University of Florida (UF) research has shown that any exercise above a walk could force acidic gastric juices up into sensitive areas of the equine stomach, which could be why ulcers develop or worsen in horses in training (affecting more than 80% of performance horses in some studies).

Alfred Merritt, DVM, MS; and Mireia Lorenzo-Figueras, DVM, have found that gastric tension changes during intense exercise can push acidic stomach contents up into the vulnerable upper squamous cell-lined portion of the stomach. The work was done at the Island Whirl Equine Colic Research Laboratory at UF’s College of Veterinary Medicine.

Lorenzo-Figueras and Merritt explored what happens in the stomach of a live, exercising horse by studying UF’s three cannulated research horses–animals with permanent external access to their stomachs. The three Thoroughbreds had Mylar bags (similar to florist balloons) equipped with barostats temporarily inserted in the proximal (upper) parts of their stomachs before treadmill exercise. The barostat maintained constant pressure in the bag, releasing air when the stomach contracted and injecting air into the bag when it relaxed. Because the Mylar bag followed the movements of the gastric wall, changes in the bag’s volume gave an indirect measurement of changes in the stomach’s volume. A computer kept track of the barostat measurements during each exercise test.

The study examined the influence exercise might have on contraction and relaxation of the stomach in horses fed two hours previously, and in those from which feed was withheld for 18 hours before exercise. Over five weeks, the three animals were put through increasingly intense treadmill sessions, culminating in a gallop of nearly two miles (3.2 km) on an uphill slope.

It didn’t take maximum exertion to produce a notable result. As soon as each horse moved from a walk to a trot, the volume of air in the bag decreased rapidly, to the point of almost emptying. The bag remained deflated when the horses galloped, and didn’t regain its original volume until they came back to a walk. This effect was most dramatic in fasted horses, but it was observable in the fed horses as well, although the food in the stomach decreased the initial volume of air in the bag.

Merritt’s team concluded that at any gait faster than a walk, either the gastric wall was becoming more rigid than normal, or external pressure was being exerted on the stomach. To investigate the latter, they inserted a catheter into the abdominal cavity through the right flank to measure intra-abdominal pressure, while a pressure transducer tracked intra-gastric pressure. When horses went from a walk to a trot, both measurements shot up and stayed there for the entire exercise bout, likely due to tensing of abdominal muscles with the faster gaits.

External pressure on the stomach during exercise, they hypothesized, forces the liquid contents of the lower stomach upward, exposing the more sensitive mucosa of the upper stomach to stomach acids and inducing ulcerous lesions. On their own, horses rarely exert themselves for long, so the upper stomach is not naturally exposed to these acids for very long. Since strenuous exercise can trigger gastro-esophageal reflux disease–heartburn–in human athletes, the UF findings are an interesting parallel.

Scientists further tested their hypothesis with a pH electrode inserted into the upper stomach just below the esophageal sphincter to monitor pH (acidity) during exercise. When horses stood or walked, the pH remained around 5-6, but as they trotted or cantered, the pH plunged as far as 1 and remained severely acidic until the horses stopped. This was a strong indicator that stomach acids were splashing on this sensitive area in response to strenuous exercise.

This study shows that gastric ulcers might be more the rule than the exception when we exercise horses. Ongoing studies should provide more ways–dietary and pharmaceutical–in which we can normalize the pH or cushion the equine GI system to minimize damage.