Insulin resistance (IR) in horses describes a condition in which the body fails to properly respond to the hormone insulin.

Impaired insulin response is the main feature of Equine Metabolic Syndrome (EMS). Horses with EMS do not metabolize sugars properly and have difficulty losing weight.

This form of metabolic dysfunction is associated with some telltale signs in horses including generalized obesity or regional adiposity, including along the neck (“cresty neck”). Other signs include subclinical or overt laminitis, increased appetite, reproductive issues and poor performance.

EMS and insulin resistance are treated with dietary intervention, exercise, and sometimes require medication. If your horse shows signs of being insulin resistant, consult with your veterinarian to obtain a diagnosis and work with a nutritionist to formulate an appropriate diet.

Overview of Insulin Resistance

Insulin resistance is diagnosed in horses with an elevated blood insulin level (hyperinsulinemia). Insulin is a hormone produced by the pancreas in response to high blood sugar levels (hyperglycemia). [1]

When a horse consumes carbohydrate-containing foods, the stomach breaks the food into components including glucose (sugar) so it can be absorbed into the blood. High levels of glucose in the blood subsequently stimulate the pancreas to release insulin.

Insulin is essential for regulating blood sugar metabolism because it facilitates glucose uptake from the bloodstream into certain tissues, including muscle, liver and adipose tissue (fat).

In healthy tissues, insulin binds to its receptor on the cell membrane and signals cells to take in glucose. In insulin resistant tissues, signaling is less effective after insulin binds to the receptor.

Failure to respond to insulin means that glucose is not adequately transported from the bloodstream into cells, resulting in high blood glucose levels. The body overcompensates by producing more insulin to promote the uptake of glucose. [1] This causes elevated insulin levels which can be picked up in a diagnostic test.

Hyperinsulinemia is an integral part of Equine Metabolic Syndrome (EMS) and the direct cause of damage to the laminae of the hoof in horses with laminitis.

The vast majority of horses with metabolic syndrome are able to maintain blood glucose in a normal zone when fed the correct diet and, if needed, medication. Animals that are sound and can be exercised do best. [2][3] If persistent glucose elevations do develop (diabetes), the only consequence that has been appreciated to date is weight loss. [4]

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8 Signs of Insulin Resistance in Horses

Not all horses with IR will show the same clinical signs or symptoms and some horses may give no outward signs that they are insulin resistant.

However, common indications of impaired insulin sensitivity include:

1) Obesity

While some IR horses are not overweight, many are overweight or obese and have a body condition score ranging from six to nine on a nine-point scale. [5][6]

While it is widely believed obesity predisposes to metabolic syndrome in people, the same is not the case with horses. Lindase et al induced obesity by feeding fat in a cohort of horses that already had high insulin. The diet resulted in a 10% increase in weight but no change in insulin. [7]

Bamford et al similarly induced obesity with a high fat diet and in one group also fed a high carbohydrate meal daily. Again, the weight gain failed to induce insulin resistance. In fact, the addition of a high carbohydrate meal improved insulin sensitivity. [8]

Obese horses may have difficulty losing weight because of leptin resistance. Leptin is a hormone produced by adipose (fat) cells and acts on the brain to control appetite and hunger. Leptin increases as fat mass increases and reduces appetite by acting on the hypothalamus. In other species, leptin resistance is a known contributing factor to insulin resistance and obesity. [9]

Although leptin resistance has not been directly demonstrated in horses, leptin and insulin levels are higher in obese horses compared to those with optimal body condition. [10][11]

2) Laminitis

Elevated insulin levels in metabolic syndrome are associated with the development of endocrinopathic laminitis in horses and ponies. [1] An abnormal insulin level may increase the risk of laminitis by altering the function of the epidermal laminar cells of the hooves.

Laminitis is a painful condition that involves damage to the hoof laminae, the internal structures that attach the wall of the hoof to the coffin bone. Despite the “itis” in its name, endocrinopathic laminitis does not involve inflammation as a cause.

Laminitis can result in lameness and downward rotation of the coffin bone (founder). In very severe cases, laminitis can cause separation between the wall of the hoof and the coffin bone. The prognosis for recovery is poor in horses with advanced cases except with intensive nursing for a prolonged period of time.

Although the exact mechanisms of how hyperinsulinemia promotes laminitis are still being researched, vasoconstriction, and endothelial damage are proposed disease pathways.

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. [12]

3) Excessive Drinking and Urinating

Healthy, non-working, non-breeding adult horses typically drink 60 mL/kg of body weight of water each day. [13] For a 500 kg (1100 lb) horse, this is approximately 30 litres per day.

Horses with high glucose levels urinate more frequently to eliminate some of the excess glucose via urine. This increases thirst and leads to increased water intake. [14]

However, horses with metabolic syndrome rarely become diabetic with high blood sugar. If excessive drinking and urinating are seen, it is likely because the horse also has PPID. [15]

4) Loss of Muscle Mass

Horses with pituitary pars intermedia dysfunction (PPID) have age-related degeneration of dopaminergic neurons in the hypothalamus of the brain. These neurons control a specific region of the pituitary gland in the brain (the intermediate lobe).

Horses with PPID can also have concurrent insulin resistance. PPID is a distinct disease that results from excessively high levels of adrenocorticotropic hormone (ACTH) that increases cortisol production which can promote muscle loss. [16]

In other species, inflammation related to insulin resistance can cause muscle atrophy. This is due to increased protein breakdown and reduced protein synthesis in skeletal muscle. [17]

However, in horses markers of inflammation in skeletal muscle are lower in obese horses and lowest of all in obese horses with high insulin. [18] Therefore, it is unclear whether inflammation plays a role in muscle atrophy in horses.

5) Abnormal Fat Deposits

Obese horses that are insulin resistant often develop excess fat deposits due to increased conversion of sugar into fat. This is particularly true in donkeys and mules but also found in ponies, minis and horses. Insulin resistance in adipose tissue also limits the ability to release fat from these storage sites.

Fat deposits are commonly seen on the neck (cresty neck), over the ribs and topline, above the eyes, and at the base of the tail. Fat deposition may occur in the sheath of geldings and around the mammary gland of mares.

6) Excessive Hunger

Horses that are obese and insulin resistant often have a higher level of leptin circulating in their blood. [19][20] Leptin is a hormone released by fat cells that helps to inhibit hunger.

Research in other species shows an association between insulin resistance and leptin resistance. When the hypothalamus in the brain becomes resistant to the effect of leptin, satiety signalling fails to occur. [21] Leptin resistance may lead to increased appetite in obese horses, although further research is needed to confirm the relationships between insulin, appetite and leptin levels in horses.

7) Inflammation

In humans, there is a clear connection between obesity, infiltration of fat deposits by immune cells and subsequent release of inflammatory cytokines. [22] However, while some studies show slight elevations of isolated cytokines [6], the consistent elevation of inflammatory cytokines found in humans does not occur in horses.

Associations have been found between serum amyloid A (involved in initiating inflammation) and insulin levels in horses. However, there was no elevation in the inflammatory cytokines typically elevated in human metabolic syndrome. [23]

Similarly, no difference in inflammatory mediators or markers were observed between normal ponies and previously laminitic ponies, or in fat deposits of normal or hyperinsulinemic horses. [24][25]

8) Depression and/or Fatigue

Depression is a common feature of PPID which can worsen insulin resistance. [26] Horses with EMS may also have a subdued attitude, likely due to constant low grade hoof pain. When not lame, performance/endurance are often affected because of poor glucose supply to the muscles and low levels of glycogen. [27]

Prevalence of Equine IR

The exact proportion of horses affected by IR is unknown. However, studies show that a large portion of the equine population is affected by high insulin levels. [28]

An Australian study reported that 27% of 200 ponies studied had hyperinsulinemia and a US study reported that 22% of 300 horses studied had the condition. [10][29]

Risk Factors

Some of the predominant risk factors for IR are:

Age

Older horses have an increased risk of developing metabolic diseases, including Equine Metabolic Syndrome (EMS) and Pituitary Pars Intermedia Dysfunction (PPID).

Horses with these metabolic conditions are more likely to be affected by insulin resistance than healthy horses, suggesting a higher prevalence of IR in senior horses. [28]

However, horses with genetic predisposition or inappropriate management can begin to show signs at a young age. [30]

Breed

Horses that are easy keepers (maintain weight easily) may be more prone to developing IR as they are more likely to become overweight. [28]

Breeds more likely to be affected by EMS include Saddlebred, Tennessee Walking, Paso Fino, Morgan, Mustang, and Arabians. EMS is less common in Thoroughbreds, Standardbreds, Warmbloods, Pure Drafts and Quarterhorses.

Research shows that some breeds may be less responsive to insulin. A small study that examined the insulin responses of equids to oral and intravenous glucose determined that ponies and Andalusians had reduced insulin sensitivity compared to Standardbreds. [28] Differences have also been shown between Paso Finos and Thoroughbreds [31]

Obesity

Obesity is a risk factor for IR because excess fat accumulation impairs cells’ ability to respond to insulin normally. Body fat releases endocrine hormones that interfere with insulin sensitivity and energy balance.

However, the link between hyperinsulinemia and obesity is very much a chicken or egg question in horses. Genetic factors are likely to be the determining factor but it has been shown that weight loss improves insulin sensitivity. [32][33]

When fat accumulates in non-adipose tissues such as in the kidneys, liver, heart, and skeletal muscle, a condition known as lipotoxicity occurs. Lipotoxicity contributes to insulin resistance by impairing metabolic pathways in the liver, pancreas and skeletal muscle. [34]

Metabolic Disease

Metabolic diseases such as EMS and PPID are correlated with a higher prevalence of IR. [28]

In horses with EMS, cells fail to respond to the hormone insulin properly, resulting in increased insulin production to keep blood sugar in normal limits. [28]

Some horses with PPID develop concurrent IR, although not all horses with PPID are insulin resistant. [35]

Excess Carbohydrate Consumption

A research study that explored the relationship between diet, obesity, and insulin dysregulation in 33 horses found that feeding cereal-rich meals appeared to be a more important determinant of insulin sensitivity than obesity. [8]

Horses appear to have a limited ability to tolerate high carbohydrate intake. Although adaptation to high carbohydrate intake can occur, [8] there is a limit beyond which the effectiveness of insulin (insulin sensitivity) decreases. Horses with a low tolerance for hydrolyzable carbohydrates will rapidly develop hyperinsulinemia on sugar and starch rich diets.

High intake of sugar and starch commonly occurs when feeding commercial feeds with a high proportion of grain.

Diagnosis of IR

To diagnose IR, your veterinarian will use a combination of physical examination and laboratory tests to assess your horse’s metabolic health and function.

Insulin status is affected by numerous factors including diet, exposure to stress, and pain. To avoid inaccurate results, testing should be completed in a low-stress environment and not during active laminitis. Stress also includes trailering horses prior to sampling. [36]

Some horses affected by IR may have a normal basal insulin level and require additional testing to receive an accurate diagnosis. Insulin testing may require your horse to be fasted for six to eight hours.
Recent studies have shown that a fasting insulin should be no higher than 5 to 9 uIU/mL. [37][38]

Common diagnostic tests performed in the field to confirm insulin resistance include the following:

  • Baseline Insulin and Glucose
  • Baseline Leptin
  • Oral Sugar Test (OST) or Oral Glucose Tolerance Test (OGTT)
  • IV Glucose Tolerance Test (IVGTT)

Treatment and Prevention

Treatment of IR in horses is foremost centred around addressing diet and management factors that can improve metabolic health. In some cases, your veterinarian may recommend medication to treat insulin dysregulation.

To treat and prevent IR, provide an appropriate diet low in hydrolyzable carbohydrate (simple sugar and starch) so your horse can achieve and maintain healthy body weight. Ensure all requirements for vitamins and minerals are met, mineral ratios are correct, and consider supplementing with nutraceuticals that have been shown to support insulin sensitivity.

Give your horse adequate exercise and provide a species-appropriate lifestyle to reduce stress. Our nutritionists can help you design a feeding and management plan for your horse to support their metabolic function.

Diet

Horses with metabolic issues must have their diets carefully managed. Feed a forage-based diet that is low hydrolyzable carbohydrates (HC) which includes ESC and starch on the hay analysis.

Overweight horses should avoid excess carbohydrates to reduce their body weight and improve insulin sensitivity. Research shows that insulin response improves in horses with EMS when they lose excess weight. [39][40][41]

Turnout on a dry lot or with a grazing muzzle can help to limit the amount of sugar they intake from pasture. Many horses with high insulin need to be kept off pasture because sugar levels fluctuate widely depending on weather and growth stage.

Feed forage with a hay net or slow feeder and consider soaking your hay to remove sugars. Analyze your horse’s hay to determine its HC content. Horses that have IR or are at risk for developing the condition should be fed forages with 10% HC or less.

Remove grain, fruits, vegetables, and treats that contain sugar from the diet as they are usually high in NSC content. If your horse is overweight, high-fat feeds may need to be restricted due to their high energy content.

Work with an equine nutritionist to develop a diet plan that will help your horse achieve a healthy weight while ensuring nutrient requirements for vitamins and minerals are met.

Nutritional Supplementation

The vast majority of equine diets are deficient in one or more essential vitamins and minerals, but this is even more common in horses on restricted diets for weight loss.

Ensure that your horse’s diet is balanced and contains adequate levels of key nutrients by feeding a concentrated vitamin and mineral supplement intended for horses with IR. Vitamins and minerals such as selenium, Vitamin E, magnesium, chromium, zinc and copper help build antioxidant defenses.

Research also shows that supplementation with magnesium, chromium, and short-chain fructo-oligosaccharides (a prebiotic) help to improve insulin sensitivity. [42][43][44] However, it is important to realize there are no magic bullet metabolic supplements. Diet and exercise are the key, with medication if that fails.

Mad Barn’s AminoTrace+ is a mineral and vitamin supplement that is specifically formulated for horses with insulin resistance or horses at risk of metabolic dysfunction. It is a comprehensive low-NSC formula that provides 100% organic trace minerals, high levels of natural Vitamin E and complete coverage of nutrients that support metabolic function.

High levels of trace minerals help combat the excessive iron in many forages. Iron overload has been documented with hyperinsulinemia. [45]

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  • Complete mineral balance
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  • Formulated for IR/Cushing's
  • Hoof growth

Spirulina

Spirulina platensis, a blue-green algae, is high in antioxidants nutrients and anti-inflammatory fatty acids that support metabolic health.

In one study, EMS horses provided with three months of spirulina supplementation lost weight and had improved insulin sensitivity. [46]

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  • Supports metabolic health
  • Rich in vitamins & protein

Jiaogulan

Jiaogulan (Gynostemma pentaphyllum) is a Chinese herb that has a long history of use in traditional medicine. Studies in mice show that Jiaogulan has an antidiabetic effect and can improving insulin sensitivity and support a healthy weight. [47][48]

Dr. Eleanor M. Kellon of the ECIR Group says that the benefits of Jiaogulan include improved hoof health and lameness scores for horses with insulin resistance.

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  • Used in laminitic horses

Gut Health

Gut health is believed to influence metabolic health in horses and other species. Research shows that horses with EMS and laminitis have differences in their gut microbiota compared to healthy horses. [49]

In a controlled study of horses on an all-forage diet, horses with EMS had less fecal microbial diversity compared to metabolically normal horses. Interestingly, healthy horses also had more Fibrobacter, a genus associated with insulin resistance in humans, compared to EMS horses. [49] The authors speculated this may be because of increased Lactobacillus in EMS horses.

Much more research is needed to understand how changes in the gut microbiome impact insulin sensitivity, but it is recommended to feed your horse a diet that optimizes gut health and to consider probiotic supplementation.

Exercise

Develop an appropriate exercise program for your horse based on his/her body condition, fitness level, and hoof health (if laminitis is present or has previously affected your horse). Any exercise is better than none! For horses or ponies, or even environments, in which riding is not feasible, ground work and ponying are great options for exercise.

Research shows that exercise reduces inflammation and blood insulin concentrations in horses with insulin dysregulation. [39][40][41] One study demonstrated that seven days of continuous light training increases obese horses’ insulin sensitivity by up to 60%. [50]

Stress

Chronic stress contributes to insulin resistance by increasing levels of cortisol and raising blood glucose levels.

You can help to manage stress in your horse’s life by giving them plenty of forage, friends and freedom. Follow our 18-point guide on how to reduce your horse’s stress level for additional strategies.

Medications

If diet and exercise strategies are not sufficient to control your horse’s insulin levels and IR, drug therapies may be helpful. The following medications can be used to treat horses with IR:

Metformin: Used to treat horses with severe insulin dysregulation, this drug prevents increases in blood glucose and insulin levels post-feeding. [51] It works quickly, within a few days, but the effect can wane over time and some horses do not respond [52]

Levothyroxine: This drug is a synthetic thyroid hormone that helps cells regulate glucose and insulin. It is helpful for treating IR in horses with EMS. It is most beneficial for horses that are resistant to weight loss. [51] Levothyroxine supplementation has no effect on insulin sensitivity [53].

SGLT2 Inhibitors: The newest addition to drug therapy is the sodium glucose transporter 2 (SGLT2) inhibitors which cause glucose to be excreted in the urine. The two drugs with published equine use are ertugliflozin and canagliflozin. [54][55] These drugs are highly effective but require close monitoring and dietary modifications because of side effects like hypertriglyceridemia. [56]

Veterinary Care

Have your veterinarian check your horse’s insulin status through blood testing on an annual basis at a minimum. Horses being treated for EMS or horses of a breed with an increased risk of metabolic dysfunction may require more frequent testing.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

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  46. Nawrocka, Daria et al. Spirulina platensis Improves Mitochondrial Function Impaired by Elevated Oxidative Stress in Adipose-Derived Mesenchymal Stromal Cells (ASCs) and Intestinal Epithelial Cells (IECs), and Enhances Insulin SensitivityinEquine MetabolicSyndrome(EMS)Horses. Marine Drugs. 2017.
  47. Huyen, VTT et al. Gynostemma pentaphyllumTea Improves Insulin Sensitivity in Type 2 Diabetic Patients. J Nutr Metab. 2013.
  48. Liu, Jie et al Gypenosides Reduced the Risk of Overweight and Insulin Resistance in C57BL/6J Mice through Modulating Adipose Thermogenesis and Gut Microbiota. J Agri Food Chem. 2017.
  49. Elzinga, S. 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.
  50. Powell DM. et al.Effect of short-term exercise training on insulin sensitivity in obese and lean mares. Equine Vet J Suppl. 2002.
  51. Frank, N. et al. Current best practice in clinical management of equine endocrine patients. Equine Vet Educ. 2014.
  52. Durham, A.E. et al. The effect of metformin on measurements of insulin sensitivity and beta cell response in 18 horses and ponies with insulin resistance. Equine Vet J. 2008.
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  54. Kellon, E.M. and Gustafson, K.M. Use of the SGLT2 inhibitor canagliflozin for control of refractory equine hyperinsulinemia and laminitis. Open Vet J. 2022.
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  56. Kellon, E.M. and Gustafson, K.M. Hypertriglyceridemia in equines with refractory hyperinsulinemia treated with SGLT2 inhibitors. Open Vet J. 2023.