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Feeding the Insulin Resistant Horse

By Dr. Martin Adams, PAS Equine Nutritionist for Southern States

Dry lots are recommended for insulin resistant horses
Dry lots are recommended for insulin resistant horses.

Insulin resistance is a problem that has recently been documented in horses, ponies and donkeys. The causes and effects of insulin resistance, as well as its diagnosis and treatment, as well as some tips on feeding the insulin resistant horse will be provided.

The horse digests and ferments carbohydrates in feedstuffs to produce glucose, which is its primary source of energy for body functions. Insulin resistance (IR) occurs when body cells that use insulin to regulate the uptake and metabolism of blood glucose become less sensitive to insulin’s effects. This means that insulin is required in greater amounts to keep blood glucose concentrations within normal amounts, especially after a meal high in starch and/or sugar.

There are many contributing factors to IR including diets high in starch and sugar, obesity, breed, age and inactivity. In one research study, horses fed a high sugar and starch feed had increased IR compared to horses fed a high fat and fiber feed, even when they were not overweight, Horses that become obese may develop IR and have an increased risk of laminitis. Horses or ponies with a history of laminitis without an obvious cause such as grain overload, heat stress, recent intestinal surgery or road founder likely have IR. IR is the major symptom of Equine Metabolic Syndrome (EMS), which is the term used to describe horses that have IR as a result of genetics and/or obesity. Horse diagnosed with EMS are characterized by abnormal fatty deposits on the neck (cresty neck), rump and above the eyes, voracious appetite and reduced stamina. Weight loss in obese EMS horses may allow them to return to a normal metabolic status, but other horses may still be symptomatic as there is likely a genetic connection with certain equine breeds and EMS.

Ponies, donkeys and “easy keeper” horse breeds may have a “thrifty gene” which is a genetic adaptation to sparse vegetation in geographic areas where they originated. An altered metabolism that allows maintenance of body weight on a less feed, and the secretion of more insulin or an insulin resistant metabolism allowed these IR-type equids a greater chance of long-term survival. Insulin has the effect of directing blood glucose into body cells and storage of glucose as fat, which is the most efficient method of storing energy when food is not available. Modern equine management providing excessive calories from improved pasture and grain feeding has resulted in obesity and the many problems associated with EMS.

Equine Cushing’s Disease (ECD), which is also known in the horse as Pituitary Pars Intermedia Dysfunction or PPID. ECD is common in horses over 20 years of age, and can include symptoms such as hirsuitism (long curly hair that won’t shed in spring), excessive sweating, muscle wasting, excessive water consumption and urination, frequent infections and infertility. Horses can have EMS or be insulin resistant and also develop ECD in their later years, with both problems causing health issues.

Regular exercise reduces blood glucose and insulin levels, allowing an active horse to prevent IR even when fed high sugar and starch meals. However, lack of activity combined with overfeeding can result in obesity and onset EMS. Prolonged high levels of blood glucose and insulin due to insulin resistance can have harmful effects on the horse’s health, so diagnosis and treatment are important.

Your veterinarian can diagnose IR in your horse by measuring blood levels of glucose and insulin. A combined glucose-insulin test may be used but more likely a resting serum insulin concentration will be utilized to diagnose IR. Levothyroxine (also called Thyro-L) has reduced IR symptoms and laminitis and increased weight loss when used over a 4 month-long period in horses diagnosed with EMS. A weight management program should also be initiated with or without the use of Thryo-L for the obese horse, with the use of a supplement pellet such as Legends CarbCare Balancer Pellet, limited hay feeding at 1.5% to 1.25% of body weight daily, and limited or no pasture for horses diagnosed with Equine Cushing’s Disease (EMS) and an increased risk of laminitis.

Prevention is always more economical than treatment, so use the following tips on feeding your horse to help prevent and/or treat IR:

  • Adjust your feeding program to maintain proper body condition of your horse, avoiding obesity. This may require limiting pasture grazing time or the use of a grazing muzzle, and limiting the amount of hay and grain fed, especially for pony breeds and horse breeds such as Peruvian Paso Fino, Paso Fino, Icelandic Horse and Morgan, and many Arabians.
  • Maintain good soil fertility of horse pastures. Research has shown that a well-fertilized pasture of cool-season grasses will have lower levels of sugars that have been implicated in causing laminitis.
  • Provide as much exercise and turnout for your horse as possible. Consider a sacrifice area or paddock, where pasture grass is not available but horses can be turned out daily for activity.
  • Avoid or severely limit access to pasture grazing for IR horses in the spring and fall. Cool season grasses are growing most rapidly during these times and contain the greatest amount of sugars, including fructan sugars, which have been implicated in causing laminitis. Horses with IR are more likely to have a laminitic outbreak when grazing on pasture during these times.
  • Select hay with a low level of soluble carbohydrates for the IR horse. Small grain hays such as oat and ryegrass have much greater sugar content than other grass hays like timothy and orchard grass. Alfalfa hay and coastal Bermuda grass are usually the lowest in soluble carbohydrate (sugar and starch) content, but alfalfa contains more calories and feeding rate must be monitored closely. Clover hay should be avoided for the IR horse as the starch content can be high as well as the caloric density. Moderate quality grass hay is preferred due to its low soluble carbohydrate content and moderate caloric content. Soaking hay for 60 minutes in cool water or hot water for 30 minutes will significantly reduce its soluble carbohydrate content.
  • Select a horse feed low in sugar and starch (20% or less) to normalize blood glucose and insulin levels for your horse with IR. Beet pulp and soy hulls contain very low levels of soluble carbohydrates, so select a feed that is based on one of these ingredients. Southern States has the soluble carbohydrate values of its horse feeds available for you to select an appropriate one for your horse, and many products with 20% or less sugar/starch content.  There are four Legends CarbCare horse feeds. They contain low levels of sugar and starch content and are recommended for IR horses. All of the Triple Crown Horse Feeds are formulated with low levels of sugar and starch and are recommended for insulin resistant horses.
  • Proper hoof care is essential for the IR horse due to increased incidence of laminitis. Select a well-fortified horse feed and make sure you are feeding an adequate amount of feed to meet requirements for protein, lysine, calcium, phosphorus, zinc, copper and iodine, these nutrients are especially important for good hoof growth. Also consider selection of a horse feed with added biotin or the use of a hoof supplement containing biotin if hoof quality is poor.
  • Magnesium is involved in insulin secretion and supplementation of magnesium has been recommended for horses with IR. Chromium has been used to reduce blood glucose levels in horses and has also been recommended for supplementing the IR horse. Select a horse feed with a higher guaranteed level of magnesium or consider using magnesium and chromium supplementation for a horse with IR. Dietary guidelines would be 2 grams of magnesium per 100 pounds of body weight and 1 gram of chromium per 100 pounds of body weight.
  • Many horses with IR are fed small amounts of horse feed to decrease calories but may not be provided with optimal amounts of required nutrients. Consider the use of a balancer pellet, such as Legends CarbCare Balancer Pellet.

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