Response to a glucose tolerance test in early-lactation Holstein cows receiving a supplementation of biotin, folic acid, and vitamin B 12 .

The aim of the study was to evaluate glucose and insulin metabolism of cows receiving a supplementation of biotin (B ₈ ), folic acid (B ₉ ), and vitamin B ₁₂ (B ₁₂ ) during the transition period. According to a 2 × 2 factorial arrangement, 32 cows were randomly assigned to 9 incomplete blocks according to their previous 305-d milk yield. Within each block, cows were randomly assigned to 1 of the following levels of biotin from -27 to 28 d relative to the parturition: (1) no biotin supplement (B ₈ -) or (2) 20 mg/d of dietary biotin (B ₈ +). Within each level of biotin, the cows received either (1) 2-mL weekly intramuscular injections of saline 0.9% NaCl (B ₉ B ₁₂ -) or (2) 2.6 g/d of dietary folic acid and 2-mL weekly intramuscular injections of 10 mg of vitamin B ₁₂ (B ₉ B ₁₂ +). An intravenous glucose tolerance test was performed at 25 d in milk. Baseline plasma glucagon, glucose, and nonesterified fatty acid concentrations did not differ among treatments. For B ₉ B ₁₂ + cows, baseline plasma insulin concentration and maximal glucose concentration after glucose administration were greater when also combined with biotin compared with no biotin combination, whereas there was no effect in B ₉ B ₁₂ - cows. There was no treatment effect on time to reach half-maximal glucose and insulin concentrations, glucose positive incremental area under the curve, and glucose and insulin clearance rates. Regarding insulin results, maximal plasma concentration and positive incremental area under the curve were respectively 51 and 74% greater for cows receiving the B ₈ supplement than for cows who did not. Moreover, plasma nonesterified fatty acid concentration nadir tended to be reached later for B ₈ cows. Insulin peak was reached earlier for cows in the group B ₉ B ₁₂ + than cows in B ₉ B ₁₂ -, regardless of B ₈ supplementation. Under the current conditions, our results suggested that cows receiving a B ₈ supplement had a reduced insulin sensitivity in early lactation. Insulin response was faster for B ₉ B ₁₂ + cows, but this was not translated into further improvements following the glucose administration challenge.
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