Equine Metabolic Syndrome

Recognizing if a horse has a weight problem is the first step in diagnosing Equine Metabolic Syndrome, this article goes into further detail.

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By: Dr. Joan Norton |

Equine Metabolic Syndrome (EMS) is a new term used to describe an old medical condition. We used to refer to these animals as “easy keepers,” the pony or horse that no matter how little you fed them were obese, with a cresty neck and fat around the tail head. Next we called these horses hypothyroid, but for reasons that will be mentioned later, this is not an accurate diagnosis. We now know that EMS is a cluster of metabolic/endocrine conditions that includes obesity, abnormal fat deposition, and on a cellular level, insulin resistance and hyperinsulinemia (too much insulin in the blood stream).

Determining how the syndrome develops is a bit like the chicken and the egg conundrum. Does obesity cause insulin resistance and hyperinsulinemia? Or does insulin resistance cause an animal to become obese? The answer is yes, to both.


There is certainly a genetic component that predisposes some horses to develop insulin resistance making it very easy for these animals to become obese once they reach maturity. These breeds may have a defective gene that alters the insulin receptor function, though no such genetic flaw has been identified. Other horses become obese because of our tendency to underestimate how much a horse eats and overestimate how much exercise they are getting. These horses develop insulin resistance because the excess fat stored in cells will stretch the membrane, altering the way glucose transporters can function.

There is also a growing amount of evidence from human medicine that obesity leads to a constant state of inflammation in the body. This inflammatory process dampens the ability of insulin to act on the cells, compounding the problem.

But why worry about the excess fat on your horse or pony? Isn’t that just more of them to love?

Obesity in horses, combined with insulin resistance and the resulting hyperinsulinemia can cause several serious health problems. It has been shown that overweight mares have lower fertility rates and shorter breeding seasons than normal weight mares, and by reducing body condition score in these horses you can significantly improve fertility.

Horses which have been obese for years are at an increased risk for developing lipomas, fatty tumors that form in the connective tissue around the intestines. These tumors can act like a bola (rock on a rope) and ensnare the nearby intestines, cutting off its blood supply. This type of colic always requires surgery to correct and the complication rates are very high.

Additionally, researchers are now investigating a link between Equine Metabolic Syndrome and early-onset of Cushing’s Disease. Pars Pituitary Intermedia Dysfunction, or Cushing’s Disease, is an age related disease in horses that is characterized by long curly haircoat, abnormal fat distribution, decreased immunity and episodes of laminitis. Typically seen in geriatric horses greater than 20 years of age, Cushing’s Disease has been diagnosed in obese horses as young as eight years old. The cause may be the chronic inflammatory state of obesity, accelerating the neuronal degeneration seen in the brain with Cushing’s Disease.

By far the most common and most severe complication that is associated with Equine Metabolic Syndrome is laminitis. This painful condition where inflammation in the connective tissue in the hoof causes separation between the coffin bone and the hard hoof wall is difficult to manage, and in many cases leads to euthanasia of the animal.

The exact cause of laminitis in these cases is not completely elucidated, but some research shows the constant high levels of insulin in the bloodstream is to blame. Insulin regulates blood flow by acting on the vessels that may compromise delivery of nutrients and oxygen to the hoof. The chronic inflammation seen with obesity may also play a role in how the hoof and the coffin bone’s attachments are damaged.

Episodes of laminitis in Equine Metabolic Syndrome cases are often directly linked to a recent intake of a high sugar/starch meal, such as lush green spring pasture or too much sweet feed in an unlocked feed room. This high spike in insulin lowers the horse’s natural threshold for laminitis, sparking a painful episode. However it is not only one-time sugary indulgences that can lead to laminitis. Chronically providing too much sugar in your horse’s diet can have the same effect. If with each meal there is too much sugar/starch, causing repeated spikes in insulin levels, the cumulative result will be inflammation in the colon and a higher propensity for laminitis. All horses can be affected by this sugar and insulin cycle but horses with EMS are much more sensitive to slight fluctuations in the sugars in their meals.


Recognizing your horse has a weight problem is the first step in diagnosing Equine Metabolic Syndrome. Excessive and abnormal fat depositions are, after all, one of the major signs of the syndrome. Fat is stored in the crest of the neck, around the tail head, behind the shoulders and even in the depressions above the eyes. Getting an accurate weight on a horse is difficult but important in monitoring your horse’s health. Unless you live near a hospital or clinic with a walk-on scale, you are limited to using a weight tape, and unfortunately those are not very accurate. A better technique is to measure your horse around the girth and from shoulder to rump in inches. Multiply the Girth x Girth x Length and divide by 330.

The most basic blood test for Equine Metabolic Syndrome is resting insulin levels. If a horse or pony has insulin levels greater than 20µU/ml, this is suggestive of insulin resistance and EMS. For the testing to be accurate the horse must be held off feed and pasture for 12-18 hours before testing. The stress of shipping can artificially increase insulin levels, so it is best to perform the testing on the farm, or have your horse spend the night before testing at the clinic. Pain and other illnesses will also increase insulin, therefore horses who are having a current episode of laminitis or any other illness should be allowed to recover before testing is done.


More advanced testing may be required for subtle cases of Equine Metabolic Syndrome. The combined glucose-insulin test (CGIT) involves administering an intravenous injection of dextrose (sugar) followed by an injection of insulin. The blood glucose and insulin levels are then monitored for 45-60min. This test carries the mild risk of hypoglycemia and many clinicians prefer to perform it using an intravenous catheter and in the safety of the clinic.

Another provocative test to evaluate the function of the insulin receptors is the oral sugar test (OST). With this test not only is the glucose-insulin pathway evaluated at the cellular level, but also the role of the GI tract in carbohydrate uptake is included. Horses are administered an oral dose of Karo syrup and insulin levels are monitored 60 and 90 minutes later. An exaggerated insulin response to the prescribed amount of glucose indicated insulin resistance. This test is now thought to be the best test available as it includes both the GI tract and cellular function.


The mainstay of Equine Metabolic Syndrome treatment is proper diet and exercise. Correcting the obesity will improve insulin sensitivity and lower the risk of laminitis. Exercise can be challenging for these horses especially if they are laminitic. You should consult with your veterinarian as to whether your horse can be exercised and then begin a slowly increasing exercise regime that suits your horse.

Access to green pasture should be limited or eliminated if possible. If a dry lot is not available, limit their access to pasture by confining them to a smaller area, give them a companion to encourage activity and reduce grazing and implement a grazing muzzle. It’s also important to schedule their access to pasture when the sugar content in the grass is low. On a daily basis, grass is the most sugary during the day when it is exposed to sun so horses should be allowed to graze early in the morning and late at night (or in the shade). Over the course of the year, sugar content in the grass is highest in the spring and the fall.

Horses on a weight loss program should be fed 1.5-2% of their body weight in hay (15-20lbs of hay a day for a 1000lbs horse). The amount of hay fed should be slowly reduced until this goal is met. Grain should be limited but does not necessarily have to be eliminated. Most feed companies produce a pelleted low-sugar/starch complete feed that provides the needed vitamins and minerals in a low calorie, low sugar formulation. If grain is removed altogether then a feed supplement should be added to ensure all the proper nutrients are being supplied.

Horses that do not lose weight despite appropriate diet and exercise may benefit from treatment with levothyroxine sodium (Thyro-LÒ). It is important to know that this is not used to treat a thyroid dysfunction. This is a common misconception amongst horse owners and veterinarians alike. True hypothyroidism in horses is extremely rare and difficult to document. Routine thyroid levels are not enough to prove thyroid dysfunction. Thyroid supplementation causes an increase in metabolism. By increasing the horse’s metabolism they are able to lose weight and correct their insulin resistance. Researchers are now recommending treating these types of horses with a much higher dose of thyroid supplementation, a “supraphysiologic dose” 4-times higher than what is recommended on the label. With this higher dose long-term use is discouraged. Once the horse has lost the weight and corrected the insulin resistance this new health must be maintained with diet and exercise alone.

Monitoring includes weighing the horse on a regular basis. Blood testing can be repeated as often as every 3-6 months to monitor insulin resistance and again once the problem is corrected any weight loss medications should be stopped. This journey may take many months, as there is no magic bullet. However, correcting your horse’s weight problem and resolving the cellular dysfunction will lead to a longer and healthier life for your horse.