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Effect of Omeprazole and Calcium Sources on Calcium Digestibility in Thoroughbred Horses

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Equine gastric ulcer syndrome (EGUS) is very common, with a prevalence estimated from 53% to 93%. A major contributor to its pathogenesis is excessive gastric acid. 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. Epidemiological studies have also found a positive association between PPI and the risk of osteoporotic fractures. It is not known how a therapeutic dose of OM affects nutrient digestibility or bone metabolism in horses. This study was conducted to determine the effect of administration of omeprazole on the digestibility of diets containing two calcium sources 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, limestone (CC) or a marine-derived Ca source (BMC™ with or without OM) over 4 x 21d periods. Treatments were assigned so that OM was not given to a horse in two sequential periods. Each 21d period had a 13-d diet adaptation phase followed by an 8-d collection phase, which consisted of a 48-h adaptation period to collection harnesses, 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, 120g of a Ca-free vitamin-trace 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. Feed and feces were analyzed for macronutrients, macrominerals and microminerals. Serum Ca, P, alkaline phosphatase, gastrin, PTH, and vitamin D were measured. Gastric fluid pH and ulcer score was measured on day 21.

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<.001), but there was no effect of Ca source on gastric pH.  Neither OM nor calcium source affected ulcer score. Serum gastrin levels were doubled in OM-treated horses compared to nontreated horses (53.8 ng/L ± 5.3 vs 26.7 ± 2.7 ng/L) (p<.05). There was a trend (p=0.12) towards higher PTH in OM-treated horses. Serum Ca, P, ionized Ca, vitamin D, and alkaline phosphatase were unaffected by OM or Ca source. OM and Ca source did not affect the digestibility of DM, CP, fat, ADF, NDF, starch, or WSC. OM and Ca source did not affect the digestibility of minerals except Ca. Ca digestibility was affected by both OM and Ca source. OM reduced apparent Ca digestibility from 52.0% to 41.4% in CC and from 55.1% to 46.5% in BMC.  This is a 20% and 15% decrease in Ca digestibility in the CC and BMC diets. Mineral source had a significant effect on Ca digestibility with 50.8% for BMC and 46.7% for CC (P<0.05).

OM reduced Ca digestibility, resulting in an increase in dietary Ca requirements. Long-term omeprazole administration combined with other factors such as high-oxalate pastures or the regular use of furosemide may compromise calcium balance. Calcium balance is of special concern due to its vital role in skeletal health. Horses receiving OM should have Ca intake reviewed and may need extra Ca. The marine-derived Ca source BMC had higher digestibility in horses than CC and was less affected by OM, making it a more effective Ca source in horses on OM.

This research was published in Proceedings of the Australasian Equine Science Symposium, 2018.

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