by Eleanor M. Kellon, VMD
Easy keepers and overweight horses and ponies have been around forever. Laminitis has also always been with us, and it is no secret that overweight animals are at high risk. We now know that the vast majority of laminitis cases are caused by high insulin levels — hyperinsulinemia — associated with Equine Metabolic Syndrome (EMS). Does this mean being overweight/obese causes insulin problems?
It might seem that way superficially but the logic is faulty.
Many horses that develop laminitis are overweight or obese. We know that the vast majority of laminitis cases are caused by high insulin levels. The correlation has always been obvious and it didn't take long for an assumption to arise that obesity is a laminitis risk factor and causes elevated insulin. There's just one thing: It's not true.
The 2015 Bamford study, published in the Equine Veterinary Journal, fed horses and ponies a control diet, or one designed to cause obesity by feeding either excess fat or excess fat and glucose. The weight gain did not reduce insulin sensitivity in either group. Dr. Bamford has also clearly shown that insulin responses to oral or intravenous glucose have marked variation by breed in horses of normal weight. You can read all of Dr. Bamford's work in detail in his thesis here.
Selim, et al., 2015, followed two groups of Finnhorse mares on either native pasture, or intensively managed improved pasture. At the end of 98 days grazing, the mares on improved pasture went from a body condition score of 5.5 to 7 and gained 145 pounds but this was not associated with insulin resistance.
If obesity isn't a cause, why is more insulin resistance seen in obese horses, 25 to 50% of high-insulin horses depending on the study, versus 10 to 15% of horses in general? The answer is simple. These horses are resistant to the effect of the hormone leptin, which results in increased appetite and weight gain. Yes, there is an association between obesity and high insulin, but obesity is the result, not the cause.
This is more than just splitting hairs. If you think obesity is a cause then weight control becomes a treatment, even possibly a cure. When you realize it is a consequence, not a cause, expectations for results of weight loss become more realistic. There are many benefits to weight loss and it should be aggressively pursued but it won't make insulin resistance go away. Approximately 50% of horses with EMS are normal weight.
For more background visit Equine Cushing's and Insulin Resistance Group Inc., (ECIR) and see the ECIR Group films Diagnosis and Diet.