The influence of a change of colonic availability of biotin on biotin status was studied. This was done by inhibition of biotin absorption by intracaecal avidin infusion. Five adult minipigs with a permanent caecal ‘T’ cannula were fed on a semi-synthetic, biotin-deficient diet for 4 months. Following an 8-week adaptation period there were nine sequential 1-week infusion periods with or without oral lactulose or antibiotics. Avidin infusion during weeks 2, 5 and 8 amounted to 18 mg/d (13 U/mg). Plasma biotin concentrations were not changed by avidin infusions. There was a significant average 84% rise in faecal biotin excretion during the avidin periods. Urinary biotin output following avidin decreased by 21 %. This is taken as evidence that biotin synthesized by colonic bacteria is available for host metabolism. A rough estimate shows that under basal conditions 1.7–17% of the metabolic allowance may be covered by this metabolic route.

Six pigs were given a biotin-deficient diet (10 μg biotin/kg) for 14 weeks. The pigs were given the same diet for a further 42 d with one of three biotin treatments: no supplement, 400 μg biotin/d given orally, 400 μg biotin/d infused into the caecum. The concentrations of biotin in the liver, kidney and heart, at the conclusion of 42 d were greater in the pigs given the oral biotin supplement than in the unsupplemented pigs. The values for the pigs given the caecal infusion of biotin were intermediate. The excretion of biotin in faeces was increased from 35 to 101 μg/d by the caecal biotin infusion, and the urinary excretion was increased from 35 to 83 μg/d. The oral dose of biotin increased urinary biotin excretion from 35 to 345 μg/d, but there was no change in faecal biotin excretion. Two pigs were fitted with ileal cannulas and catheters in the vena cava. The pigs were given a dose of [14C] biotin either into the caecum, when the ileum was occluded by a bladder catheter, or orally. [14C] biotin was measured in the urine and plasma and showed that post-ileal biotin absorption was 8 % as efficient as the absorption of biotin after oral dosing.

The initial distribution and absorption of biotin in chickens was studied in vivo from either changes in the composition of intestinal contents or analysis of blood draining the gut. The progressive decline in biotin concentration and flow along the small intestine demonstrated absorption throughout this organ. Quantitatively the duodenum was the most significant site of biotin absorption when crystalline biotin was included in the diet and when dietary ingredients contained highly available biotin. In contrast, when diets of low biotin availability were given the distal segments of the small intestine contributed a relatively greater amount to biotin absorption. During passage of digesta from the ileum to the caecum there was a tenfold increase in biotin concentration. About half the amount was of ileal origin with the remainder presumably resulting from microbial synthesis. About 20% of caecal biotin was in a ‘free’ form but the degree of absorption from the caeca was not determined. The amount of biotin disappearing from the gut of birds given maize-based diets was similar to the quantity appearing in portal blood. Biotin absorption was not detected from wheat-based diets using arteriovenous differences.