For some horses, an all-hay diet may reduce the efficacy of omeprazole in treating ulcers, according to a new study from Australia.
Omeprazole, a so-called proton-pump inhibitor, reduces the amount of acid released in the stomach; low gastric pH is associated with the development of gastric ulcers. Developed for use in human medicine, omeprazole (trade name GastroGard) is the most commonly prescribed ulcer medication for horses.
To see whether diet influences how omeprazole is absorbed by the horse’s body, Ben Sykes, BVMS, PhD, of the University of Queensland, measured changes in stomach pH in six healthy horses receiving oral omeprazole under a variety of conditions. Two diets were studied: an all-hay ration in which horses had constant access to forage, and a high-grain, low-fiber ration, in which the horses effectively fasted overnight.
Two dosing protocols were analyzed: 1 mg/kg given once daily (the dose indicated on the drug label for prevention of ulcers) and 4 mg/kg given once daily (the dose indicated on the drug label for treatment of existing ulcers). Each horse was monitored on each combination of diet and dose for six consecutive days, and its gastric pH was continually measured via gastrostomy tubes. The researchers also drew blood on the first and fifth day of the study to measure levels of omeprazole.
The data revealed that the horses typically had better absorption of omeprazole while on the high-grain diet as opposed to the all-hay rations. The impact of an all-hay ration was dramatic in some horses, with minimal increase in pH observed, even at the higher dose, in these animals. The horses also tended to absorb more and respond best to the higher dose of the medication.
Exactly how an all-hay diet affects drug absorption and efficacy in some horses wasn’t addressed by this study, but Sykes says the finding isn’t without precedent. “It is well documented in other species that feeding practices can impair the absorption of this class of drugs, so the effect is not unexpected.”
Sykes admits that the idea that hay may interfere with omeprazole treatment seems counterintuitive, given that a primary recommendation for managing a horse with gastric ulcers is to reduce his grain ration and provide free-choice hay. “The provision of an all-forage diet remains an important part of ulcer prevention, especially for squamous disease,” he says. “However, our findings somewhat disrupt the standard treatment recommendations that we have used for years as, based on the recent studies, it does appear that feeding such a diet during the treatment phase may be counterproductive in some animals.”
What this means, explains Sykes, is that certain horses may require a different dosing approach. “Some may respond better if they are subject to an overnight fast (assuming morning medication) prior to administration of omeprazole, with feeding occurring approximately 60 to 90 minutes later---this is the peak time for drug absorption,” he says. “Feeding roughage throughout the day remains logical but the nighttime feeding regimen becomes more critical to drug efficacy the following day. Identifying the non-responding horse is challenging, but if one fails to respond to treatment the role of diet should be considered as one possible factor.”
Sykes adds that the development of injectable omeprazole may eventually solve these dosing challenges. “It will be some time before it is commercially available and there is a lot of work to do to determine its most appropriate use,” he says. “But it is an exciting development.”
Reference: “The effects of dose and diet on the pharmacodynamics of omeprazole in the horse,” Equine Veterinary Journal, In Press, 2016
This article first appeared in EQUUS issue #472, January 2017.