of the same order of magnitude as in serum and seemed to increase with the interval postpartum. Methylcobalamin was the most abundant Cbl of milk. Human milk from well fed mothers contains adequate amounts of Cbl, but the Cbl may become available only if there are sufficient proteolytic enzymes to release it from binding to R binder.