Written By Kris Hiney
Imagine a bright spring day. You excitedly turn your horse out to indulge in the fresh spring grass as a special treat. You return in a few hours to collect your companion, but instead are met by an unhappy painful horse, slowly limping its way back to the gate.
Sound familiar? Unfortunately for some owners, this is an all too real scenario. Many horses suffer from carbohydrate sensitivities, or metabolic syndrome, which make them extremely susceptible to changes in carbohydrates in the diet. One may also hear these horses referred to as insulin resistant, almost like Type II diabetes in humans. In recent years there has been an upsurge in the number of studies and articles written about metabolic syndrome in horses. While awareness in the general public has increased, many horsemen still wonder if their horse is, indeed, one of these individuals. Should they be paying strict attention to every type of carbohydrate their horse consumes? Should horses no longer consume grass? Does their horse need medication? How do you know if your horse truly has metabolic syndrome?
Classically, horses with metabolic syndrome are described by a certain appearance. They are typically obese horses which gain weight readily, and are considered “easy keepers”. Breeds with a higher prevalence of metabolic syndrome include the traditional easy keepers such as ponies, Morgans, and Paso Finos. However, metabolic syndrome can be seen in a wide spectrum of breeds including Quarter Horses, Arabians and Thoroughbreds. Beyond just being obese, metabolic horses tend to have regional adiposity, or specific fat deposits on the crest of their neck, over their tailhead, the sides of their abdomen and also in the scrotal or mammary area. The size of the crest of the neck is often the best physical predictor of metabolic syndrome. The thicker the crest, the more likely the horse truly fits into this category. However, it is important to note that it is possible for leaner horses to also suffer from metabolic syndrome. Despite being lean these horses still demonstrate regional adiposity, along with a susceptibility to pasture associated laminitis, as well as insulin resistance. Therefore, if your horse shows symptoms, it may be wise to have it tested, despite it not being overly obese.
Unfortunately the most common way horses are diagnosed with metabolic syndrome is the frequency of laminitic bouts. Usually this is seen following grazing on pasture, especially in the spring or fall. These horses may be young or middle aged, which sets them apart from horses who suffer from Cushings disease. However, horses who suffer from metabolic syndrome early in life are certainly more likely to develop Cushings later on. Cushing horses are also distinct in the prevalence of hair coat which does not shed or long curly hair while the metabolic horse has a normal hair coat.
Physiologically, these horses demonstrate insulin resistance. Essentially they must secrete larger amounts of insulin compared to a normal horse, in order to stabilize their blood glucose levels. Therefore, their insulin levels remain higher in their bloodstream, which can have a cascade of effects on their body. They also present with elevations in blood lipids, as well as an increase in leptin. Leptin is a hormone secreted by fat cells or adipocytes, that normally helps in the feeling of satiety (or fullness). However, increased concentrations of leptin may contribute to inflammation in the body. Metabolic horses also have a lower resting thyroxine levels (T4) then their normal counterparts. However, the low level of T4 does not cause insulin resistance and metabolic syndrome, but rather is merely a consequence of altered metabolic profiles.
So why are these horses so susceptible to laminitis? What could insulin resistance possibly have to do with painful feet? One of the commonalities between the myriad of disorders that can result in laminitis in horses is a disruption of the circulation to the hoof. Insulin is most commonly recognized for its role in glucose disposal, but it is a hormone with systemic effects. It is presumed that sustained hyperinsulinemia promotes vasoconstriction. It is already known that carbohydrate overload induces laminitis by creating vasoconstriction in the hoof, so the hyperinsulinemic horse may be even more susceptible to shifts in carbohydrate intake. This disruption of blood flow to the foot results in hypoxia and tissue damage to the sensitive laminae. Severe bouts may render the hoof wall unstable and allow the coffin bone to rotate downwards within the foot. This may lead to permanent alterations of the hoof structure.
Testing for metabolic syndrome frequently involves blood sampling after a short period of fasting (typically 6 hours). Blood is analyzed for glucose and insulin levels that are above normal. The presence of altered adrenocorticortropin releasing hormone can also be tested if Cushings is suspected in an older horse. Further testing can be done if horse’s insulin levels are within the normal range, but metabolic syndrome is suspected. Horses are again removed from feed, and a standard blood sample is taken. Horses are then given a bolus of glucose and then insulin to determine how the body metabolizes these compounds. This provides a more dynamic picture of the horse’s metabolic response to carbohydrates.
If your horse has been diagnosed with metabolic syndrome, or has show signs of pasture associated laminitis, it is important to start them on a rigorous management protocol. First, as these horses have sensitivities to carbohydrates, concentrates should be removed from the diet. As these horses are typically obese anyhow, there is little need to supply concentrates to them anyhow. If the owner is concerned with mineral and vitamin intake, there are many products which are intended to complement forage only diets. Typically these are pelleted supplements which are fed at very low levels of intake. The obesity issue in the horse should also be addressed. Exercise should be increased to 5 days a week. Not only will this aid in reducing the body weight of the horse, but exercise also enhances glucose clearance from the blood in a non-insulin dependent manner. However, be sure that the horse is not recovering from any laminitic episodes. Pasture intake should also be limited in these horses. Horses should only have access to pasture for a short time or have access to a very small area. If more movement of the horse is desired, a grazing muzzle should be employed to prevent overconsumption of grass. The horse should receive an all forage diet, preferably of grass hay, with intake reduced in order to encourage weight loss. If weight loss is not able to be achieved at an intake of 2% of the body weight, then reduce feed intake to 1.5% of bwt. Unfortunately simple diet restriction may take a long time due to the efficiency of the horses prone to metabolic syndrome. If the horse has greater degrees of insulin resistance, it is advisable to monitor the non-structural carbohydrate composition of the hay, with it ideally below 10%.If horses have persistent issues with metabolic syndrome after calorie restriction, decrease in adiposity, alteration of diet, limitation of pasture intake and exercise have all been employed, then there are medical therapies which can be used. Levothyroxine is effective in improving insulin sensitivity. If all of these measures are followed faithfully, there is no reason that these horses cannot be returned to a metabolically normal state and enjoy a long healthy life.
Next month: We will discuss other strategies that have been employed to assist the metabolic horse.