Relative Bioavailability of Phosphorylated Ascorbic Acid in Lactating Dairy Cows

Abstract Text: Rumen degradation limits ascorbic acid (AA) absorption from the small intestine.  The objective of the present study was to determine the relative bioavailability of AA and phosphorylated AA (PAA) introduced into the abomasum of dairy cows.  Four rumen fistulated lactating (207 DIM; 33.3 ± 1.8 kg/d milk) Holstein cows were fed a TMR once daily for ad libitum DMI (17.3 ± 0.7 kg/d).  Cows initially received one of 4 treatments in a balanced 4 X 4 Latin Square design experiment with one week periods.  Treatments were control, 10 g AA in 250 ml saline injected rapidly into a jugular vein (JV) catheter, 10 g AA in 2 gelatine capsules placed into the abomasum via the rumen, or 10 g of AA as PAA (Rovimix® Stay-C® 35) in 2 gelatine capsules placed into the abomasum.  After period 2, abomasal AA and PAA dose was increased to 100 g AA in 750 ml water.  Controls were JV saline injection and abomasal gelatine capsules or water.  Blood (JV) was sampled at intervals from 1 h before to 8 h after dosing.  Urine was collected and sampled at intervals before and for 10 h after AA doses.  Blood and urine were sampled at 24, 48 and 72 h after dosing.  Samples were treated to convert dehydroascorbic acid to AA, stored at -80° C, and analyzed for total AA within 10 days.  The area under the curve (AUC) for plasma AA concentration was calculated and the decrease in plasma AA concentration following the JV dose used to calculate AA kinetics.  Data were analysed using mixed models for effects of cow, period, and treatment (n = 6).  Comparison of the AUC for abomasal versus JV doses suggests the relative bioavailability of AA (7%) and PAA (5%) were similar but very low.  The half-life of the JV dose averaged 66 ± 23 min, with an estimated pool size of 178 ± 42 L and a clearance rate of 2.07 ± 0.34 L/min.  Urinary excretion accounted for only 7 % of the JV dose, and plasma AA concentration remained elevated for 48 h.  Estimates of the relative bioavailability of AA and PAA in lactating dairy cows suggest absorption from the small intestine is low.  This may in part be due to a lack of previous exposure of small intestinal enterocytes to AA or differences in metabolism of intravenously injected versus absorbed AA. 

Keywords: Ascorbic acid, dairy cows, absorption