1. A study was made of the absorption of [58Co]cyanocobalamin in suckling piglets. Cyanocobalamin given at birth and at 7 d of age was efficiently absorbed from the intestine and retained within the body, mostly in the liver. A 10 μg test dose was absorbed no less efficiently than 0.5 μg, despite the virtual absence of intrinsic factor in the gut. In piglets given a 10 μg test dose at different ages between 0.5 and 56 d, there was a marked decrease in the efficiency of retention between about 7 and 21 d of age.
2. Vitamin B12-binding capacity in the gastric mucosa increased with age, from 40 ng at birth to about 2000 ng at 14 d and 7000 ng at 35 d. This binder-protein was largely endogenous, whereas much of the unsaturated binder-protein in intestinal mucosa was apparently derived from milk.
3. The chyme in the stomach and small intestine contained unsaturated binder-protein, partly endogenous and partly deriving from milk, which prevented uptake of added [G-3H]-cyanocobalamin into the ‘solids’ phase of the intestinal contents. The intestinal chyme contained large numbers (log10 7.0–9.1/ml) of bacteria, some of which were isolated and shown to take up cyanocobalamin or folic acid or both.
4. The findings are discussed in relation to the concept that for some days or weeks after the cessation of transport of intact protein across the neonatal gut epithelium (‘closure’), protein-bound vitamins may continue to be taken up into the epithelial cells and there released for transport into the circulation. It is concluded that unsaturated vitamin-binders may strongly influence the ecology of the intestinal microflora.