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Feeding the Insulin Resistant Horse

Dr. Martin W. Adams, PAS – Equine Nutritionist for Southern States

Insulin resistance has recently been documented in horses, ponies and donkeys. This article provides information on causes and effects of insulin resistance, diagnosis and treatment, and some tips on feeding the insulin‐resistant horse.

Horses digest and ferment carbohydrates to produce glucose, which is their primary source of energy. 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.

The exact cause of IR is not known. Several possible causes or contributing factors include diets high in starch and sugar, obesity, age and inactivity. In a recent 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 are overweight may develop IR and have an increased risk of laminitis. Horses or ponies with a history of laminitis that develop it without an obvious cause like grain overload or road founder likely have IR. But other horses with IR may be diagnosed with Equine Metabolic Syndrome (EMS), which includes other symptoms such as abnormal fatty deposits along the crest, rump and above the eyes, voracious appetite and reduced stamina. Weight loss in obese horses diagnosed with IR may allow them to return to normal metabolic status, but other horses may still be symptomatic because 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. A lowered metabolism that allows maintenance of body weight on less feedstuffs and the secretion of more insulin or an insulin‐resistant metabolism enabled a greater chance of long‐term survival. Insulin has the effect of directing blood glucose into body cells and storage of that glucose as glycogen or fat, which can be used when food is not available. Modern equine management, which provides excessive calories from improved pasture and grain feeding, has resulted in obesity and the many problems associated with EMS.

Many older horses can be insulin‐resistant due to the hormonal effects of Equine Cushing’s Disease (ECD), which is more common in horses over 20 years of age, and can include all of the symptoms of EMS and others 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.

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 of IR or 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 is important.

Your veterinarian will be able to diagnose IR 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. There have been two drugs that veterinarians have used on IR horses: levothyroxine (also called Thyro‐L) has reduced IR and increased weight loss when used over 3‐4 month periods, and metformin reduced obesity and laminitic pain in 78% of horses in a 220‐day study.

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, 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 and Morgan, and some lines of Arabians, quarter horses and thoroughbreds.

• 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 orchardgrass. Alfalfa hay and coastal bermudagrass 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 will reduce its soluble carbohydrate content by 30%.

• Select a horse feed low in sugar and starch to reduce 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. MFA Incorporated has the soluble carbohydrate values of its Legends© horse feeds available for you to select an appropriate one for your horse.

• 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, like Legends© Horse Feeds, or the use of a hoof supplement containing biotin.

• 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, such as Legends© manufactured by MFA, or consider using magnesium and chromium supplements for a horse with IR.

• 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 or a highly fortified, low starch Legends© horse feed manufactured by MFA.