Insulin Sensitivity Compared among Breeds

Researchers confirmed that metabolic differences exist between breeds, even if equids aren't overweight.

Photo: Thinkstock

Think insulin resistance is just for ponies that are fat?
Don’t think that.
Now scientists know that the breed’s where it’s at.

Previously, scientists believed easy keepers—horses and ponies that gain and keep weight easily—metabolized insulin differently than their slim herdmates because of their increased body fat. But British and Australian researchers have determined that metabolic differences from one breed to another are present even in equids that aren’t fat.

“By comparing nonobese animals in moderate body condition, we have found that there are underlying fundamental differences in insulin responses between horse types as well as between horses and ponies,” said Simon Bailey, BVMS, PhD, FHEA, Dipl. ECVPT, MRCVS, associate professor in preclinical veterinary sciences at the Faculty of Veterinary Science at the University of Melbourne, in Australia.

This means certain breeds—such as Andalusians and ponies, which were used in the study along with several other breeds including Morgans, Tennessee Walkers, Paso Finos, and possibly some Quarter Horses—are more prone to developing problems related to excessive insulin production, partly because of their genetic makeup, Bailey said.

Insulin resistance in humans can lead to diabetes; in horses it is associated with equine metabolic syndrome (EMS). The high insulin levels resulting from EMS can increase a horse's risk of developing the hoof disease laminitis, said Bailey. Insulin normally helps clear glucose from the bloodstream; however, Bailey said, insulin resistance causes the horse or pony to produce more insulin, leading to a condition known as hyperinsulinemia. Hyperinsulinemia can also promote fat storage and prevent loss of body fat—which is typical of the “easy keeper” body type, he said.

In their study, Bailey and colleagues worked with 23 horses and ponies—seven Andalusians (purebreds and cross breeds), eight Standardbred trotters, and eight mixed-breed ponies—in moderate body condition. The researchers tested the animals' insulin sensitivity using two glucose tests: one high-glucose meal, and one intravenous glucose infusion. In both cases the Andalusians and the ponies produced much higher insulin concentrations than the Standardbreds, Bailey said. However, all the horses and ponies had the same blood glucose levels, indicating that the Andalusians and ponies used much more insulin to clear their glucose levels after a meal, he explained. In other words, they had a higher degree of insulin resistance than the Standardbreds.

“The fact that we found these differences between breeds and types of equids—even in the nonobese state—links in to the idea that ponies and certain types of horses are adapted (over thousands of years) to living off rather sparse food resources (e.g., thin scrubby pastures) and therefore have subtle genetic differences which manifest as metabolic differences,” Bailey said.

The problem is expounded when owners place these animals in lush pastures or on other high-carbohydrate diets that aren’t consistent with what their bodies have adapted to, he said. In such situations, “the underlying tendency to become insulin-resistant is magnified, and their insulin can shoot up very high,” Bailey said.

“It’s a combination of a genetic predisposition plus environmental, dietary, and management factors (a combination known as “epigenetics”) which are likely to lead to the development of the EMS phenotype,” he said.

The research group worked in collaboration with Pat Harris, MA, VetMB, PhD, Dipl. ECVCN, MRCVS, head of the WALTHAM Equine Studies group in Leicestershire, England.

The study, "Breed differences in insulin sensitivity and insulinemic responses to oral glucose in horses and ponies of moderate body condition score," was published in Domestic Animal Endocrinology.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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