Obesity, Insulin Resistance, and Laminitis

Nicholas Frank, DVM, PhD, Dipl. ACVIM, associate professor of large animal clinical sciences at the University of Tennessee, discussed the causes, clinical signs, and management of insulin resistance in horses, and its link to laminitis at the 2006 AAEP Convention.

"Insulin resistance can be defined as failure of tissues to respond appropriately to insulin," said Frank. "Insulin is secreted by the pancreas to move glucose (sugar from digestion of food) into tissues when it's readily available (after meals)."

There are three types of insulin resistance.

"Compensated IR is the most common form; this is when the pancreas secretes more insulin to achieve the same effect (hyperinsulinemia)," he explained. "Uncompensated IR is when pancreatic beta cells (the source of insulin) fail, so blood glucose concentrations rise and insulin levels are variable; this is fairly rare. An extremely rare event is Type 2 diabetes mellitus (caused by insufficient production of insulin or by resistance of target tissues to the effects of insulin), which describes advanced pituitary pars intermedia dysfunction (PPID, or Cushing's). This results in hyperglycemia (high blood sugar) and glucosuria (sugar in the urine)."

Insulin resistance is a part of equine metabolic syndrome (EMS).

Said Frank, "There are three criteria for identifying the horse with EMS: Insulin resistance, prior (founder lines) or current laminitis, and general obesity or regional adiposity (areas of abnormal fat deposition such as a cresty neck or fat pads near the tailhead). It has a genetic predisposition--the 'easy keeper,' or the horse that could stay fat on fresh air, is more likely to have EMS."

Insulin Resistance and Laminitis

There are three theories on why insulin resistance might contribute to laminitis, according to a report by Frank.

  • It decreases the amount of glucose getting into hoof tissue cells, which could starve them and hamper their function.
  • Insulin resistance causes decreased peripheral vasodilation (contraction of blood vessels at the extremities, such as in the hoof). Decreased blood flow to the foot means less nutrition for the tissues and likely less healthy tissues.
  • When adipose tissues reach their capacity for fat storage, they can become stressed and release cytokines, causing a pro-inflammatory state. This could lower a horse's threshold for laminitis.

Thus, a smaller trigger could cause laminitis--less of a carbohydrate overdose, for example. Whatever its mechanism of action might be, insulin resistance has been linked to laminitis.

Frank found insulin sensitivity could even predict laminitis: "Measuring their insulin sensitivity predicted laminitis would occur in 13 ponies, and it actually developed in 11 (85%). This was the first paper saying insulin sensitivity had something to do with laminitis."



Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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