Insulin resistance in the horse: Definition, detection, and dietetics1,2

Specific quantitative methods for de- termining insulin resistance have been applied to obe- sity, activity/inactivity, reproductive efficiency, and exer- cise in horses, but only nonspecific indications have im- plicated insulin resistance as a risk factor or component of equine diseases. Insulin resistance derives from insu- lin insensitivity at the cell surface, which regulates glu- cose availability inside the cell, or from insulin ineffec- tiveness 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 insuli- nemia, or best by a specific quantitative method. Sub- jects 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 inter- vention 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 stud- ies 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 preven- tion or treatment of insulin resistance in an equine dis- ease. 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 distur- bances 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 ex- change 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 meth- ods for protecting health and promoting horse per- formance.