Stress, Gastric Ulcers, and Metabolic Dysfunction in Horses: The Hidden Link Every Owner Should Understand

Gastric ulcers are one of the most common health issues in domesticated horses, affecting every discipline and lifestyle. Studies show 93% of endurance horses develop ulcers during competition season, 90–100% of racehorses are affected, and even 53% of leisure horses considered “healthy” show significant ulceration when scoped. Ulcers are not just a performance‑horse problem, they’re a modern equine epidemic.

But while diet and management play huge roles, one factor is often underestimated: stress. And stress doesn’t just influence ulcer risk, it also drives metabolic dysfunction through powerful hormonal pathways.

Factors of horse lifestyle that often contribute to stress

• Being housed alone, in their stable to field
• Frequently moving between different herds
• Weaning
• Travelling
• Competition
• Infrequent turnout
• Limited access to forage/grazing
• Intense exercise
• Pain
• Restricted Food or Water access

The above causes of stress, are also stated in many research papers as being directly correlated to the risk of gastric ulcer development - proving environmental stresses and gastric ulcers are intrinsicly linked. 

Ulcer symptoms can be subtle. Horses may display:

• Irritability or aggression
• Nervousness or tension
• Teeth grinding
• Reduced appetite
• Weight loss
• Diarrhoea
• Acute or recurring, chronic colic
• Poor coat condition
• Reduced performance

These signs overlap with other conditions, making diagnosis tricky - but stress is often the underlying trigger.

Why Stress Matters: The Cortisol Connection

When a horse experiences stress, whether that it physical, emotional, environmental, or management‑related, the body releases cortisol, the primary stress hormone. Cortisol prepares the horse for “survival mode,” but chronic elevation has consequences.

Cortisol increases gastric acidity

Cortisol stimulates pathways that increase stomach acid production. More acid + an empty stomach = a perfect storm for ulcer formation, especially in the unprotected squamous region.

Cortisol disrupts metabolic balance

Cortisol directly antagonises insulin, pushing the horse toward metabolic dysfunction. It:

• Restricts insulin release by suppressing pancreatic function and reducing GLP‑1
• Blocks glucose uptake by reducing insulin sensitivity in muscle and fat
• Triggers glucose production in the liver, raising blood sugar

Over time, this forces the pancreas to work harder, increasing the risk of insulin resistance, EMS, laminitis, obesity, and inflammatory disease.

Stress isn’t just an emotional state: it’s a biochemical cascade with real physical consequences.

Why Ulcers Develop: Stress + Physiology + Management

The equine stomach produces acid continuously. The glandular regions are protected, but the squamous region is not. Ulcers form when acid or volatile fatty acids contact this unprotected tissue.

Stress amplifies every risk factor:

• Increased acid production
• Reduced mucus and bicarbonate protection
• Impaired circulation
• Higher oxidative stress
• Behavioural changes that reduce forage intake
• Management changes (transport, exercise, weaning, isolation)

Combine this with modern feeding practices — high‑starch meals, abrupt diet changes, long periods without forage — and the stomach becomes even more vulnerable.

Diet Still Matters

High‑starch diets lower stomach pH and increase VFA production, allowing acids to penetrate the squamous mucosa. Horses with ulcers or metabolic issues should consume low sugar and low starch diets, as excess starch increases acidity and disrupts hindgut bacteria.

But even the best diet cannot fully protect a horse living in chronic stress.

The Takeaway: Species‑Appropriate Management Reduces Stress

Stress is a major driver of both gastric ulceration and metabolic dysfunction, through the shared pathway of cortisol overproduction. By feeding and managing horses in a species‑appropriate way - consistent forage access, low‑starch diets, social contact, movement, predictable routines, and reduced environmental stressors - we support both their physical health and mental wellbeing.

(c) Nia Cooke 2026