We're familiar with diabetes in humans, but not many people know that horses can also have diabetes and blood-sugar problems. "Insulin resistance" is sometimes called "pre-Cushing's" because it was felt that insulin-resistant horses are in the early stages of Cushing's disease. You may also hear it called "equine metabolic syndrome," because there are similarities with human insulin-resistant conditions. It has only recently been recognized that horses can be insulin resistant without having a pituitary tumor (Cushing's disease).
What Is It?
Insulin is a hormone secreted by the pancreas. It signals the body's cells to take in glucose, "blood sugar." Every cell has a minimum requirement for glucose, which is the body's major fuel. The skeletal muscles consume most of the glucose in your horse's body.
Signs Suggestive of Insulin Resistance
- Family history of easy weight gain
- Family history of laminitis Easy weight gain beginning soon after the horse stopped growing
- Gains or holds weight on much less feed than other horses
- Fatty crest on the neck (Note: This is never normal. It may very well be extremely common in some breeds, but it's not normal.)
- Fat deposits at the base of the tail or elsewhere on the body, may have a dimpled appearance (like cellulite)
- Bulges above the eyes in the area that is normally hollow (supraorbital fossa)
- History of grass-related laminitis
- History of laminitis when pregnant
Insulin resistance (IR) is exactly like it sounds - the cells are resistant to the effects of insulin. To compensate for this, the pancreas has to put out an abnormally large amount of insulin to get the job done. If the pancreas is unable to produce enough insulin, the blood sugar rises abnormally high. At that point, insulin resistance becomes diabetes.
It's very important to realize that an insulin-resistant metabolism is not a disease per se. It has allowed many of the very hardy breeds to survive under harsh conditions. It only becomes a problem when they are fed inappropriately.
How Do Horses Get It?
It's been known for more than 30 years that many ponies are naturally insulin resistant to some degree. This gave them a survival advantage in the wild because they would gain weight very easily when food was available and use that fat reserve to sustain themselves when there was less to eat.
Some people are the same way, and this tendency for easy weight gain was dubbed "the thrifty gene" by some human researchers. Several genetic markers have been identified in people. No genetic work has been done in horses yet, but evidence is accumulating that the easy keeper breeds are more likely to be insulin resistant.
Overfeeding a horse to the point it becomes obese can also result in insulin resistance, although not all fat horses are insulin resistant. Cushing's disease, a condition common in older horses, occurs when a tumor has grown in the pituitary gland in the brain and puts out large amounts of adrenocorticotropic hormone (ACTH). ACTH in turn makes the adrenal gland secrete the hormone cortisol. Cortisol blocks the action of insulin on cells, creating insulin resistance. Not all horses with Cushing's disease are insulin resistant, at least initially, but that condition does put them at high risk.
IR initially causes horses to be overweight, if not obviously obese. Many people equate a horse being "round" with being well cared for and healthy, but this is not the case. The extra weight puts unnecessary stress on your horse's back, joints and heart.
IR also causes your horse's body to react with exaggerated inflammatory responses. This combined with a tendency for blood vessels to spasm easily is thought to be why these horses are more prone to laminitis. In the advanced stages of IR, the horse actually begins to lose weight and muscle mass. His cells are "starving" for glucose that he is unable to get into them.