Omeprazole Reduces Calcium Digestibility in Thoroughbred Horses

Omeprazole (OM) is a proton pump inhibitor (PPI) that inhibits gastric acid secretion in horses and is the most popular treatment for EGUS. PPIs are also widely used in humans to treat acid-related conditions and have been associated with a reduction in the digestibility of several nutrients, including protein, fat, calcium (Ca) and iron. This study was conducted to determine the effect of omeprazole on the digestibility of diets containing either limestone (CC) or a marine-derived Ca source (BMC) and to assess changes in blood parameters associated with gastric acid production and Ca status. Four mature Thoroughbred geldings (age 11.25 ± 4.11 y; BW 585.4 ± 25.4 kg) were used in a 2 x 2 factorial design to evaluate the digestibility of diets containing two different Ca sources with or without OM over four 21-d periods. Each 21-d period had a 15-d diet adaptation phase followed by a 6-d collection phase, which consisted of a 5-d total fecal collection period and a final day for gastroscopy and blood sampling. Horses were fed 6 kg timothy hay, 3 kg unfortified sweet feed, 120 g vitamin-mineral supplement and 30 g loose salt per day. Horses were also given either 60 g/d BMC or 50 g/d CC so the total diet supplied ~45g Ca. Horses on OM were given a full tube of Gastrogard® once daily for the final 14 d of each 21-d period which supplied 3.91 ± 0.17 mg/kg BW/d of OM. On day 21, blood samples were taken and gastric fluid pH was measured 8 hr after OM administration. Omeprazole administration had a profound effect on gastric fluid pH compared to nontreated horses (pH 6.0  ±  0.3 vs 2.0  ±  0.2) (p<0.001). Serum gastrin levels were doubled in OM-treated horses compared to nontreated horses (53.0 ng/L  ±  4.7 vs 25.5 ±  2.7 ng/L; p<0.05). Omeprazole and Ca source did not affect the digestibility of DM, CP, fat, ADF, NDF, starch or WSC. Omeprazole and Ca source also did not affect the digestibility of minerals measured except Ca. Calcium digestibility was affected by both OM and Ca source. Omeprazole reduced apparent Ca digestibility from 52.0% to 41.4% in CC and from 55.1% to 46.5% in BMC. This equaled a 20.3% and 15.6% decrease in Ca digestibility in the CC and BMC diets, respectively. Mineral source had a significant effect on Ca digestibility with BMC at 50.8% and CC at 46.7% (P<0.05). Horses receiving routine OM should have Ca intake reviewed and may need extra Ca in the diet. The marine-derived Ca source BMC had higher digestibility in horses than CC and was less affected by OM.