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Insulin is a hormone produced by special cells within the pancreas in order to do many important jobs, one of which is the transportation of glucose from the bloodstream into the insulin-sensitive cells of the liver, adipose (fat), and skeletal muscle.
When foods high in glucose enter the body, more insulin must be released. Insulin resistance (IR) is a condition where the cells of the body do not respond normally to the hormone, causing many serious health problems.
Insulin resistance can be induced by use of corticosteroid drugs or the hormonal abnormalities associated with Cushing’s disease/PPID. IR may also accompany chronic infections such as Lyme disease.
Pregnancy also triggers or worsens IR, preserving precious glucose supplies for the fetus.
However, most cases of IR are associated with equine metabolic syndrome. There is strong evidence that this is genetic. [1][2][3][4]
Equine Insulin Resistance
Insulin resistance is a metabolic type that developed to conserve glucose for key organs such as heart and brain when horses were evolving under conditions of sparse vegetation.
Many modern breeds such as Thoroughbreds, Standardbreds, Quarter horses, full size Drafts and Warmbloods no longer have insulin resistance but those that remain true to their ancestral type still do.
If these horses, ponies, donkeys and mules could live a life similar to their ancestors – working all day or covering 20 miles a day to find sufficient food and water – they wouldn’t have any problems. Few domesticated horses get even close to that level of exercise.
On top of that they are eating diets much higher in hydrolyzable carbohydrates, ESC and starch, which are the components of NSC which cause an insulin response.
Symptoms of Equine Insulin Resistance
Equine insulin Resistance can cause several symptoms such as:
- unusual fat deposits
- generalized obesity
- laminitis
Laminitis is a disease that affects the hooves of the horse, causing elongation and weakening of the laminae and lameness.
Testing for blood insulin and glucose levels is a simple way to diagnose a horse with IR. Once diagnosed, several steps must be taken to manage this condition.
What's your top priority with your horse's health?
What's your top priority with your horse's health?
Treatments
The most obvious change that needs to be implemented is an improvement in diet. Feeding the horse hay that is low in starch and sugar can help prevent laminitis. Hay for horses with IR/EMS should have less than 10% ESC + starch.
One trick to make the hay healthier is to soak it in water which will lower sugar (ESC). Various soaking times have been used but a common guideline is 30 minutes in hot water or an hour in cold water.
Another important change is cutting down the amount of fat the horse consumes; therefore, food rich in fat such as oils and rice bran are not helpful except for amounts needed to provide essential fatty acids and as a carrier for vitamin E. Mad Barn’s w-3 oil provides 900 mg of the anti-inflammatory omega-3 fatty acid DHA and ~890 IU of natural vitamin E in 60 ml (2 oz).
Specific vitamins and minerals, such as magnesium, can benefit a horse with insulin resistance.
Horses with EMS/IR are also prone to iron overload which is known to worsen IR in other species. [5] Higher intakes of copper and zinc are helpful in counteracting iron’s competition for absorption.
Mad Barn’s AminoTrace+ mineral and vitamin supplement has been specifically formulated to meet the needs of insulin-resistant horses.
Management
Although not all horses with IR are obese, most usually gain weight, so an increase in exercise is very important for the affected horse, as long as it is not laminitic. Exercise is actually an excellent way to increase sensitivity to insulin, better than diet alone. [6][7]
Physical activity cannot be ignored when dealing with insulin resistance; it must be combined with a diet change to help improve the horse’s condition.
Lastly, because laminitis is a serious threat linked to equine insulin resistance, the horse’s hooves must be given proper attention in order to minimize the effects of high insulin.
It has been known since the early days of studying this condition that the laminae are weakened and lengthened even in horses not showing any pain. [8]
Maintaining a trim with a short rounded toe and palmar angle no higher than 5 degrees helps to avoid undue traction and pressure on the laminae.
Equine insulin resistance is a serious condition that is connected with Cushing’s disease and laminitis that must be managed thoroughly and carefully.
Nevertheless, with a balanced diet, good hoof care, and a healthy amount of exercise, horses with insulin resistance can live a normal life.
Submit your horse’s diet for a free diet evaluation by our University-trained nutritionists and veterinarians.
Frequently Asked Questions
Here are some frequently asked questions about insulin resistance in horses:
Insulin resistance in horses is a metabolic condition in which body tissues do not respond normally to insulin. Insulin normally helps move glucose from the bloodstream into insulin-sensitive tissues such as liver, fat, and skeletal muscle. When tissues become less responsive, the body must produce more insulin to manage blood glucose. Most cases are associated with Equine Metabolic Syndrome and appear to have a strong genetic basis. [1][2][3][4]
Insulin resistance in horses can be caused or worsened by genetics, Equine Metabolic Syndrome, Pituitary Pars Intermedia Dysfunction, corticosteroid use, chronic infections, and pregnancy. It originally developed as a metabolic adaptation that helped horses conserve glucose during periods of sparse forage. In modern management, limited exercise and diets higher in sugar and starch can make this trait harmful. Pregnancy can also trigger or worsen insulin resistance because the mare's body prioritizes glucose availability for the fetus.
Horses most at risk for insulin resistance are easy keepers, ponies, donkeys, mules, and horses that retain ancestral metabolic traits. These animals are efficient at conserving energy and may be poorly suited to modern diets that are high in hydrolyzable carbohydrates. Limited exercise or turnout further increases risk because many domestic horses do not move nearly as much as their ancestors did. Some modern breeds are less likely to develop insulin resistance, but individual risk still depends on genetics, diet, body condition, and management.
Signs of insulin resistance in horses can include unusual fat deposits, generalized obesity, and laminitis. Fat may accumulate along the neck, over the tailhead, or in other regional deposits even when the horse is not obviously overweight. Laminitis is the most serious complication because high insulin can weaken and lengthen the laminae inside the hoof. Some horses with insulin resistance may not show obvious signs until hoof pain or abnormal fat deposits develop.
Insulin resistance in horses is diagnosed with blood testing for insulin and glucose levels. These tests help determine whether the horse is producing abnormal insulin responses or has poor glucose regulation. Diagnosis should be made with veterinary guidance, especially when laminitis, obesity, Pituitary Pars Intermedia Dysfunction, or chronic illness is also suspected. Once insulin resistance is identified, diet, exercise, hoof care, and metabolic management should be adjusted to reduce risk.
Insulin resistance is related to laminitis because high insulin levels can damage the hoof laminae even before obvious pain appears. The laminae are the tissues that help suspend the coffin bone within the hoof capsule, and weakening of these tissues can cause lameness and long-term hoof changes. Laminar weakening and lengthening have been documented even in horses not showing pain. [8] This makes hoof care and early metabolic management essential for insulin-resistant horses.
Exercise can help horses with insulin resistance by improving insulin sensitivity, often more effectively than diet alone. [6][7] Increased physical activity is especially important for horses that gain weight easily, provided they are not currently laminitic or too painful to exercise. Exercise should be introduced safely and adjusted to the horse's soundness, fitness, and veterinary recommendations. Diet change and exercise work best together for improving body condition and metabolic health.
Not all insulin-resistant horses need to lose weight, although many are overweight or prone to easy weight gain. Some horses may have abnormal fat deposits or metabolic dysfunction even when their overall body condition does not appear extreme. Body condition scoring, weight tracking, and assessment of fat deposits can help determine whether weight loss is needed. If weight loss is appropriate, it should be gradual and based on low-sugar forage, controlled calorie intake, and safe exercise.
Important nutrients for horses with insulin resistance include balanced vitamins and minerals, adequate protein, essential fatty acids, magnesium, copper, zinc, and vitamin E. Horses with Equine Metabolic Syndrome and insulin resistance may also be prone to iron overload, which is known to worsen insulin resistance in other species. [5] Higher copper and zinc intake may help counteract iron's competition for absorption. A balanced forage-based diet is still the foundation for horses with insulin resistance, with supplements used to correct gaps rather than add unnecessary calories.
Hoof care in insulin-resistant horses should focus on reducing mechanical stress on the laminae and monitoring for early laminitis changes. Maintaining a short rounded toe and a palmar angle no higher than 5 degrees can help avoid excess traction and pressure on weakened laminae. Regular farrier care is especially important because insulin resistance can affect hoof structures before obvious pain is present. Veterinary and farrier support should be coordinated if the horse has current or previous laminitis.
Summary
Insulin resistance (IR) occurs when a horse’s cells respond poorly to insulin, leading to elevated blood glucose and insulin that can drive serious health problems.
- IR can be triggered or worsened by corticosteroids, PPID/Cushing’s disease, chronic infections, and pregnancy, but most cases are linked to genetically driven Equine Metabolic Syndrome.
- Easy-keeper types on high-NSC diets with little exercise are at highest risk, and common signs include regional fat deposits, obesity, and a strong association with laminitis.
- Diagnosis is made with blood insulin and glucose testing, and management starts with a low-sugar, low-starch forage program (ESC + starch < 10%), including soaking hay to lower sugars and avoiding unnecessary added fats.
- Targeted nutrition supports IR management, including adequate magnesium, higher copper and zinc to counter iron’s interference, and vigilance for iron overload, alongside a balanced vitamin–mineral supplement.
References
- Weckman, M.J. et al. Genome-wide association study suggests genetic candidate loci of insulin dysregulation in Finnhorses. Vet J. 2024. View Summary
- Stefaniuk-Szmukier, M. et al. Equine Metabolic Syndrome: A Complex Disease Influenced by Multifactorial Genetic Factors. Genes (Basel). 2023. View Summary
- Patterson-Rosa, L. et al. Metabogenomics reveals four candidate regions involved in the pathophysiology of Equine Metabolic Syndrome. Molec Cell Probes. View Summary
- Norton, E. et al. Genome-Wide Association Analyses of Equine Metabolic Syndrome Phenotypes in Welsh Ponies and Morgan Horses. Genes (Basel). 2019. View Summary
- Kellon, E.M., and Gustafson, K.M. Possible dysmetabolic hyperferritinemia in hyperinsulinemic horses. Open Vet J. 2020. View Summary
- Moore, J.L. et al. Effects of Diet Versus Exercise on Morphometric Measurements, Blood Hormone Concentrations, and Oral Sugar Test Response in Obese Horses. J Equine Vet Sci. 2019. View Summary
- Bamford, N.J. et al. Influence of dietary restriction and low-intensity exercise on weight loss and insulin sensitivity in obese equids. J Vet Intern Med. 2018. View Summary
- Johnson, P.J. et al. Endocrinopathic laminitis in the horse. Clin Tech Equine Pract. 2004.
