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, 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 for free.
Equine Metabolic Syndrome
The clinical signs associated with EMS were previously referred to as peripheral Cushing’s disease, hypothyroidism, prelaminitic syndrome, or Syndrome X. The term EMS was first suggested in research in 2002. 
The predominant feature of EMS is insulin dysregulation (ID), which refers to an imbalance between the levels of insulin, glucose, and fatty acids in blood plasma.
In horses with EMS, cells do not respond to the hormone insulin properly which can result in higher than normal blood sugar levels and increased insulin production.
Horses may have one or more of the following:
- High blood insulin levels (basal hyperinsulinemia)
- Insulin resistance (IR)
- High triglycerides (hypertriglyceridemia)
This often coincides with obesity and increases the risk of debilitating laminitis.
Veterinary examination is necessary to diagnose EMS in horses. Common blood tests used to evaluate horses for the condition include:
- Baseline insulin and glucose (blood sugar)
- Oral Glucose Tolerance Test (OGTT)
- Baseline leptin
Multiple factors including genetics, environment, diet, exercise and the gut microbiome influence the development of EMS.
Research indicates insulin resistance is associated with age.  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.
EMS often occurs in senior horses affected by pituitary pars intermedia dysfunction (PPID) and could be a predisposing factor for the condition. PPID was previously referred to as Equine Cushing’s Disease.
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. 
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 carbohydrates, 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.  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
- Unusual fat deposits
- Loss of muscle mass
- Changes in appetite
- Changes in exercise tolerance
Clinical Signs of EMS
Horses with EMS are affected by multiple physiological abnormalities that can be detected by blood tests and physical examination.
When your veterinarian performs an assessment for EMS, common clinical findings in horses with EMS include:
- Insulin resistance with normal blood glucose concentrations
- High blood insulin levels (hyperinsulinemia)
- Arterial hypertension (high blood pressure)
- Abnormally high levels of fatty acids in the blood
- A high level of leptin (a hormone which is produced by fat cells)
- A low level of adiponectin (a fat-derived hormone that normally improves insulin sensitivity)
- Infertility or altered ovarian activity
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:
Research demonstrates that horses affected by EMS have genes that predispose them to develop the condition.  These genes may have assisted the survival of the ancestors of modern-day horses by promoting increased energy efficiency.
Storing energy reserves in the form of excess fat may have enabled ancestral equines to live in harsh environments. However, this can cause problems for modern horses that are provided with readily available calorie-dense food sources.
When obesity develops in horses with EMS, the amount of fat stored in their liver increases. Excess fat stored in the liver may promote insulin resistance by interfering with the normal signaling activities of the hormone.
The fatty tissues of obese horses with EMS produce high levels of adipokines, a group of hormones produced by fat cells that have a variety of effects on the body, including an increase in inflammation.
A prolonged inflammatory state in the body affects how cells respond to insulin and leads to insulin resistance and high blood glucose and insulin levels.
Elevated insulin levels is a known cause of laminitis in horses and ponies. An abnormal insulin level may alter the function of the epidermal laminar cells of the hooves and its ability to uptake glucose, increasing the risk for horses with EMS to develop laminitis.
The influence of management practices on the development of metabolic issues is greatest during the first decade of a horse’s life. 
Horses that are relatively inactive and fed high-calorie diets have a greater risk of developing EMS.
The gut microbiome is believed to play a role in the development of metabolic disease and obesity in humans and other species.
A small study of EMS horses showed less diversity in their fecal microbiota compared to healthy horses and an increase in biomarkers that are associated with glucose intolerance in humans. 
Endocrine disrupting chemicals (EDCs) have been proposed as a risk factor for the development of EMS in horses.
Data collected from horses living on farms located within 30 miles of EDC disposal sites showed a higher incidence of laminitis and insulin dysregulation. 
How is EMS Diagnosed?
Definitive diagnosis of EMS requires an examination from a veterinarian. Diagnosis of EMS is based on physical examination, diagnostic testing, and an evaluation of the horse’s dietary history. 
Prior to starting a treatment plan, a baseline neck circumference and body condition score should be established for horses suspected of having EMS.
The body condition scoring system assesses horses’ body condition based on the amount of fat that has accumulated in areas of their body including the ribs, shoulder, withers, loin, tailhead, and neck.
Body condition score refers to a numerical value ranging from 1 to 9. A score of 5 is considered the ideal body condition for a mature horse.
Horses suspected of having EMS should have their hooves carefully examined. Signs of laminitis include growth rings in the feet that widen at the heels, increased digital pulse, and warm hooves.
When laminitis is severe enough, horses may be unable to walk or have a shortened stride.
Radiographs are useful for determining if a horse has previously experienced a bout of laminitis.
Lateral images of the pedal bone are recommended to detect changes such as rotation and sinking of the coffin bone, reduced sole depth, and accumulated pockets of gas/fluid in the hoof capsule.
Diagnostic Blood Testing
The most common diagnostic blood tests used to diagnose and monitor EMS and the resulting ID that occurs with the condition are Baseline Insulin and Glucose, the Oral Sugar Test (OST), Oral Glucose Tolerance Test (OGTT), and Baseline Leptin.
High insulin levels are often present in horses with EMS. A blood insulin concentration of over 20 mU/L indicates insulin resistance. 
Insulin testing should be conducted after the horse being evaluated has fasted for six to eight hours.
Testing should be completed in a low-stress environment and not during active laminitis to avoid false results. Insulin is affected by numerous factors including diet, exposure to stress, and pain.
Although one blood draw may be sufficient to determine insulin resistance in a horse suspected of having EMS, additional tests increase diagnostic accuracy. Some horses affected by EMS may have a normal resting insulin level and require additional testing to receive a diagnosis.
Horses with EMS typically present with blood glucose (sugar) concentrations in the normal range or slightly elevated.
PPID or diabetes mellitus may be present if glucose levels are well above the normal range.
Oral Sugar Test (OST)
A more sensitive diagnostic test, the Oral Sugar Test involves feeding a sugary solution such as corn syrup to a horse after a fast of three to twelve hours.
Blood samples are collected after 60 and 90 minutes and are analyzed to determine insulin concentration. An insulin concentration over 60 mU/L is abnormal. 
Oral Glucose Tolerance Test (OGTT)
The Oral Glucose Test (OGTT) assesses blood insulin concentration after the horse has consumed feed with dextrose powder added to it at a rate of 1g/kg.
This test requires horses to be in a fasted state. Blood samples are collected two hours after eating the feed. An insulin concentration over 87 mU/L in the blood sample is abnormal. 
A hormone produced by fat cells, leptin is typically present at a higher concentration in the blood of horses affected by EMS. 
This test is useful for tracking weight loss in horses because reductions in weight result in a lower leptin concentration.
Insulin Tolerance Test
The insulin tolerance test is useful for determining if insulin can stimulate normal glucose uptake by peripheral tissues.
A blood sample is collected before and 30 minutes after administering insulin to the horse. A second blood glucose measurement that fails to decrease to 50% or less than the baseline concentration indicates insulin resistance.
IV Glucose Tolerance Test (IVGTT)
The intravenous glucose tolerance test is the most reliable test available for diagnosing insulin resistance.
This test requires taking a blood sample prior to IV administration of sugar (dextrose) and subsequent collection of additional samples at 1, 15-, 30-, 45-, and 60-minutes post-injection.
Blood samples are analyzed to determine glucose and insulin values. In healthy horses, the concentrations of these substances return to normal quickly. Glucose and insulin levels remain elevated for a longer time in horses with EMS.
Combined Glucose-Insulin Response Test
The combined glucose-insulin response provides information on how quickly insulin is cleared from the bloodstream and how much insulin is produced by the pancreas, a reflection of tissue insulin sensitivity.
This test involves the IV administration of dextrose and insulin and measures the rate of decrease of these substances in the blood.
Testing to Rule Out PPID
It is important to test horses with EMS for PPID, since this condition can worsen insulin resistance. Tests for PPID include measuring ACTH and thyroid releasing hormone concentrations.
Treatment of EMS
The first step in treating Equine Metabolic Syndrome is to make changes to your horse’s feeding program to help them achieve a normal body weight. Dietary management strategies may involve selecting lower energy density forages and removing excess grains and sugars.
In addition, exercise can help horses maintain a healthy body condition and improve insulin sensitivity.
Some horses affected by EMS may require medication to improve their condition if diet and exercise strategies are insufficient.
Overweight horses should be fed carbohydrate-restricted diets to reduce their body weight and improve insulin regulation. Studies show that horses with EMS that lose excess weight see improved insulin response. 
Grain, fruits, vegetables and sugary treats should be removed from the diet because they have high non-structural carbohydrate (NSC) content. High-fat feeds may need to be reduced in the diet due to their high energy content.
Overweight horses with EMS may require restricted or no access to pasture until they achieve a normal body weight. The use of a grazing muzzle can help to reduce intake while still providing turnout.
Forages provided to horses with EMS should be analyzed to determine NSC content. A hay sample report will show values for the percentages of starch and water-soluble carbohydrates in the forage. These values can be added together to arrive at an NSC value.
The NSC value of forage fed to horses with EMS should ideally be less than 10% of dry matter. Soaking hay reduces the concentration of water-soluble carbohydrates, but may also lead to loss of some nutrients. 
The ideal rate of weight loss in obese horses with EMS is between 0.5% and 1.0% of body mass per week.
It is recommended to consult with an equine nutritionist to design a weight loss feeding program for your horse. A nutritionist can help your horse achieve weight management goals while ensuring nutrient requirements for vitamins and minerals are met.
Horses affected by EMS and on restricted diets may require supplemental vitamins and minerals to avoid nutrient deficiencies that result from soaking hay and or reducing intake.
In some cases, additional dietary protein may be required if the forage fed contains an insufficient amount.
Mad Barn’s AminoTrace+ mineral and vitamin supplement is specifically formulated for horses with EMS or inuslin resistance. It is a low-NSC concentrated formula that provides comprehensive coverage of the nutrients that EMS horses require to support healthy metabolic function.
Designing the right exercise program for your horse will depend on body condition, fitness level, and the stability of the hooves if laminitis is present or has occurred in the past.
If diet and exercise do not sufficiently improve the condition of horses with EMS, drug therapies may be considered as an additional strategy. The following medications may be used to treat horses with EMS.
Metformin: Used to treat horses with severe insulin dysregulation, this drug prevents increases in blood glucose and insulin levels post-feeding. 
Levothyroxine: This drug is beneficial in horses that are resistant to weight loss. 
Pergolide: Used in horses with EMS and PPID, this drug helps to minimize the overproduction of endocrine hormones by the pars intermedia.
EMS Prevention & Recovery
Horses being treated for EMS should be assessed regularly by a veterinarian to determine if management strategies are helping. Retesting for insulin dysregulation is also required to monitor improvements in health.
The following strategies are recommend to help your horse recover from EMS and to prevent the recurrence of this condition:
- Provide a forage-based diet with low-NSC content to prevent obesity.
- Adopt a suitable exercise program for sound horses to aid in body condition management.
- Work with an equine nutritionist to create a nutritionally balanced diet with an appropriate energy intake.
- Regularly check the hooves of your horse to detect early signs of laminitis such as an increased digital pulse.
- Reduce or eliminate access to lush pastures that are high in sugars.
- Use a grazing muzzle to reduce grass consumption. Use a hay net when providing free choice hay.
- Keep affected horses in a dry lot to permit exercise while preventing grass consumption.
- Soak hay for 15 to 30 minutes to reduce the amount of soluble carbohydrates.
For a free diet plan to prevent EMS and/or support improved insulin sensitivity, submit your horse’s diet for evaluation by our equine nutritionists.
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- Asplin, KE. et al. Induction of laminitis by prolonged hyperinsulinaemia in clinically normal ponies. The Veterinary Journal.
- McCue, M. et al. Equine metabolic syndrome: a complex disease influenced by genetics and the environment. J Equine Vet Sci. 2015.
- Vick, MM. et al. Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse. J Anim Sci. 2007.
- Frank, N. Equine metabolic syndrome. Vet Clin North Am Equine Pract. 2011.
- Morgan, R. et al. Equine metabolic syndrome. Vet Rec. 2015.
- Morgan, RA. et al. Prevalence and risk factors for hyperinsulinaemia in ponies in Queensland, Australia. Aust Vet J. 2014.
- Johnson, PJ. The equine metabolic syndrome peripheral Cushing’s syndrome Vet Clin North Am Equine Pract.
- Frank N. et al. Insulin dysregulation. Equine Vet J. 2014.
- Lewis, SL. et al. Genomewide association study reveals a risk locus for equine metabolic syndrome in the Arabian horse. J Anim Sci. 2017.
- Elzinga, SE. et al. Comparison of the fecal microbiota in horses with equine metabolic syndrome and metabolically normal controls fed a similar all-forage diet. J Equine Vet Sci. 2016.
- Durward-Akhurst, SA. et al. The association between endocrine disrupting chemicals and equine metabolic syndrome. Dorothy Russell Havemeyer Foundation Equine Endocrinology Symposium. 2017.
- Frank, N. et al. Current best practice in clinical management of equine endocrine patients. Equine Vet Educ. 2014.
- Pleasant, RS. et al. Adiposity, plasma insulin, leptin, lipids, and oxidative stress in mature light breed horses. J Vet Intern Med. 2013.
- Delarocque, J. et al. Weight loss is linearly associated with a reduction of the insulin response to an oral glucose test in Icelandic horses. BMC Veterinary Research. 2020.
- Vervuert, I. et al. Effects of chromium yeast supplementation on postprandial glycaemic and insulinaemic responses in insulin-resistant ponies and horses. 2010.
- Respondek, F. et al. Dietary supplementation with short-chain fructo-oligosaccharides improves insulin sensitivity in obese horses. J Anim Sci. 2011.
- Winter, JC. et al. Oral supplementation of magnesium aspartate hydrochloride in horses with equine metabolic syndrome. 2016.
- Morgan, RA. et al. Treatment of Equine Metabolic Syndrome: a clinical case series. Equine Veterinary Journal.
- Menzies-Gow, NJ. Et al. The effect of exercise on plasma concentrations of inflammatory markers in normal and previously laminitic ponies. Equine Vet J. 2014.
- Bonelli, F. et al. How swimming affects plasma insulin and glucose concentration in Thoroughbreds: a pilot study. Vet J. 2017.
- Bochnia, M. et al. Effect of Hay Soaking Duration on Metabolizable Energy, Total and Prececal Digestible Crude Protein and Amino Acids, Non-Starch Carbohydrates, Macronutrients and Trace Elements. J Equine Vet Sci. 2021.