Equine Metabolic Syndrome: The Hidden Cause of Laminitis

The Morgan gelding was easy to keep. Too easy. He held weight on air, developed a thick, hard crest that didn't wobble when he trotted, and one spring morning after a night of unrestricted grazing, he couldn't walk. Bilateral forelimb laminitis. The owner was bewildered. No grain overload. No black walnut shavings. No retained placenta. Just grass. But the real problem had been building for years inside the horse's own bloodstream: runaway insulin levels that had been quietly priming his laminar tissue for catastrophic failure. Equine Metabolic Syndrome isn't dramatic until it is, and by the time the horse is standing in the classic laminitic stance, rocked back on the heels with a pounding digital pulse, the damage is already done.

What Equine Metabolic Syndrome Actually Is

EMS is not a single disease with a single cause. It's a clinical phenotype, a cluster of metabolic abnormalities that tend to occur together. The core features are insulin dysregulation, regional or generalized adiposity, and a predisposition to laminitis. Think of it as the equine parallel to metabolic syndrome in humans, where obesity, insulin resistance, and cardiovascular risk cluster together.

The central problem is insulin dysregulation (ID), which encompasses several related abnormalities. Some EMS horses have elevated resting (basal) insulin levels. Others have normal resting insulin but produce a wildly exaggerated insulin spike in response to dietary carbohydrates. Still others have impaired insulin sensitivity at the tissue level, meaning their cells don't respond to insulin normally, forcing the pancreas to produce more and more to achieve the same glucose uptake. Many EMS horses have all three problems simultaneously.

The terminology has evolved over the years. Older literature uses "insulin resistance" as the umbrella term, but current research favors "insulin dysregulation" because it captures the full spectrum of abnormalities, including horses with excessive insulin secretion that aren't classically insulin resistant at the receptor level.

How Hyperinsulinemia Causes Laminitis

This is the piece that changed everything in our understanding of endocrine laminitis. For years, researchers assumed that the laminitis in EMS horses was caused by vascular dysfunction, reduced blood flow to the laminae triggered by metabolic derangement. That theory has largely been replaced.

Experimental work, most notably by Chris Pollitt's group at the University of Queensland and by researchers at the University of Tennessee, demonstrated that sustained hyperinsulinemia alone, without any change in glucose levels, directly causes laminitis in healthy horses. When researchers infused insulin intravenously to maintain supra-physiologic blood insulin concentrations while clamping glucose at normal levels, the horses developed laminitis within 48 to 72 hours. The insulin itself was the toxin.

The mechanism appears to involve insulin's effects on lamellar epithelial cells. At high concentrations, insulin activates insulin-like growth factor-1 (IGF-1) receptors on lamellar cells, disrupting their normal proliferation and differentiation. The lamellar basal epithelial cells essentially lose their structural organization. Cell stretching, abnormal mitosis, and eventually mechanical failure of the dermal-epidermal bond follow. The coffin bone loses its suspension within the hoof capsule.

This is why spring grass is so dangerous for EMS horses. Rapidly growing pasture can contain 15% to 30% nonstructural carbohydrates (NSC) by dry weight. An EMS horse grazing freely on this pasture absorbs a massive sugar load, triggers a disproportionate insulin spike that may stay elevated for hours, and the laminae suffer cumulative damage. One bad grazing episode can push an already-compromised horse over the edge into clinical laminitis.

The EMS Phenotype: What These Horses Look Like

Not every overweight horse has EMS, and not every EMS horse is obviously obese. But regional adiposity is the hallmark physical finding. The cresty neck score (CNS), developed by Carter and colleagues, grades neck crest thickness on a 0 to 5 scale. EMS horses typically score 3 or above: the crest is thickened, hard, and doesn't easily fall to one side. At a score of 4 or 5, the crest is so enlarged it may develop a permanent lateral fall.

Fat deposits also accumulate over the tailhead, behind the shoulder, in the sheath or mammary region, and as retroperitoneal fat pads. The distribution matters. Generalized obesity (a horse that's simply fat all over from overfeeding and underexercising) doesn't carry the same metabolic risk as regionalized adiposity. The cresty neck in particular correlates strongly with insulin dysregulation, more so than overall body condition score.

Certain breeds are dramatically overrepresented. Ponies of all types, including Welsh, Shetland, and mixed-breed ponies, top the list. Morgans, Paso Finos, Arabian crosses, Tennessee Walking Horses, and many draft crosses carry higher risk. Thoroughbreds and Standardbreds appear to be relatively resistant, though individual exceptions exist. The genetic component is strong enough that breed alone should raise the index of suspicion in an easy keeper with a thick neck.

Diagnosing EMS

A clinical suspicion of EMS based on phenotype is a starting point, but confirmation requires laboratory testing. The challenge is that a single resting insulin measurement can be misleading. Stress, recent feeding, and sample handling all affect results. A horse tested after an overnight fast may have a normal resting insulin despite having profound postprandial hyperinsulinemia.

Resting insulin: A baseline blood sample taken after a period of fasting (ideally 6 to 8 hours on hay only, no grain or pasture) can identify horses with elevated basal insulin. Values above 20 to 40 microIU/mL (depending on the laboratory and assay) are considered abnormal. But many EMS horses have resting insulin in the normal range.

Oral sugar test (OST): This is the most practical dynamic test for general practice. The horse receives a measured dose of corn syrup (typically 0.15 mL/kg body weight of Karo syrup) orally, and blood insulin is measured 60 to 90 minutes later. An insulin level above 60 microIU/mL at the 60-minute mark is considered abnormal. The OST evaluates the horse's insulin response to an oral sugar challenge and catches the horses that have normal resting insulin but exaggerated postprandial responses.

Insulin-modified frequently sampled intravenous glucose tolerance test (FSIGTT): The gold standard for research but impractical in clinical settings. It requires an intravenous glucose bolus followed by an insulin bolus with serial blood sampling over three hours and mathematical modeling of the results.

It's also critical to test for pituitary pars intermedia dysfunction (PPID, or Cushing's disease) in any horse being evaluated for EMS, particularly if the horse is over 10 years old. PPID and EMS frequently coexist, and PPID can exacerbate insulin dysregulation. An ACTH level should be part of the workup. Seasonal variation in ACTH must be accounted for, as levels naturally rise in the fall.

Dietary Management: The Foundation of Everything

Diet is the single most important intervention for EMS horses, and it's non-negotiable. Without dietary control, no medication or exercise program will adequately manage the disease.

Forage: Hay should be the dietary staple, and it needs to be low in nonstructural carbohydrates. The target is hay with less than 10% NSC on a dry matter basis. The only way to know your hay's NSC content is to have it tested through an equine forage laboratory. Visual inspection and "type" of hay (timothy vs. orchard grass vs. bermuda) don't reliably predict NSC content, which varies enormously with harvest conditions, maturity at cutting, and storage.

If your hay tests above 10% NSC, soaking it in cold water for 30 to 60 minutes before feeding leaches out a portion of the water-soluble carbohydrates. Studies show that soaking reduces WSC content by 20% to 40% on average, though the reduction is variable. Drain the water before feeding, as it contains the extracted sugars. Hot water soaking is more effective but produces rapid fermentation and spoilage, so cold water is standard practice.

No grain. Period. Cereal grains (oats, corn, barley) and sweet feeds are concentrated sugar and starch sources that provoke exactly the insulin spikes you're trying to avoid. Commercial "low-starch" feeds can be used if the horse needs supplemental calories, but only products with guaranteed NSC values below 10%. Most EMS horses don't need supplemental calories at all. They need fewer calories.

Pasture restriction: This is the hardest part for many owners. Lush pasture, particularly in spring and fall when grass NSC peaks, is the most common trigger for laminitis in EMS horses. Options include dry lot turnout (no grass), grazing muzzles (which reduce intake by approximately 30% to 80% depending on the muzzle and the horse's determination), and limited grazing during early morning hours when grass NSC tends to be lowest (before photosynthesis increases sugar production through the day).

For a broader look at equine nutritional requirements, our nutrition guide covers the fundamentals.

Exercise Protocols

Exercise improves insulin sensitivity through mechanisms that are independent of weight loss. Skeletal muscle contraction increases glucose uptake through GLUT4 transporter translocation, bypassing the insulin signaling pathway. Regular exercise essentially creates an alternative route for glucose clearance that doesn't require insulin.

The practical challenge is that many EMS horses are presented after a laminitis episode, and you can't exercise a laminitic horse. The acute phase requires stall rest, pain management, and hoof support. Exercise enters the picture only after the laminitis has resolved and the horse is comfortable.

For non-laminitic EMS horses, the goal is consistent, moderate exercise at least 5 days per week. Thirty to forty-five minutes of trotting work, whether ridden, lunged, or driven, provides measurable improvement in insulin sensitivity. The effect is transient, lasting roughly 24 to 48 hours after each session, which is why consistency matters more than intensity. A horse exercised three times a week gets about half the metabolic benefit of one exercised six times a week.

Ground work, hand walking, and ponying from another horse are options for horses that aren't under saddle. Even daily turnout in a large space where the horse moves consistently is better than standing in a stall or small paddock, though it doesn't replace structured exercise for metabolic improvement.

Pharmaceutical Options

Medications play a supporting role in EMS management but don't replace diet and exercise. Two drugs see the most clinical use.

Levothyroxine sodium: Oral thyroid hormone supplementation at supra-physiologic doses (0.1 mg/kg/day) promotes weight loss and improves insulin sensitivity. The horse is not hypothyroid; the levothyroxine is being used pharmacologically to increase metabolic rate. Most clinicians use it as a short-term tool (3 to 6 months) to help an obese EMS horse lose weight when diet and exercise alone are insufficient. Long-term use is generally unnecessary once target weight is achieved. Thyroid function returns to normal after discontinuation.

Metformin: Borrowed from human type 2 diabetes treatment, metformin has a complicated history in equine medicine. Early enthusiasm was tempered by pharmacokinetic studies showing very poor oral bioavailability in horses (around 7%). Blood levels achieved after oral dosing are well below the concentrations needed for the insulin-sensitizing effects seen in humans. However, some researchers have proposed that metformin may work in horses by reducing intestinal glucose absorption rather than through systemic insulin sensitization. The clinical evidence is mixed. Some horses appear to improve on metformin; controlled studies have been largely disappointing. Most equine internists consider it a second-line option to be tried when other measures are insufficient.

SGLT2 inhibitors: Newer to equine medicine, sodium-glucose cotransporter-2 inhibitors (such as ertugliflozin and canagliflozin) promote renal glucose excretion, lowering blood glucose and consequently insulin levels. Early clinical use has shown promising results, particularly for acute management of severe hyperinsulinemia. These drugs are still being studied and are used off-label.

The EMS-PPID Connection

EMS and PPID are distinct conditions with different underlying pathologies, but they overlap in frustrating ways. EMS is primarily a peripheral metabolic disorder driven by adipose tissue dysfunction and genetic predisposition. PPID is a neurodegenerative disease of the pituitary gland, caused by loss of dopaminergic neurons in the hypothalamus. But PPID causes its own form of insulin dysregulation through cortisol and other POMC-derived peptide effects, and many older horses have both conditions simultaneously.

A horse that was well-managed for EMS through its teens may develop worsening insulin dysregulation as PPID emerges. If the PPID component isn't identified and treated (with pergolide), dietary and exercise management of EMS becomes increasingly ineffective. This is why ACTH testing should be repeated annually in EMS horses over the age of 10.

Visit our interactive models to examine the hoof structures affected by laminitis and understand how the laminae support the coffin bone within the hoof capsule.

Frequently Asked Questions

Can a thin horse have EMS?

It's uncommon but not impossible. Most EMS horses display obesity or regional adiposity. However, insulin dysregulation can exist in horses that are not overtly overweight, particularly in certain predisposed breeds. A lean horse with unexplained laminitis should still be tested for insulin dysregulation.

Is EMS curable?

EMS is manageable but not curable. The underlying genetic predisposition to insulin dysregulation doesn't change. With appropriate diet, exercise, and monitoring, most EMS horses can avoid laminitis episodes and live comfortable, functional lives. But the management is lifelong. Complacency during a "good phase" almost always leads to relapse.

Can I feed my EMS horse any treats?

Traditional treats like carrots, apples, sugar cubes, and commercial horse treats are high in sugar and should be avoided or given in very small quantities. A single carrot won't trigger laminitis, but a bag of carrots daily adds up. Hay cubes, a handful of unsweetened beet pulp, or celery make safer alternatives.

How do I know if my hay is safe for an EMS horse?

Test it. Submit a sample to an equine forage testing laboratory (Equi-Analytical or Dairy One are commonly used in the US) and request an NSC analysis. Look for combined ethanol-soluble carbohydrates (ESC) and starch below 10% on a dry matter basis. If testing isn't an option, soaking hay for 30 to 60 minutes reduces sugar content as a precaution.

At what age does EMS typically appear?

EMS most commonly presents clinically in horses between 5 and 15 years of age, though the metabolic tendencies are likely present from a young age in predisposed individuals. Horses that are "easy keepers" from youth in predisposed breeds should be monitored proactively rather than waiting for a laminitis episode to prompt testing.

Sources

• Frank, N. et al. (2010). "Equine Metabolic Syndrome." Journal of Veterinary Internal Medicine, 24(3), 467-475.
• Asplin, K.E. et al. (2007). "Induction of laminitis by prolonged hyperinsulinaemia in clinically normal ponies." The Veterinary Journal, 174(3), 530-535.
• de Laat, M.A. et al. (2010). "Equine laminitis: induced by 48 h hyperinsulinaemia in Standardbred horses." Equine Veterinary Journal, 42(2), 129-135.
• Durham, A.E. et al. (2019). "ECEIM consensus statement on equine metabolic syndrome." Journal of Veterinary Internal Medicine, 33(2), 335-349.
• Carter, R.A. et al. (2009). "Apparent adiposity assessed by standardised scoring systems and morphometric measurements in horses and ponies." The Veterinary Journal, 179(2), 204-210.
• American Association of Equine Practitioners (AAEP). Endocrine Disease Guidelines.