Bioavailability of dietary cyanocobalamin (vitamin B₁₂) in growing pigs

The present project aimed to estimate bioavailability of dietary vitamin B₁₂, for which little information is available in growing pigs. Two approaches, each using 2 quantities of dietary cyanocobalamin, were compared; the first was based on whole body retention for 8 d and the second was based on nycthemeral portal net flux of vitamin B₁₂. In the first trial, 15 blocks of 3 pigs (31.7 ± 0.5 kg of BW) were formed according to their vitamin B₁₂ status. Within each block, 1 pig (CONT) was killed and tissues were sampled for vitamin B₁₂ determination. The remaining 2 piglets were fed 25 (B₁₂-25) or 250 (B₁₂-250) μg daily of cyanocobalamin for 8 d. Urine was sampled twice daily, and the pigs were killed and sampled as CONT pigs. The total content of vitamin B₁₂ in the carcass, urine, and intestinal tract was affected by the dietary treatments (P < 0.01) but not in the liver (P > 0.019). The whole body retention of vitamin B₁₂ was greater (P = 0.02) in B₁₂-250 than B₁₂-25 pigs, but the corresponding bioavailability was estimated to be 5.3 and 38.2%, respectively. In trial 2, 11 pigs (35.1 ± 4.0 kg of BW and 75.4 ± 5.9 d of age) fed a diet unsupplemented with vitamin B₁₂ from weaning at 28 d of age were surgically equipped with catheters in the portal vein and carotid artery and an ultrasonic flow probe around the portal vein. Each pig received 3 boluses of 0 (B₁₂-0), 25, and 250 μg of dietary vitamin B₁₂ according to a crossover design. Postprandial nycthemeral arterial plasma concentrations of vitamin B₁₂ reached minimum values (P < 0.01) between 15 and 18 h postmeal that were 29.6, 15.6, and 10.0% less than the premeal values for B₁₂-0, B₁₂-25, and B₁₂-250 pigs, respectively (linear, P < 0.01). The cumulative net flux of vitamin B₁₂ for 24 h corresponded to 2.4 and 5.1 μg for B₁₂-25 and B₁₂-250 treatments, respectively, and the corresponding bioavailability was estimated to be 9.7 and 2.0%, respectively. Although bioavailability estimates varied according to approaches, both showed the inverse relationship between dietary vitamin B₁₂ and bioavailability of the vitamin. The dietary supplement of 25 μg was sufficient to maximize hepatic vitamin B₁₂ retention and to attenuate the nycthemeral decrease of arterial plasma concentration of the vitamin.