The Horse

OCT 2017

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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28 TheHorse.com THE HORSE October 2017 How Insulin Works (or Doesn't Work) Glucose, a type of sugar we get from the foods we eat, supplies energy to all our cells—muscle, skin, brain, etc. But it needs help getting from the digestive sys- tem into the bloodstream and then into the body's various tissues and cells. That's where insulin comes in—the pancreas secretes this hormone when we ingest glucose and makes sure the glucose gets wherever it needs to go. Without insulin the body and its cells can't survive. It plays a vital role in life, so clearly it's a good thing to have. But problems can occur when the body's signaling for how much insulin the pancreas needs to produce goes awry, says Andrew van Eps, BVSc, PhD, MACVSc, Dipl. ACVIM, associate profes- sor of equine musculoskeletal research at the University of Pennsylvania's School of Veterinary Medicine, in Kennett Square. And, unfortunately, it is possible to have too much of a good thing. Some people and horses are just born that way: They produce far more insulin than they need. In equids, Arabians, Morgans, ponies, and some Warmbloods might even carry a genetic risk for insulin overproduction, van Eps says. Others produce more insulin in response to glucose, he says, usually in association with obesity. The body becomes less sensitive to insulin, so it calls for more insulin to manage incom- ing glucose. Over time it keeps calling for greater quantities in a never-ending cycle of production increase. "That's true insulin resistance (IR)," van Eps says. "The body keeps making more insulin to compensate for the fact that the insulin isn't working well." Scientists classify insulin sensitivity and resistance as insulin dysregulation (ID). It's important to study ID horses as one group, but it's also important to recognize their distinctions, says Nicholas Frank, DVM, PhD, Dipl. ACVIM. Frank is associate dean for academic affairs and professor of large animal internal medicine at Tufts University's Cummings School of Veterinary Medicine, in North Grafton, Massachusetts. "You can't assume you know why those insulin concentrations are high," he says. "Certainly many of the horses that have high insulin concentrations have some degree of IR, but that might not be the whole picture." How Horses Differ From Humans We hear about certain human patients requiring insulin injections, but our veterinarians always seem to be trying to reduce horses' insulin levels. So are we supposed to add insulin or remove it? The answer seems to be species-related. Our sources say equids behave differently than humans in the face of IR. "Like IR horses, IR humans keep producing more and more insulin in re- sponse to that insensitivity," van Eps says. "But in humans, the pancreas eventually just can't keep up the pace anymore and wears out, shutting down insulin produc- tion," and creating a deficit. This is what's known as Type 2 diabetes. (In Type 1, or "juvenile" diabetes, the pancreas fails to produce insulin because an autoimmune process destroys the cells.) While diabetes can happen in horses, it's rare, our sources say. "The horse seems to have a much greater capacity to keep secreting insulin," says Frank. "That's partly because when the human pancreas secretes insulin, it also creates a protein that damages the pancreas. But that doesn't seem to occur in the horse. So horses can carry on with this remark- able production of insulin for years." The Woes of Excess Insulin Horse people already know that over- feeding sugars can lead to laminitis. But let's take a moment to find out why. Insulin causes a reaction that makes cells take in glucose. Biochemically speaking, that means it's activating a spe- cial protein called a receptor. Specifically, insulin activates the insulin receptor— and when that happens, all is well. Unfortunately, however, there's a very similar receptor with a very different job—growth—appropriately called the insulinlike growth factor (IGF) receptor. The exact details of what occurs when insulin "accidentally" sets off that IGF re- ceptor remains unknown. But scientists have a good guess. "The latest evidence suggests that the lamellar cells (in the hoof, also called lamellae) get pushed into a growth cycle, disrupting the way they normally attach to each other," van Eps says. The result, of course, is laminitis, the most devastating side effect of ID. Other effects are uncertain. Insulin anomalies don't seem to cause changes in attitude or activity level, van Eps says. But excess insulin is known to damage Grasping Insulin Resistance Some breeds, such as Arabians, Morgans, and Warmbloods, might carry a genetic risk of insulin overproduction. ISTOCK.COM Certainly many of the horses that have high insulin concentrations have some degree of IR, but that might not be the whole picture." DR. NICHOLAS FRANK

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