Bioavailability of vitamin B12 in cows' milk

The natural source of vitamin B-12 in human diets comes from animal products. For example, one glass (250 ml) of milk provides approximately 50% of the RDA (2.4 mu g/d). It was hypothesised that the provision of vitamin B-12 from milk is more efficiently absorbed than the synthetic form used in vitamin supplements. Pigs (n 10) were used as a model for intestinal absorption of vitamin B-12 in humans to compare the net fluxes of vitamin B-12 across the portal-drained viscera (PDV; an indicator of intestinal absorption) after ingestion of meals complemented with conventional and vitamin B-12-enriched (via injections to cows) milk (raw, pasteurised or microfiltrated) or with equivalent amounts of cyanocobalamin, the synthetic form used in supplements or unsupplemented. Net flux of vitamin B-12 across PDV after the ingestion of milk was positive, though not influenced by milk enrichment (P>0.3) or technological processes (P=0.8) and was greater than after ingestion of equivalent amounts of cyanocobalamin (cyanocobalamin v. all milk, P <= 0.003). In fact, net fluxes of this vitamin were not different from 0 after either cyanocobalamin or the meal devoid of vitamin B-12 (unsupplemented v. cyanocobalamin, P=0.7). The cumulative PDV fluxes during the 24 h following ingestion of meals complemented with milk varied from 5.5 to 6.8 mu g. These values correspond to an efficiency of intestinal absorption of vitamin B-12 from milk varying between 8 and 10%. Therefore, vitamin B-12, which is abundant in cows' milk, is also substantially more available than the most commonly used synthetic form of this vitamin.