What's at the root of insulin resistance?

It's long been know that obese horses are more likely to develop insulin resistance. Now we may know why.
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It's long been know that obese horses are more likely to develop insulin resistance. Now we may know why.

It’s common knowledge that obese horses are more likely to develop insulin resistance, but a new study from Australia suggests that the root of the problem may lie with an individual’s diet rather than his weight.


“We know that insulin resistance is associated with the risk of laminitis in ponies and certain breeds of horses, and so is obesity. Together, this syndrome is known as equine metabolic syndrome (EMS),” explains Simon Bailey, BVMS, PhD, of the University of Melbourne. “However, we didn’t know what caused insulin resistance. From other species it has been suggested that obesity might be the cause, but we speculated that it may be the nonstructural carbohydrates (sugars and starches) in the diet that might be more important. Previous studies had made horses obese using high-grain diets, but we wanted to separate the two factors.” Insulin is a hormone that enables the body to utilize glucose from carbohydrates.

Bailey’s team devised an experiment using Andalusians and ponies--breeds susceptible to EMS and laminitis---and Standardbreds, which are not prone to those conditions. The horses, 33 in all, were divided into three groups. One group was fed a high-starch, grain-rich diet, while the second group received a ration that provided the same amount of calories but through fats. The third group, which served as controls, was given only hay and a protein/vitamin/mineral balancer ration.

The horses were kept on the diets for 20 weeks, long enough for those on the high-calorie rations to become obese. The researchers tested each horse’s insulin sensitivity at the start of the study and again at the end to determine whether insulin resistance had developed.

The results showed that every horse on the calorie-dense diets gained weight, but those on the high-grain rations were much less sensitive to insulin than were those on the high-fat diets. In fact, there was no difference in the insulin regulation among horses fed high-fat diets and those in the control group, which had not become obese. “[This confirms] that, in horses, obesity itself does not cause insulin resistance, but sustained carbohydrate absorption from the diet and the prolonged insulin response is the main cause of insulin dysregulation in otherwise healthy horses,” explains Bailey.

These findings suggest that a horse’s risk of EMS-related laminitis will decrease when dietary changes are made, even before such changes lead to significant weight loss. “Cutting down the starches and sugars will not only reduce the insulin dysregulation (and therefore risk of EMS and laminitis) but will also be the most important factor in driving weight loss,” says Bailey. “Weight loss and insulin sensitivity would probably change at similar rates, although you would be preventing the insulin peaks straight away, so that would reduce the risk of high insulin causing laminitis.”

Bailey stresses that obesity is always unhealthy for horses. “Even though obesity does not directly appear to cause the insulin dysregulation that leads to laminitis, usually it is an indicator that insulin levels may be high because insulin promotes obesity,” he says. “Therefore, aiming for a moderate body condition is going to be the best advice for long-term management.” Bailey adds that this study underscores the influence of genetics on insulin sensitivity.

“Comparing breeds, we saw marked breed differences between the responses. Even though the Standardbreds became slightly more insulin resistant on the high-grain diet, the ponies and Andalusians were relatively insulin resistant even at a moderate body condition score and became even more insulin resistant on the high-grain diet (but not on the high-fat diet),” he says.

Taking these findings into consideration, says Bailey, “the general advice for owners of EMS-prone horses and ponies would be to avoid high-grain concentrate feeds and spring pasture which may produce the sorts of high insulin levels that drive insulin resistance and increase the risk of laminitis. When animals are in work, then using some oil or a proprietary low-glycemic supplementary feed may be the best way to supplement calories.”

Reference: “Effect of increased adiposity on insulin sensitivity and adipokine concentrations in different equine breeds adapted to cereal-rich or fat-rich meals,” The Veterinary Journal, May 2016

This article first appeared in EQUUS issue #470, November 2016.