Background: Gastric disease is highly prevalent in sport horses and may lead to poor performance, cause behavioral changes and impact welfare. Hypothesis: Assess whether diet affects gastric disease and pain expression during riding, and whether it has an impact on physiological and locomotor variables during an exercise test, including jumps. Animals: Nine healthy show-jumping Warmbloods trained at the same stable. Methods: Prospective observational cohort study. The horses receiving a pelleted diet, high in sugar and starch (>30%), were examined at T0 and after 12 weeks (T12) of changing to a cooked, muesli-type low-starch (11%) diet. Each time, the horses underwent a standardized exercise test (SET) and a ridden pain score (Ridden Horse Pain Ethogram [RHpE]) was calculated by 2 blinded observers. The next day, horses underwent gastroscopy and gastric lesions were scored blindly. Results were analyzed using Wilcoxon and Spearman tests. Results: After 12 weeks of a low starch diet, the Equine Gastric Disease (EGD; 4 [3–5] at T0 vs 1 [0-1] at T12, P < .01) and RHpE scores (6 [3–13] at T0 vs 3 [0-6] at T12, P < .01) improved significantly. Squamous, glandular, and EGD scores were positively correlated with RHpE scores (respectively, r = .747, P < .01; r = .743, P < .01 and r = .867, P < .01). Conclusion and Clinical Importance: Gastric disease and pain scores correlated positively in ridden horses. A low starch diet significantly decreases the severity of gastric disease and associated pain score during riding in horses. Gastric ulcers may be mitigated and the comfort of equines athletes improved by dietary adjustments.

Equine gastric disease (EGD) is a common condition in performance horses (Equus caballus), potentially compromising behaviour, performance, and welfare. EGD is often attributed to high-starch, high-sugar feeds and limited forage. Evidence for diet-induced changes on digestive microbiota is lacking. Nine elite showjumping horses were housed at the same performance yard with standardised diet and management throughout the study. Horses were transitioned from a high-sugar and -starch (31%) feed to a low-starch and -sugar (16.5%) concentrate feed. Gastroscopies, blood, and faecal samples were taken pre- and 12 weeks post-diet change. Squamous and glandular ulceration was blindly graded a posteriori using 0–4 scores and faecal microbiota profiled using 16S rRNA gene amplicon sequencing. Total (t_(1,8) = −6.17, p < 0.001; Pre: 4 [0–5], Post: 1 [0–2]), squamous (t_(1,8) = −5.32, p < 0.001; Pre: 1 [0–3], Post: 0 [0–1]), and glandular (t_(1,8) = −2.53, p = 0.04; Pre: 2.5 [0–4], Post: 0 [0–2]) disease improved following the introduction of a low-starch diet. Diet change did not impact microbiota communities (PERMANOVA: F_(1,16) = 1.37, p = 0.15, r² = 0.08), but Firmicute to Bacteroidota (F/B) ratio reduced (t_(1,8) = −3.13, p = 0.01; Pre: 2.07 ± 0.21 vs. Post: 1.29 ± 0.14). Lower F/B ratios were associated with reduced total EGD scores (ChiSq_(1,17) = 3.83, p = 0.05). Low-starch diets did not influence faecal microbiota diversity but aided gastric disease healing and reduced F/B ratios in elite showjumpers during a competition season without medication.