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 obesity, abnormal fat deposits along the neck, increased appetite and excessive thirst and urination.
Insulin resistance also increases the risk of laminitis in horses and can lead to excessive inflammation, reproductive issues, poor performance, loss of topline and high blood pressure.
EMS and insulin resistance are treated with dietary intervention, exercise, and medication. If your horse shows signs of being insulin resistant, consult a veterinarian to obtain a diagnosis and work with a nutritionist to formulate an appropriate diet.
Overview of Insulin Resistance
Equine insulin resistance or insulin dysregulation is diagnosed in horses with an elevated blood insulin level (hyperinsulinemia). Insulin is an endocrine hormone produced by the pancreas in response to high blood sugar levels. 
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). The liver and muscle use glucose for energy or to make glycogen – a stored form of carbohydrates used for energy production. The liver can also use glucose to make fat which is then stored in adipose tissue.
In healthy tissues, insulin binds to its receptor on the cell membrane and signals cells to take in glucose. In insulin-resistant tissues, signalling is less effective after insulin binds to the receptor.
This 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 responds by producing more insulin to promote the uptake of glucose.  This causes elevated insulin levels which can be picked up in a diagnostic test.
The persistently high blood glucose levels can lead to health issues. Rather than entering cells designed to handle a large amount of sugar (i.e. muscle, liver and adipose), the glucose enters cells of other tissues that can not handle glucose effectively. This can include the eyes, nerves, and kidneys. With prolonged insulin resistance, excess sugar in these cells can be damaging leading to issues with eyesight, sensation and kidney function.
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:
Obese horses may have difficulty losing weight because of leptin resistance. Leptin is a hormone produced by adipose 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. 
Obesity is a risk factor for IR as it promotes the production of inflammatory substances that interfere with the function of insulin in the body. 
Elevated insulin levels and IR are associated with the development of endocrinopathic laminitis in horses and ponies.  An abnormal insulin level may increase the risk of laminitis by altering the function of the epidermal laminar cells of the hooves.
Laminitis involves inflammation of the hoof laminae, the internal structures that attach the wall of the hoof to the coffin bone. 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.
Although the exact mechanisms of how IR promotes laminitis are still being researched, inflammation, vasoconstriction, and endothelial damage are three proposed disease pathways.
3) Excessive Drinking and Urinating
Healthy, non-working, non-breeding adult horses typically drink 60 mL/kg of body weight of water each day.  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.
4) Loss of Muscle Mass
Horses with pituitary pars intermedia dysfunction (PPID) may or may not be insulin resistant. These horse have excessively high levels of adrenocorticotropic hormone (ACTH). A high ACTH level increases cortisol production which in turn can promote muscle loss. 
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.  PPID and IR horses may have accelerated muscle loss due to high cortisol levels, inflammation and insulin resistance.
5) Abnormal Fat Deposits
Obese horses that are insulin resistant often develop excess fat deposits due to increased conversion of sugar into fat. 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.
Over-conditioned horses can have excess fat accumulation in their liver, which may promote IR by interfering with the normal signalling responses to insulin.
6) Excessive 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. 
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.
Obese horses have more adipose tissue and have higher levels of adipokines. Some of these are released by immune cells present within fat tissue and contribute to inflammation.
The presence of chronic inflammation in the body influences how cells respond to insulin.  Prolonged inflammation can lead to insulin resistance and promotes high blood glucose levels.
8) Depression and/or Fatigue
Research in humans and laboratory animals has found that inflammatory signalling molecules (cytokines) can have a negative effect on mood regulation, which can cause depression and fatigue. 
Horses with IR may suffer a similar effect although further research is required to confirm this.
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. 
Some of the predominant risk factors for IR are:
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. 
Breeds more likely to be affected by EMS include Saddlebred, Tennessee Walking, Paso Fino, Morgan, Mustang, and Quarter horses. EMS is less common in Thoroughbreds and Standardbreds.
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. 
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.
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. 
Metabolic diseases such as EMS and PPID are correlated with a higher prevalence of IR. 
In horses with EMS, cells fail to respond to the hormone insulin properly, resulting in higher-than-normal blood sugar levels and increased insulin production. 
Some horses with PPID develop concurrent IR, although not all horses with PPID are insulin resistant. 
Excess Carbohydrate Consumption
Diets that provide a high amount of carbohydrates (sugar/starch) can promote weight gain and subsequently contribute to IR. Examples of high carbohydrate feeds include commercial feeds with a high proportion of grain or molassed sweet feeds.
Diets that are high in sugar and starch lead to higher blood glucose concentrations, resulting in increased insulin secretion by the pancreas. Long-term excess consumption of carbohydrates can promote insulin resistance in tissues.
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. 
Diagnosis of IR
To diagnose IR, a veterinarian will perform several tests to understand your horse’s health and metabolic function. Veterinarians will diagnose insulin resistance after completing a physical examination and laboratory tests.
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.
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.
Diagnostic strategies used to confirm if a horse is insulin resistant include the following tests:
- Baseline Insulin
- Oral Sugar Test (OST)
- Oral Glucose Tolerance Test (OGTT)
- Insulin Tolerance Test
- IV Glucose Tolerance Test (IVGTT)
- Combined Glucose-Insulin Response Test
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 low-sugar diet so your horse can achieve and maintain healthy body weight. Ensure all requirements for vitamins and minerals are met 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.
Horses with metabolic issues must have their diets carefully managed. Feed a forage-based diet that is low in non-structural carbohydrates (NSC) including starch and sugar.
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. 
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.
Analyze your horse’s hay to determine its NSC content. Horses that have IR or are at risk for developing the condition should be fed forages wiith 10% NSC or less.
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.
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 and fight against the inflammation associated with IR.
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.
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. 
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. 
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.
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. 
In a controlled study of horses on an all-forage diet, horses with EMS had less fecal microbial diversity compared to metabolically normal horses. Healthy horses also had more Fibrobacteria, a genus associated with insulin resistance, compared to EMS horses. 
More research is needed to understand how changes in the gut microbione impact insulin sensitivity, but it is recommended to feed your horse a diet that optimizes gut health and to consider probiotic supplementation.
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).
Research shows that exercise reduces inflammation and blood insulin concentrations in horses with insulin dysregulation.  One study demonstrated that seven days of continuous light training increases obese horses’ insulin sensitivity by up to 60%. 
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.
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. 
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 also beneficial for horses that are resistant to weight loss. 
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.
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