Although documented in ponies over a quarter century ago, the existence and clinical significance of insulin resistance in equines have only recently become widely recognized and accepted.
Insulin resistance in horses
While insulin resistance can be induced in any horse by pregnancy, corticosteroid therapy or pituitary pars intermedia dysfunction, this complex condition has multiple causes related to diet and exercise.
A recent study by scientists at Michigan State University explored the link between insulin resistance and iron overload in captive rhinoceroses using horses which have a digestive tract simlar to that of rhinos. The study used horses because of the impractical aspects of working with rhinos.
The scientists aimed to determine if an association between high ferritin concentrations indicative of iron stores in the body and insulin resistance exists in horses and to what extent.
The team used 15 healthy horses and one insulin resistant horse in their study. The horses were fed a grass hay diet with no concentrates. All horses were administered two treatments after a 12-hour fast.
The study showed that the insulin resistant horse had the highest serum ferritin concentration and the highest insulin response. Based on these results, Nielsen determined, that for an overweight horse not given sufficient exercise, giving feed stuffs lower in iron may be appropriate, or, more importantly, not giving the horse as much feed or feed of higher quality may be appropriate.
While insulin resistance can be induced in any horse by pregnancy, corticosteroid therapy or pituitary pars intermedia dysfunction, also known as Equine Cushing's Disease, it appears there may also be a genetic basis.
Insulin resistance should be thought of as a metabolic type rather than a disease state per se in those horses.
Whether insulin resistance actually becomes a significant health issue depends on the interaction between this metabolic type and the environment, specifically diet and exercise.
Insulin resistant horses may also be predisposed to allergic or exaggerated inflammatory reactions because of elevated levels of inflammatory cytokines. These problems, rather than weight issues, may be what brings the horse to the attention of the veterinarian. The worst case scenario is the acutely laminitic horse with uncontrolled insulin resistance.
Controlling the consumption of simple carbohydrates is essential. In the acute phases it is usually necessary to restrict the combined dietary intake of simple sugars and starch to 10% or less. Pastures often exceed this amount. Because sugar/starch levels fluctuate constantly over the course of the day and in response to weather conditions, there is no way to guarantee any grazing strategy is consistently safe.
For reasons that are not entirely clear, some laminitic horses are sensitive to alfalfa. This may be because some amino acids can trigger an insulin response, or it may be related to the higher starch and glucose in alfalfa hay. Horses on alfalfa that do not respond well should be switched to grass hay.
Even grass hays should be analyzed to ensure safe levels of sugar and starch and determine protein and mineral intakes. Diets should be formulated to provide approximately 150% of the NRC minimum requirements.