Equine metabolic syndrome (EMS) is a collection of risk factors centered on the horse’s inability to properly regulate insulin levels. It’s not a single disease but rather a pattern of hormonal imbalance, abnormal fat deposits, and a dangerously increased risk of laminitis. If your horse has been diagnosed with EMS or you suspect it, understanding the condition is the first step toward managing it effectively.
The Three Hallmarks of EMS
EMS revolves around three interconnected problems: insulin dysregulation, regional fat deposits, and laminitis susceptibility.
Insulin dysregulation is the central feature. In a healthy horse, eating sugars and starches triggers a controlled rise in insulin that helps cells absorb glucose from the blood. In a horse with EMS, this system misfires. The body either produces too much insulin in response to sugars, fails to clear insulin efficiently, or both. The result is chronically elevated insulin levels, a state called hyperinsulinemia. Research has shown that sustained high insulin can trigger laminitis in as little as 48 hours, even in otherwise healthy horses.
The second hallmark is abnormal fat distribution. Most EMS horses carry a body condition score above 6 on a 9-point scale, but it’s not just overall heaviness that matters. Fat tends to accumulate in specific regions: along the crest of the neck (giving that characteristic “cresty” appearance), over the ribs, behind the shoulder, and around the tailhead. The cresty neck is so common in EMS horses that veterinarians use a dedicated 0-to-5 scoring system for it, ranging from no visible crest to a large crest that droops to one side.
The third, and most serious, consequence is laminitis. This painful condition involves inflammation and structural failure of the tissues connecting the hoof wall to the coffin bone inside the hoof. Laminitis is considered the primary clinical consequence of EMS, and it’s often the crisis that leads to a diagnosis in the first place. Horses with EMS are especially vulnerable when they have access to lush pasture or high-carbohydrate feeds.
What Causes EMS
Genetics play a significant role. While any horse can develop EMS, certain breeds that evolved to thrive on sparse forage are far more susceptible. Donkeys, ponies, Arabians, Morgans, Saddlebreds, Paso Finos, Spanish Mustangs, and Warmbloods all carry higher risk. Thoroughbreds and Standardbreds rarely develop the condition. These “easy keeper” breeds are genetically efficient at storing energy, which becomes a liability when they’re fed modern diets and get limited exercise.
On top of genetics, lifestyle factors push horses over the edge. Overfeeding (especially sugar-rich grains and lush grass), insufficient exercise, and prolonged obesity all worsen insulin dysregulation. Fat tissue itself is metabolically active, releasing inflammatory signals that further impair insulin function at the cellular level. This creates a vicious cycle: excess fat promotes insulin resistance, which promotes more fat storage.
How EMS Is Diagnosed
Diagnosis starts with a physical exam and blood work. Your vet will assess body condition, look for regional fat deposits, and check for signs of current or past laminitis (such as divergent hoof rings or a stretched white line).
The simplest blood test is a resting insulin level, drawn while the horse is on its normal diet (not fasted). A result below 32 mIU/L generally suggests insulin dysregulation is unlikely. A result above 45 mIU/L points strongly toward insulin dysregulation. Values between those two numbers fall into a gray zone that warrants further testing.
The oral sugar test (OST) is a more revealing follow-up. Your vet gives the horse a measured dose of light corn syrup by mouth, roughly 75 mL for an average-sized horse, then draws blood at 60 or 90 minutes to see how insulin responds. A horse with EMS will show an exaggerated insulin spike that a healthy horse would not. This test mimics what happens when the horse eats sugary pasture grass, making it a practical gauge of real-world risk.
For horses over 10, vets often test for pituitary pars intermedia dysfunction (PPID, commonly called Cushing’s disease) at the same time. The two conditions can coexist, and PPID itself can cause or worsen insulin dysregulation.
How EMS Differs From Cushing’s Disease
EMS and PPID are often confused because both involve hormonal imbalance and laminitis risk, but they’re distinct conditions. EMS is a metabolic problem driven by insulin and fat tissue, typically appearing in younger to middle-aged horses. PPID is a neurodegenerative condition of the pituitary gland, most common in horses over 15, and it produces a broader set of symptoms including a long, curly coat that doesn’t shed properly, muscle wasting, and increased drinking and urination. A horse can have both conditions simultaneously, which is why testing for both matters in older horses.
Diet: The Foundation of Management
Dietary changes are the single most important intervention for an EMS horse. The goal is to reduce the sugar and starch load that drives insulin spikes while still meeting the horse’s nutritional needs.
Hay should be the dietary foundation, fed at roughly 1.5 to 2% of the horse’s ideal body weight per day. The key metric is non-structural carbohydrate (NSC) content, which combines sugars and starch. Hay for EMS horses should contain less than 10% NSC. You can have your hay tested through a forage analysis lab, and if the NSC is too high, soaking it in water for 30 to 60 minutes before feeding leaches out a portion of the soluble sugars.
Grain concentrates, sweet feeds, and treats high in sugar or starch should be eliminated. A low-calorie ration balancer can provide vitamins and minerals without excess calories. Pasture access is one of the biggest risk factors for laminitis in EMS horses because grass, especially during spring growth or after a frost, can contain very high sugar levels. Restricting or eliminating turnout on lush pasture, or using a grazing muzzle, significantly reduces the danger.
Exercise and Weight Loss
Exercise directly improves insulin sensitivity in a way that diet alone does not. In a 12-week study of obese horses, animals that combined dietary restriction with five days per week of low-intensity exercise showed significantly greater improvement in insulin sensitivity compared to horses on diet alone. Both groups lost weight and had lower resting insulin levels, but only the exercise group achieved measurably better cellular response to insulin.
The exercise protocol in that study was straightforward: 25 minutes per session consisting of 5 minutes of walking, 15 minutes of brisk trotting, and 5 minutes of walking to cool down. This is manageable for most horse owners using a lunging ring, horse walker, or under saddle. The critical caveat is that horses currently experiencing a laminitis episode cannot be exercised. The hooves need to stabilize first, and your vet should clear the horse before starting any exercise program.
Weight loss itself brings measurable benefits. As horses shed excess fat, their baseline insulin and leptin (a hormone linked to appetite and fat storage) both drop, while adiponectin (a hormone that improves metabolic function) rises. These shifts reduce laminitis risk even before the horse reaches an ideal body condition.
What Long-Term Management Looks Like
EMS is a lifelong condition. Horses don’t outgrow it, and the genetic predisposition never goes away. But with consistent management, many EMS horses live comfortable, sound lives. The practical routine involves maintaining a low-sugar diet year-round, keeping the horse at a healthy body condition (ideally a 5 on the 9-point scale), providing regular exercise, and monitoring insulin levels periodically through blood work.
Seasonal vigilance matters. Spring and fall pasture growth tends to be highest in sugar content, so these are the periods when grazing restrictions need to be tightest. Any sudden change in diet, a new batch of hay, unexpected access to grain, or turning out on rich pasture can trigger a laminitis episode in a horse that had been well-controlled for months.
Medications are sometimes used as part of the management plan when diet and exercise alone aren’t bringing insulin levels down sufficiently. These work alongside lifestyle changes, not as a replacement for them. Your vet can determine whether pharmaceutical support is appropriate based on repeated insulin testing and the horse’s clinical response to dietary management.