Have you ever wondered why some horses are tractable part of the time, but hard to control or "hot" other times? Or why some foals have skeletal problems when everything possible was done to provide nothing but the best feed and care? Or why some performance horses tie up (azoturia) periodically? While still controversial in the world of equine nutrition, studies reveal that a high-carbohydrate diet, which produces a high glycemic response (the level of blood glucose that rises in response to a meal), might be the culprit in some of these problems.

According to Joe Pagan, PhD, president of Kentucky Equine Research (KER) in Versailles, Ky., "There is growing evidence that when certain horses eat feeds that produce large amounts of blood glucose, it may affect the horses' behavior or health."

Pagan, who founded KER in 1988, emphasizes that glucose is necessary for the life function of a horse. But, he says, "Consider the diet of horses in the wild. They eat high-forage diets which contain very little sugar. When horses eat forage, the bacteria in the large intestines break down the plant cell walls and produce volatile fatty acids as a by-product of this fermentation. It is these volatile fatty acids that can be used to produce glucose. Horses make glucose from fiber fermentation, but it is a more steady production of glucose that does not cause large fluctuations in blood glucose. However, today's domestic horses, due to smaller pastures and higher athletic expectations, are often fed concentrated feeds, many of which are high in starch and sugars.

"It is not the glucose, per se, that causes some potential problems," he clarifies. "It is the resulting insulin. Glucose increases the production of insulin, which is an anabolic hormone that sends the glucose to the muscles and liver to be stored as glycogen, both of which are important fuels for performance horses.

"There is reason to believe that an increase in insulin also affects other hormones and neurotransmitters that may affect behavior or the health of some horses," he says.

While not all horses might be affected, Pagan has discovered that some horses are more sensitive to high levels of glucose. "For example, certain horses become more excitable and less tractable, something that many horse owners have observed for years. And we've known for a long time that some performances horses on high-carbohydrate feeds are more prone to tying-up and laminitis."

Skeletal Problems

Pagan has questioned how much glycemic response contributes to skeletal disorders, such as osteochondritis dissecans (OCD) in young horses.

In vitro studies done at the University of Cambridge with fetal and foal chondrocytes (cartilage cells) suggest that the role of insulin in the growth of cartilage might be to promote chondrocyte survival or to suppress differentiation. Therefore, high levels of insulin in the blood might be a contributory factor to equine osteochondrosis. This work was stimulated by work at Rutgers University by Sarah Ralston, VMD, PhD, Dipl. ACVN, associate professor in Rutgers University's (the State University of New Jersey) Animal Science Department, who reported a correlation between high insulin and an increase in incidence of osteochondrosis in Standardbred weanlings in 1996.

Pagan explains the process of how OCD lesions develop. "OCD lesions occur on the articular cartilage, which is the cartilage at the end of the bones that protects the joints," he says. "Since the skeleton starts as cartilage, we think there is a problem in the maturation of the cartilage that can set the stage for lesions to develop on the cartilage."

Kentucky Equine Research conducted a study with 218 young Thoroughbred horses on six farms in Central Kentucky to assess the relationship between glycemic response and OCD. Pagan used the young horses' normal feed as their test meal.

"Each horse was fed a measured amount of their normal feed that delivered a standardized level of carbohydrate (1.4 g non-structural carbohydrate per kilogram of body weight, or NSC/kg bw). Two hours after the weanlings had finished this meal, a single blood sample was taken and the levels of glucose and insulin were measured. The overall incidence of OCD on these farms was recorded until the horses were sold as yearlings in July or September at ages ranging from 16 to 20 months. (For the purpose of the study, OCD was defined as osteochondrotic lesions occurring in the fetlock, hock, shoulder, or stifle that were treated surgically. Horses with radiographic lesions that were not corrected surgically were included in the "non-affected group.")

The glycemic index (the extent of increase in blood glucose, and the hormone insulin, concentrations following a feed's consumption) of each feed was measured, and the study found that there was an increased incidence of OCD that was treated surgically with feeds that had a high glycemic index.

Ralston has recently developed a low-dose oral glucose test to measure a foal's metabolic response to feed. It can easily be administered to foals as young as two to three months, before OCD lesions tend to appear. This could identify animals predisposed to OCD due to a relative insulin resistance, which results in abnormally high insulin responses to the glucose challenge. By using this test, a horse owner or farm manager potentially could adjust the rations of at-risk foals to reduce the risk of OCD.

Reduced Glycemic Response

There was yet another unexpected discovery during a study to measure the glycemic response--this time in adult horses. Pagan explains, "If we added oil to the grain or sweet feed as a means of increasing caloric intake, we got a very different glycemic response--one that was very low. We'd feed the same amount of grain, but top-dressed with corn oil. Although the oil increased the number of calories, the glycemic response was much lower than if we did not add the oil.

"Wondering why adding the oil changed the glycemic response, we then looked at how quickly the stomach emptied," continues Pagan. "We found that when the oil was added, it reduced the rate of gastric emptying, and we think this might contribute to the lower glycemic response. Therefore, adding vegetable oil to a horse's ration can reduce glycemic response in two ways. First, oil is a rich source of calories that can be substituted for a portion of the carbohydrate in the ration. Additionally, the oil will reduce the glycemic response of the remaining carbohydrate in the ration (by slowing its digestion)."

Studies done by Ralston and David Kronfeld, PhD, DSc, MVSc, MRCVS, Dipl. ACVN, Dipl. ACVIM, professor of veterinary medicine at Virginia Polytechnic Institute (VPI) and State University in Blacksburg, Va., also documented that adding fat and fiber to concentrate feeds can reduce the glycemic index. Ralston cautions that studies done at VPI indicate that feed with over 10% fat might adversely impact calcium metabolism in the young, growing horse. She recommends using feeds that contain 6-10% fat, at least 12% fiber, and little or no added molasses, but that contain higher mineral and protein than the high fat/fiber feeds formulated for performance horses.

Pagan also cautioned that simply adding oil onto a grain mix might imbalance the nutrient:calorie ratio of the ration. He advised getting help from an equine nutritionist on how much oil can be safely added. Or, better yet, he recommended using a commercial feed that has been formulated both for growth and low glycemic response.

Pagan says, "After a foal gets to be about 15 to 18 months of age, the chances of him developing these skeletal lesions becomes very low. So once they become yearlings, or long yearlings, you don't have to worry about it."

Perfomance Horses

Pagan emphasizes that for the vast majority of riding horses, you don't need to run out and change their feeds. "When we talk about feeding low-glycemic feeds," he says, "we are talking about only a small percentage of horses with specific problems."

Research has indicated that horses which are genetically prone to tying up, more specifically with polysaccharide storage myopathy (PSSM), will need a low-glycemic feed.

Horses with PSSM store excess glycogen (sugar) in their muscles--the excess of sugar and its byproducts disrupts the balance of energy metabolism and the muscles of these horses cramp and become stiff. However, these horses can be treated by feeding feeds with low glycemic indices.

For horses which tie up easily and/or are glucose intolerant due to Cushing's disease or obesity, high-starch, high-sugar feeds such as processed grains and especially sweet feed should be avoided as much as possible. Horses that get excitable and nervous when fed high amounts of grain might also benefit from switching to a ration with a lower glycemic index.

High-fat/fiber pelleted and extruded feeds tend to cause lower glucose/insulin response than textured grain mixes, while still providing the energy and nutrients necessary for performance. High-fat supplements, such as processed rice bran (20% fat) or edible oils, are also good choices to provide energy without increasing blood glucose and insulin.

Pagan states, "The overall picture is forming that our dependence on cereal grains as the preferential energy source for a lot of our horses is too high.

"What the research findings suggest is that young, growing foals, horses that are prone to tying-up or laminitis, those with Cushing's disease, and some that tend to be excessively 'hot' mannered could possibly be managed better on low-glycemic diets," concludes Pagan.

About the Author

Genie Stewart-Spears

Genie Stewart-Spears resides with her husband on Runamuck Ranch in southern Illinois, in the Shawnee National Forest. Now a pleasure rider, she competed in endurance for 10 years and has served as the Media Chairperson for the American Endurance Ride Conference. Her photography and articles appear in several equine magazines and many books, brochures, and advertisements.

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