Equine Metabolic Syndrome

Equine metabolic syndrome (EMS), Cushing's disease, insulin resistance (IR), glucose intolerance, and glycemic indices of feeds have gotten a lot of press lately. The terms are taken from the human literature, where they have very specific connotations with regard to the clinical entities they denote. In human medicine:

Metabolic syndrome is characterized by increased glucose and insulin response to standardized glucose tolerance tests plus hypertension (high arterial blood pressure), which has not been documented in EMS horses.

Cushing's disease is classically due to hypersecretion of adrenocorticotrophic hormone (ACTH) from pituitary tumors, which causes overly high cortisol release from the adrenal gland. The high cortisol is considered to be the primary cause of the clinical signs of hyperglycemia, polydipsia (excess thirst), polyphagia (excess eating), polyuria (excess urination), hair coat changes, etc., in other species. In horses, the syndrome is often associated only with IR in the early stages and has been attributed to pituitary dysfunction instead of actual tumor activity.

Insulin resistance, a reduced sensitivity of the body's cells to insulin's facilitation of glucose uptake, is ideally documented by hyperglycemic or euglycemic (normal) insulin "clamp" techniques, where both insulin and glucose are infused intravenously and, via complex mathematical models, the amount of insulin required to reduce or maintain blood glucose is calculated. These are direct measures of the cellular sensitivity and are considered to be the "gold standards" for diagnostic purposes. Simpler tests of insulin sensitivity involve administration--either orally or intravenously--of a standardized glucose challenge and measurement of the glucose and insulin responses. These, however, assume normal pancreatic beta cell function (necessary for insulin release), which is influenced by a variety of factors such as glucagon-like peptide-1 (GLP-1) release by enteric cells and other neural/hormonal inputs.

Glucose intolerance is tested by administration of a standardized glucose challenge and documentation of the area under the glucose response curve (AUC-glucose) by repeated sampling, with or without measuring concomitant insulin responses. The glycemic index of foods is defined as the AUC-glucose response to a standardized "meal" of the food in relation to that of a standard "meal" of white bread. In equine studies we use oats, starch/sugar solutions (2 g/kg), and sweet feeds as the "standard" feed, and the glycemic index has been, in at least one study, based on a single blood sample taken at what was presumed, but not verified, as the time of peak response.

In horses, the only basis often used for "diagnosis" of EMS, Cushing's, or IR has been the presence of high insulin and/or glucose in a single blood sample, and many vets have recommended placing such horses on high fat and fiber rations. This can be grossly inaccurate and misleading.

Hyperinsulinemia, which can be defined as an unusually high blood or plasma insulin concentration, with or without concomitant hyperglycemia, is common in horses. Pituitary dysfunction and obesity cause IR and are correlated with an increased risk of laminitis. Hyperinsulinemia has been correlated with the presence of OCD lesions in weanlings less than 12 months of age, but cause and effect have not been proven! In every study of insulin responses in horses for the past 20 years, at least one research animal has had exaggerated insulin responses and was reported as an "outlier." Many of these hyperinsulinemic horses are clinically normal.

If you take a single blood sample, the glucose/insulin results will be meaningless unless you know the following:

  • When the animal was last fed, since glucose/ insulin remain high for up to four hours after a meal of grain;
  • Type of feed it ate, for example, high sugar/starch feeds would cause greater responses than high fat/fiber concentrates;
  • Ration to which it was adapted, since horses on predominantly hay rations will have a higher insulin response to a meal of grain or glucose challenge than those adapted to high-grain rations;
  • Time of day and level of stress, since morning responses are higher than evening responses in unstressed horses due to normal diurnal fluctuations in cortisol (which induces IR). Abnormal results should be followed by further, more definitive tests before instituting drug or drastic dietary measures, unless, of course, the horse is obviously obese. Even then, "diet" rations should not be high-fat--they don't need the extra calories!

About the Author

Sarah Ralston, VMD, PhD, Dipl. ACVN

Sarah L. Ralston, VMD, PhD, Dipl. ACVN, is a professor in the Department of Animal Sciences at Rutgers' School of Environmental and Biological Sciences, specializing in equine nutrition. Her research has focused on the effects of diet on metabolism, behavior, and the development of orthopedic disease in young horses, and she has additional interests in nutritional modulation of stress, metabonomics (the study of metabolic responses to drugs, environmental changes, and diseases), and pasture management. Previous research highlights were the pioneering work she did in nutrition for geriatric horses and post-surgical colics while at Colorado State University in the 1980s and the discovery of the correlation of hyperinsulinemia with development of osteochondrosis in young Standardbreds.

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