Comparison of nonspecific indications and quantitative methods for the assessment of insulin resistance in horses and ponies

Abstract

I resistance (IR) is a component of certain metabolic conditions and a risk factor for the development of several diseases of humans and domestic animals, including Equus caballus (horses and ponies). In humans, those conditions and diseases include diabetes mellitus type 2, obesity, dyslipidemia, hypertension and stroke, surgery or injury, stress, sepsis, aging and longevity, coronary heart disease, polycystic ovary syndrome, cancer, inflammation, oxidative stress, and endotoxemia. Insulin resistance is a component of several of the same conditions and diseases in horses (Table 1). Insulin resistance is regarded generally as a decrease in tissue responses to insulin and more specifically as a decrease in insulin-mediated uptake of glucose by the liver, muscles, and adipose tissues. These responses to insulin are usually measured as insulin sensitivity, so IR may simply be regarded as the inverse of low sensitivity of insulin receptors on the cell surface. In addition to glucose transport into a cell, however, IR may involve ineffectiveness as a result of various disruptions of intracellular glucose metabolism. Controversy about IR has been provoked by the hypothesis that genetic predispositions are exacerbated by diets high in carbohydrate content. Notable examples are the high-carbohydrate, low-fat diets recommended to prevent coronary heart disease in humans and grain-molasses concentrates for horses. Allusions to IR in the medical literature on horses have been based on various kinds of evidence, ranging from speculative analogies via common associations to various ambiguous indications on the basis of plasma concentrations of glucose and insulin and to specific quantitative measurements that separate function of pancreatic β-cells from peripheral actions of insulin. Our objective was to evaluate types of evidence that have been used to document IR in horses and ponies and thereby identify opportunities for improvement in the assessment of IR, especially as it relates to nutritional recommendations for the treatment and prevention of clinical conditions, the avoidance of nutritional risk factors, and the promotion of production and performance.