Equine metabolic syndrome--dietary and medical management (Proceedings)

Prevention of EMS centers on maintaining normal weight in horses, particularly those that are high risk breeds. Since these animals may be more efficient users of calories than others, it is imperative to feed to maintain an ideal condition score and not to use arbitrary guidelines. The feeding amounts suggested on the bags of most complete feeds or supplements are usually much too much for a horse prone to EMS. Particular care needs to be used when turning animals on pasture as the carbohydrate concentration of the grasses is ever changing. Restrictions are most important during times of high soluble carbohydrate content such as spring and autumn.

Treatment

Treatment for EMS has two arms –management of caloric intake and exercise and, if diet and exercise is not sufficient to manage the condition, medical therapy. If the history reveals that the horse is being overfed, correction of the diet may be all that is needed to return the animal to normal body weight.

Many owners do not realize that the “typical” American horse diet of free choice hay or turnout supplemented by sweet feed is the equine equivalent of a person consuming a large bowl of sugar coated cereal at every meal. Even “high fiber” grains such as whole oats contain over 50% carbohydrate. While this is better than corn at over 70%, it still cannot be considered a suitable feedstuff to recommend when managing a horse with EMS.

Exercise

Muscle tissue is the largest organ in the body with insulin-mediated uptake of glucose, and thus is extremely important in the whole body regulation of insulin and glucose dynamics. There are several types of glucose transporter proteins in the body. Glucose transporter 4, commonly referred to as GLUT4, is the predominant glucose transporter on muscles cells. There are not fixed amounts of GLUT4 receptors on cell membranes. In low insulin situations, the receptors are kept within vesicles inside the cells. When insulin is present, the vesicles rapidly fuse with the muscle cell membranes where they facilitate the transport of glucose into the cell.

In humans with type 2 diabetes, the failure of insulin to stimulate the transport of GLUT4 from the vesicles to the muscle cell membrane is believed to be one of the key factors in the pathogenesis of their insulin-resistance. Exercise stimulates the translocation of GLU4 in both normal and type 2 diabetic humans (and thus the influx of glucose into the muscle cells and out of the general circulation). The increased uptake of glucose associated with exercise may occur in horses with EMS in a similar manner. If the horse does not have laminitis, then 30 minutes or more of daily trotting and cantering should be instituted. This level of exercise has been shown to improve insulin sensitivity in horses.

Obviously, the more a horse is exercised, the more calories it will consume and the more quickly it will return to a normal weight. If lameness prevents that level of exercise, 20 or 30 minutes of walking may help as well. Studies using GPS technology have demonstrated that horses in small pastures walk several miles a day as they move around. Thus, any turnout will increase a horse's exercise level when compared to stabling. Spreading the horse's daily hay ration broadly over the turnout area will also encourage movement. Often it is difficult to find a large turnout area that is not pasture, however, so each situation must be assessed individually.

Diet

The current recommendation is to encourage weight loss in horses with EMS by feeding a forage, preferably a hay that has been tested and is known to be 10% or less of nonstructural carbohydrate, at a rate of 1.5% of the horse's body weight per day. A non-caloric vitamin mineral supplement can be added to balance the diet for essential nutrients. Other supplements and pasture access should be eliminated. If no appreciable weight loss occurs, the hay intake can be decreased to 1% of body weight. Further restriction my lead to hyperlipemia and exacerbate insulin resistance. If 1% intake is not sufficient to induce weight loss, either the amount of exercise should be increased or the addition of a pharmacologic agents should be considered.

Forage

One cannot properly feed a horse with EMS by making assumptions about the nutrient content of the hay being fed. Values for grass or alfalfa hay from tables are averages and very general. Grass hay is better forage than alfalfa because it is less nutrient dense, but there is no absolute contraindication against feeding alfalfa.  Timothy and orchard grass hay tend to have favorable carbohydrate profiles when compared to other forages, but selecting a desirable grass type is no substitute for laboratory analysis.

There are several commercial laboratories that will perform hay analysis at a very reasonable cost. Equine-analytical Laboratories is one such company that reports the carbohydrate content of the feed in a way that is easy to understand. The goal is to find and feed a hay that has less than 15% of non-fiber carbohydrate and less than 10% of non-structural carbohydrate on a dry matter basis. Beet pulp with no molasses added is a good source of low starch feed, and can be used to supplement hay once the horse's weight has reached a desirable level.

Soaking hay in water for 30 minutes in warm water or 60 minutes in cold water has been recommended to lower water soluble carbohydrate concentrations. Unfortunately, but the actual amount reduced is extremely variable and this is not a reliable method to produce a low NSC forage.

Definitions of carbohydrate type

·         Crude Fiber (CF) is the historical method of fiber analysis used to divide carbohydrates into digestible and indigestible fractions. Crude fiber accounts for most of the cellulose and only a portion of the lignin. It is not the most accurate method for quantifying fiber in forages. It is still used today as the legal measurement of fiber in grains and finished feeds.

·         Neutral Detergent Fiber (NDF) is a measure of hemicellulose, cellulose and lignin representing the fibrous bulk of the forage. These three components are classified as cell wall or structural carbohydrates. They give the plant rigidity enabling it to support its growth. NDF can be negatively correlated with intake.

·         Acid Detergent Fiber (ADF) is a measure of cellulose and lignin.

·         Lignin is indigestible plant component. As lignin content increases, digestibility of cellulose decreases thereby lowering the amount of energy potentially available to the animal.

·         Pectin is a cell wall polysaccharide. It is also known as "soluble fiber."

·         Starch is a polysaccharide found primarily in the grain or seed and/or root portions of plants.

·         Water Soluble Carbohydrates (WSC) are carbohydrates solubilized and extracted in water. This includes monosaccharides, disaccharides and some polysaccharides (mainly fructan). Fructan is a major storage carbohydrate in grasses, and is the substance most highly correlated with the development of laminitis in insulin-resistant horses.

·         Ethanol Soluble Carbohydrates (ESC) are carbohydrates solubilized and extracted in 80% ethanol. These include primarily monosaccharides and disaccharides. Many companies not report ESC rather than WSC values.

Non Fiber Carbohydrates (NFC) is a mathematical estimate of non-cell wall (non-fiber) carbohydrates consisting of starch, sugar, pectin and fermentation acids that can serve as energy sources for the animal. NFC is calculated as 100% - (CP% + NDF% + Fat% +Ash%)

Dietary supplements

There are numerous dietary supplements that contain specific ingredients because they have been theorized to increase insulin sensitivity. These include herbal preparations, cinnamon, chromium, and magnesium. To date, none have been shown to improve insulin sensitivity in horses in experimental situations.

Complete feeds

Several companies now manufacture low starch feeds that are recommended for horses with EMS and/or PPID. While expensive, a low starch complete feed has the advantage of allowing owners to feed a measured amount of a substance with a known nutritional value. Some horses do not find low starch complete feed palatable. In such instances, trying another brand or going back to hay may be needed.

In addition to complete feeds, various low starch supplements are on the market. Some are primarily designed for horses with muscle problems, and these replace carbohydrate calories with isocaloric amounts of fat. The key ingredient in such feeds is rice bran, a high fat material. EMS horses do not need a nutrient dense supplement of any form, and generally do better on a no-calorie vitamin/mineral preparation.

Pasture

Recent research has documented that a motivated horse can consume a huge amount of calories in a very short period of time when turned out on a normal pasture. Ideally pasture access should be eliminated while a horse with EMS is on a weight loss regime. If this is not possible, restricting pasture access to 1 hour a day or using a grazing muzzle will help decrease intake. A grazing muzzle covers the horse's muzzle leaving only one small hole on the bottom.

A horse needs to work to push a small amount of forage through the hole to eat it. Some horses adapt to grazing muzzles well and seem very happy to work all day for a small amount of feed. Others quickly learn how to dislodge the muzzle. Once normal weight is achieved, pasture access can be increased very gradually. Time on pasture should be increased by no more than 1 hour every few days.

Medical therapy

The thyroid hormone, thyroxine, in the form levothyroxine sodium improves insulin sensitivity and produce weight loss in horses. Horses over 350 kg can be given 48 mg/day PO while smaller horses and ponies should receive 24 mg/day PO. Treatment periods between three and six months are often needed to achieve desirable weight loss. At that time the horse can be weaned off the medication over a 3-4 week period. If feed intake is not limited concurrently, treatment with levothyroxine will be unlikely to resolve the horse's clinical signs.

Various medications formulated to improve insulin sensitivity in humans have been attempted on horses. To date, none has provided consistent results. Metformin increases insulin sensitivity via inhibition of gluconeogenesis and glycogenolysis. It has may lead to improvement in hyperinsulinemic horses at a dose of 15 mg/kg B.I.D. PO. Pioglitazone at a dose of 1 mg/kg PO SID has been shown to be safe in horses, but has little effect on insulin sensitivity.