Equine metabolic syndrome is characterized foremost by insulin resistance, defined as a peculiar physiological response to the ingestion of foods that are eventually broken down to glucose or other sugar molecules. Abundant in certain feedstuffs commonly fed to horses, glucose causes a normal state of hyperglycemia or elevated sugar in the blood. The elevated glucose prompts the release of insulin from the pancreas, which encourages the removal of glucose from the bloodstream by fat or skeletal muscle cells. Once in the cells, glucose can be put to work immediately to fuel exercise or growth or be stored as glycogen or fat for later use. Insulin resistance implies that either the central tissue (liver) or the peripheral tissues (the skeletal muscle or the fat cells) are relatively insensitive to the action of insulin or that the quantity of insulin released by the pancreas in response to hyperglycemia is diminished. This leaves glucose circulating in the blood. Because glucose levels do not drop, the pancreas continues to discharge insulin, leading to elevated concentrations of insulin in the bloodstream, a condition known as hyperinsulinemia.
In the equine model, obesity appears to be related to the onset of metabolic syndrome. The suggested cause is that certain fat cells produce cortisol, among other hormones, which interferes with the ability of insulin to move the glucose into cells. Obese horses having more fat cells to produce cortisol results in greater interference with insulin. This explains why weight reduction is effective in increasing insulin sensitivity. Not all fat horses are insulin resistant and not all insulin resistant horses are overweight. Current beliefs hold that horses whose fat cells produce high levels of leptin as well as cortisol are the ones prone to insulin resistance. Leptin is not believed to cause insulin resistance but is found to be higher in horses that are insulin resistant. Age and diet may be directly related to the development of equine metabolic syndrome. Age is thought to decrease the horse's sensitivity to insulin. Meals high in starch and sugar cause significant spikes in blood glucose and insulin, and years of consuming such meals might lead to insulin resistance.
At this time, diagnosis of equine metabolic syndrome is based on description and physical characteristics, results of glucose-tolerance testing, and elimination of similar conditions. Affected horses are usually between the ages of eight and 18. Horses and ponies of nearly all breeds have been diagnosed, though Morgans, Peruvian Pasos, Paso Finos, domesticated Spanish Mustangs, and Warmbloods appear to be especially predisposed to the syndrome. As a group, ponies tend to become overweight more readily than horses and are often inclined to suffer from laminitis. What's more telling than either age or breed of the patient is distribution of exterior body fat. Areas of unusual fat accumulation include the top of the neck (commonly called the crest), over the shoulders, and the rump (including deposits over the croup and just above the tail head). Significant fat sometimes settles in the sheaths of geldings, so much so that they may appear swollen. Affected broodmares show unusual estrous cycling, which makes them incredibly difficult to get pregnant. Anecdotal evidence by owners is also instrumental in diagnosing equine metabolic disease. Owners frequently describe their horses as easy keepers, finding it virtually impossible to reduce the weight of these horses by calorie restriction alone. Many report that high-calorie feeds such as grain are not being fed.
An overwhelming secondary symptom of equine metabolic syndrome is chronic laminitis. The laminitis exhibited by these obese, middle-aged horses tends to be mild. On occasion, so minimal are the laminitic episodes that knowledgeable, conscientious horse owners cannot vouch definitively for any clinical signs of lameness. The hard evidence speaks a different tale, however. Abnormal hoof growth occurs. Dropped soles, unusual growth lines, and separation of the hoof at the white line are frequently observed.
The most important factor to helping horses with metabolic disease is getting the extra weight off. These animals should be taken off grain. Pasture time should also be limited. In springtime when the pasture conditions are very conducive to rapid grass growth, the pasture has a very high carbohydrate content, resulting in very high circulating insulin concentrations for these animals. Soaking hay for 30 minutes (and pouring off the water) helps to leech out extra calories. Reducing calories along with regular exercise can help take off the extra weight.