by: Betsy Wagner
PhD, PAS, Department of Animal Science, Auburn University

Insulin resistance (IR) has been getting a lot of attention in both the research and popular press publications of the horse world for the last several years. Insulin resistance, or reduced insulin sensitivity, is a condition where the body produces adequate-to-elevated amounts of insulin but the cells are slow to respond to it and utilize glucose correctly. A good comparison to human health is the development of Type II diabetes, and if you consider the risk factors, associated medical problems, and dietary management of Type II diabetes, one can begin to understand how IR can be managed in horses.

Several risk factors have been associated with the development of IR in horses. Genetics is part of the picture, with stock-type and pony breeds more susceptible as are horses with an "easy keeper" metabolism. Horses that consume a diet high in soluble carbohydrates, including concentrates rich in starches and sugars, may be predisposed. Weight and physical activity also playa role. Obese horses are obviously at a greater risk. However, it is important to note that horses in light to moderate exercise, regardless of body condition, generally have a lower risk of developing IR than those which are sedentary or on pasture turnout.

Insulin resistance may occur by itself although it is usually seen in conjunction with other metabolic problems. It is one of several symptoms along with obesity and elevated plasma lipid concentrations that characterize equine metabolic syndrome (EMS). Horses with pituitary pars intermedia dysfunction, better known as PPID or Cushing's disease, may or may not have IR. Pasture-associated laminitis may also occur in conjunction with IR, EMS, or PPID. Unfortunately laminitis is often the first sign to the owner that the horse has a metabolic problem.

The only way to diagnose IR is with a thorough veterinary exam including blood work. Horses that have experienced pasture associated laminitis or that have PPID should have their insulin sensitivity evaluated. It also may be a good idea to evaluate horses that are chronically obese or perennial easy keepers. Make this part of the annual veterinary exam along with rabies vaccination and Coggins testing. If IR is identified and managed early it may be possible to avoid the development of some of these other more serious medical problems.

To manage as well as prevent IR begin by maintaining the horse at an ideal body condition. Most horses should be maintained at a moderate to moderately fleshy body condition, which is a score of 5 to 6 on the 9-point Henneke scoring system. This means a horse where the ribs can be easily felt but not seen, the back is level or has a slight crease through the loin area, spongy fat deposits can be felt around the tail head, and the neck and shoulders blend in well with the rest of the body.

Ideal weight can be achieved through calorie restriction and an increase in activity. Concentrates should be severely restricted if not completely eliminated from the diet. If it is necessary to feed a concentrate because the forage does not provide enough protein, vitamins, or minerals offer a balancer product or a pelleted, low soluble carbohydrate concentrate that is labeled for use in horses with IR or other metabolic problems. Consider keeping the horse in a dry lot or using a grazing muzzle to restrict the intake of forage. Increase the activity level of the horse by slowly introducing regular lunging or light riding. In a study using apparently healthy animals, horses ridden at a walk for 60 minutes three times a week or walk and trot for 90 minutes three times a week had improved insulin sensitivity compared to when the same horses were turned out to pasture for eight hours a day. Another study using idle, obese horses found improved insulin sensitivity when horses were lunged at the trot for 30 minutes, three times per week. Horses with a history of pasture-associated laminitis or other lameness issues should be cleared by a veterinarian before beginning an exercise program.

In most cases feeding good quality warm season forage such as bermuda-grass or bahiagrass hay or pasture will meet all of the horse's nutritional needs. These grasses are safe for horses with metabolic problems because of the way they store carbohydrates within the plant cells. Cool season grasses such as fescue, ryegrass, and timothy should be avoided. These grasses tend to store carbohydrates in the form of sugars and fructans which can easily upset the horse's metabolism. If cool season forages are the only hay option, soak the hay for 30 minutes prior to feeding in order to draw out as much of the sugars as possible.

Horses diagnosed with insulin resistance can be managed by reducing concentrate intake, increasing activity levels, and selecting safe forage sources. These same practices can minimize the risk of developing IR in other horses.

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