Equine metabolic syndrome (EMS) is a condition that affects how horses metabolize sugars from the diet and use them for energy.

It involves an inability to properly respond to the hormone insulin, similar to Type 2 diabetes in humans. In addition to insulin resistance, EMS also results in increased obesity, abnormal fat deposits (such as cresty neck), and a higher risk of laminitis.

Metabolic syndrome commonly affects ponies and horses referred to as “easy keepers“. But not all horses that are obese have EMS, which is why testing is important to diagnose this disorder.

The treatment for EMS includes reducing starches and sugars in the diet, decreasing caloric intake to promote weight loss, safely increasing exercise and, in some cases, using medication. Regular veterinary checks are important for preventing and treating EMS.

If your horse has been diagnosed with EMS or you are looking for strategies to prevent EMS, contact our equine nutritionists to formulate an appropriate feeding plan.

Equine Metabolic Syndrome

Equine metabolic syndrome (EMS) is a collection of risk factors related to metabolic dysfunction. [1][2][3]

The clinical signs associated with EMS were previously referred to as peripheral Cushing’s disease, hypothyroidism, prelaminitic syndrome, or Syndrome X. The term Equine Metabolic Syndrome was first suggested in research in 2002. [4]

The predominant feature of EMS is hyperinsulinemia, which refers to abnormally high levels of insulin in the blood.

In horses with EMS, cells do not respond to the hormone insulin properly which results in higher than normal insulin required to bring glucose into cells. Blood sugar levels remain normal in most cases.

Horses may have one or more of the following:

  • High insulin levels (hyperinsulinemia)
  • Insulin resistance (IR)
  • High triglyceride levels (hyperlipidemia)
  • Fatty neck crest
  • Other abnormal fat deposits, including above the eyes, at the tail base, on the withers or anywhere on the body
  • Abnormal estrus cycles, sometimes with pain and laminitis flare-ups

This often coincides with obesity and increases the risk of debilitating laminitis.

Additionally, horses with IR and EMS may be more sensitive to steroid administration (e.g. joint injections, steroids given for skin or respiratory allergies) and could be inadvertently pushed into a laminitic episode in situations of otherwise routine steroid use. [5]

Veterinary examination is necessary to diagnose EMS in horses. Common diagnostic tests used to evaluate horses for the condition include:

  • Baseline insulin, glucose, and leptin or adiponectin
  • ACTH to rule out PPID / Cushing’s disease as the cause of hyperinsulinemia
  • Oral Sugar Test (OST) or Oral Glucose Tolerance Test (OGTT)
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Risk Factors

Multiple factors including genetics, environment, diet, exercise, sex and possibly the gut microbiome influence the development of EMS.

Genetics appears to play a strong role in the risk of EMS. A fat pony has always been the poster child for pasture laminitis. In 2006, Treiber et al. were the first to examine the genetics of EMS in a year-long study of ponies at pasture. [23] In comparison, modern breeds such as Thoroughbreds, Standardbreds, Quarter horses, full Drafts and Warmbloods are unlikely to develop EMS unless they have PPID.

Research indicates insulin resistance is associated with age. [6] Insulin concentrations are significantly higher in older horses and ponies compared to those that are younger. Horses between the ages of 5 and 16 are most likely to be affected by EMS. However, signs of EMS have emerged in horses as young as 3 years old, when the period of rapid growth has passed.

Other endocrine diseases, particularly pituitary pars intermedia dysfunction (PPID), can increase the risk of developing concurrent EMS. PPID was formally referred to as Equine Cushing’s Disease. PPID occurs in horses 15 years and older (although it has been diagnosed in horses as young as 7), and becomes more likely to develop as the horse ages.

Younger horses with EMS typically have a normal pituitary gland, whereas older horses with EMS and PPID are likely to have lesions on their pituitary gland. [7]

EMS is far less likely to occur in stallions than geldings or mares, unless the horse has PPID. Dr. Kellon of the Equine Cushing’s and Insulin Resistance Group reports than in over 2 decades of following thousands of metabolic horses there has never been an incident of EMS in a stallion that did not have PPID.


Over 90% of horses presenting with laminitis as their predominant clinical sign have EMS. [8]

Horses with EMS often have an obese body condition, although the condition can also occur in thin horses. Thin horses with EMS typically exhibit abnormal fatty deposits on various regions of their body, such as their neck and tailhead regions.

Objectively monitoring your horse’s body condition over time is critical for assessing their risk of developing EMS.

EMS occurs in donkeys, ponies, and equine breeds such as Saddlebred, Tennessee Walking, Paso Fino, Morgan, Mustang, and Quarter horses. The condition is less common in Thoroughbreds and Standardbreds.

Gender does not predispose horses to have EMS.

Insulin Resistance – A Key Feature of EMS

Most horses diagnosed with EMS are insulin resistant, which means they are unable to properly metabolize carbohydrates in the diet.

This puts them at risk of laminitis when fed high-starch or sugar-rich feeds or when given access to lush pasture.

Insulin is a hormone produced by the pancreas in response to high levels of sugar in the blood. Insulin enables the body to use sugar for energy by helping your muscle and fat cells take up glucose from the blood.

When a horse ingests starch, it gets broken down into glucose in the stomach and absorbed into the blood. Glucose subsequently stimulates the pancreas to release insulin.

Insulin has many roles in the body, including enabling the movement of glucose into muscle and adipose (fat) cells of the body. When insulin binds its receptor on these cells, it makes channels available to move glucose out of the blood and into the cell.

Insulin resistance refers to a condition in which normal insulin concentrations fail to stimulate the tissues to uptake glucose.

When cells become resistant to insulin, the pancreas responds by releasing more of the hormone to prevent blood glucose levels from becoming too high.

This leads to high blood levels of insulin (hyperinsulinemia) in horses with metabolic syndrome.

Signs and Symptoms of EMS

Horses affected by EMS are typically overweight with increased fat deposition on the neck, over the ribs and topline, above the eyes, and at the base of the tail. [9] Their body condition score typically ranges between six to nine on a nine-point scale.

Geldings with EMS often have increased fat deposition in their sheath, whereas mares with the condition may have fatty deposits around the mammary gland. Affected horses often have a history of gaining weight easily after reaching adulthood.

Laminitis commonly occurs in horses with EMS following the ingestion of feed sources that are high in soluble carbohydrates. Bouts of laminitis are most likely to occur in horses with EMS after consuming lush pasture or high carbohydrate feeds.

Signs of EMS-related laminitis in the hooves include abnormal hoof growth rings, radiographic evidence of rotation in the coffin bone, and demineralization in the coffin bone (pedal osteitis).

Horses with this condition may also show symptoms related to high levels of circulating glucose and insulin resistance including:

  • Excessive drinking and urination
  • Loss of muscle mass
  • Changes in appetite
  • Changes in exercise tolerance
  • Infertility or abnormal ovarian activity

Supporting findings on bloodwork submitted by your veterinarian may include:

  • High blood insulin levels (hyperinsulinemia)
  • High fatty acids in the blood (hyperlipidemia)
  • High leptin (a hormone produced by adipose or fat cells)
  • Blood glucose levels may be normal or high (hyperglycemia)

Causes of EMS

The exact cause of metabolic syndrome in horses is unknown. However, the development of the condition is believed to be influenced by multiple factors including: