Many horses are insulin resistant and have a condition called equine metabolic syndrome in which they have an abnormal response to insulin. Increased insulin in the blood can lead to laminitis and subsequent founder.  Laminitis and founder are painful foot conditions that can be deadly in horses.  Some horses are genetically predisposed to have insulin resistance and have high insulin levels after ingesting high-carbohydrate diets, such as high grain diets or, more commonly, lush green pasture.  However, some new research has revealed that older horses are susceptible to insulin dysregulation even if they have not been diagnosed with insulin resistance and therefore need an appropriate diet to prevent laminitis.  Insulin is produced by the horse’s pancreas and when insulin is released, cells remove glucose from the blood and a high level of insulin in the blood can be accompanied by insulin resistance. 

The Horse Magazine reports researchers at the Waltham Centre for Pet Nutrition in England and at Michigan State studied insulin dysregulation in older horses, and the best diets to feed and aging.  They studied the insulin response to healthy adult horses and healthy older horses and found the insulin response increases with age even in healthy horses, regardless of the diet they were fed prior to evaluation.  Clare Barfoot from Spillers Nutrition indicated that energy sources used in the diet of senior horses and their effects on insulin need to be carefully considered.  Basically, this means restricting the amount of sugar and starch or nonstructural carbohydrates in the diet of all older horses.  So even if you have an older horse that does not appear to be insulin resistant or have Cushing’s disease, feeding a low carbohydrate food is a good idea.