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

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, Triple Crown Lite Formula or 30% Ration Balancer, 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:

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