Insulin resistance in the horse: Definition, detection, and dietetics

Specific quantitative methods for determining insulin resistance have been applied to obesity, activity/inactivity, reproductive efficiency, and exercise in horses, but only nonspecific indications have implicated insulin resistance as a risk factor or component of equine diseases. Insulin resistance derives from insulin insensitivity at the cell surface, which regulates glucose availability inside the cell, or from insulin ineffectiveness due to disruption of glucose metabolism inside the cell. Interplay of insensitivity and ineffectiveness should be considered in regard to patterns of disease, such as laminitis. Detection of insulin insensitivity is made weakly on the basis of fasting hyperinsulinemia, more strongly with a statistically validated surrogate, such as the logarithm of the reciprocal of basal insulinemia, or best by a specific quantitative method. Subjects found to be at risk can be managed to improve their insulin sensitivity by dietetics. Claims for dietetic prevention of a disease should be distinguished from claims for avoidance of a dietary risk factor. The evidence required for a claim of prevention is a controlled intervention trial as for a therapeutic drug, according to the U.S. FDA. In contrast, the evidence required for a claim of avoidance is association revealed by population studies plus causation shown by mechanistic experiments, as formulated in the Surgeon General of the Public Health Office's (1988) Report on Nutrition and Health. In this view, no appropriate evidence is available for the prevention or treatment of insulin resistance in an equine disease. Evidence is available, however, to justify avoidance of high-glycemic feeds, such as high starch intakes in grains, clover, and alfalfa, and high fructan intakes in grasses, to decrease the risk of acute digestive disturbances associated with rapid fermentation, and chronic metabolic disorders associated with insulin resistance. During submaximal exercise, high-glycemic meals have been shown to increase glucose utilization immediately. On the other hand, chronic adaptation to feeds that exchange corn oil and fiber sources for sources of sugar and starch confers benefits to athletic performance that may be due to several aspects of fat adaptation, including the regulation of insulin sensitivity, as well as glycolysis and lipid oxidation by signals from insulin receptors. Information regarding insulin resistance suggests methods for protecting health and promoting horse perf