What is insulin resistance?
It is still often equated with EMS (Equine Metabolic Syndrome) by many horse owners, but these are two completely different conditions.
EMS means that my horse is significantly overweight, and in cases of “true” EMS, the excess weight is primarily due to fat.
A horse with fat deposits can also have insulin resistance, but this does not necessarily have to be the case. Similarly, many people with significant overweight also have insulin resistance.
But not all. Insulin resistance (Type 2 diabetes) is a distinct condition that can be accompanied by other metabolic issues.
Many horses with overweight (EMS, Pseudo-EMS) also have insulin resistance – but not all. Many horses with Cushing’s symptoms also experience insulin resistance, as well as many horses with a history of laminitis.
Background
There are also occasional horses that do not show any noticeable symptoms of metabolic issues but can still be insulin resistant. These horses often only become apparent when they develop laminitis “out of nowhere.”
Insulin resistance means that the pancreas responds to fluctuations in blood sugar levels and produces insulin accordingly, but the receptors on the cells that should absorb and store the sugar (mainly muscle and liver cells) no longer respond adequately to the insulin. The result is a consistently elevated or poorly regulated blood sugar level.
In a healthy horse, blood sugar levels are kept constant within a very narrow range for good reason. Both a too low and too high blood sugar level can have fatal consequences for the organism. A chronically poorly regulated blood sugar level can lead to the gradual destruction of capillaries in the tissues, which, in humans, can result in kidney failure and the death of the lower limbs (known as “diabetic foot”).
In horses, it is assumed that the consequences are similarly severe, and currently, many scientific studies are underway to understand the effects of poorly regulated blood sugar on a molecular level, particularly in the hoof area.
Insulin resistance and laminitis
It is already clear that high insulin levels, as well as dysregulation of blood sugar levels, can be involved in laminitis, although we do not yet fully understand the mechanisms down to the last detail.
Often, horses with insulin resistance show no noticeable symptoms as long as they are fed only hay with a moderate sugar content (<10%). However, when a new batch of hay is opened that contains higher sugar levels, or when the grazing season begins with sugar-rich young spring grass, the body is unable to properly regulate this influx of sugar. A laminitis flare-up can be the unfortunate consequence.
Diagnostic Methods
Unfortunately, there is still no reliable diagnostic test to detect insulin resistance in horses. However, one can at least infer a tendency from the so-called “EMS profile.” It is important that the horse is not fasting (i.e., should not undergo a “fasting test”) beforehand, as fasting can lead to false-negative results, potentially masking an insulin resistance due to the prolonged period of hunger.
For the 12 hours prior to sample collection, no sugar- or starch-containing feed should be given (meaning no apples or carrots), but the horse should be allowed to continuously eat its normal hay.
©Charlymorlock / AdobeStock
The blood glucose is then determined from stabilised NaF blood (not from serum, as that gives false-negative results), along with insulin levels (for which the sample must be centrifuged and frozen within 30 minutes after collection, otherwise the insulin breaks down, leading to false-negative results).
Currently, it is assumed that a horse has insulin resistance when both blood glucose and insulin levels are elevated. Type 2 diabetes (essentially the end stage) is diagnosed when a high blood sugar level coincides with a low insulin level.
Unfortunately, both glucose and insulin levels are only momentary measurements. A long-term blood sugar value like HbA1c in humans has not yet been established for horses. However, these values do provide an indication. Therefore, if insulin resistance is suspected, it is better to have an EMS profile test done rather than end up dealing with laminitis at a later stage.
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