Glucosamine in a Cushing's Horse?

Q. As soon as my 20-year-old gelding was diagnosed with equine Cushing's disease, I eliminated glucosamine from his diet. However, he appears to have become gradually and increasingly stiff and short-strided without it. I really think that it helped him, and I'd love to put him back on the supplement if I could safely do so.

Based on current thinking, would it be at all possible to start him back on glucosamine, perhaps at a lower level? Is the risk of something negative occurring a substantial one or a minimal one? If it makes any difference, my horse's only symptoms of Cushing's are a high insulin level and an abnormal cortisol rhythm test. He's not laminitic and has a normal haircoat.

Joyce, via e-mail

A. You ask some great, but also very challenging, questions. That is because minimal research has been done regarding glucosamine and how it affects glucose and insulin in the equine patient. I will share some of my viewpoints with you.

The concerns about glucosamine in Cushing's disease patients have been taken from human medicine, where the supplement has been shown, in some studies, to increase insulin resistance. We must remember that people and horses do not necessarily break down supplements in the exact same manner. The risks of keeping an equine Cushing's disease patient or insulin-resistant patient on daily glucosamine are really not known.

With that in mind, I think it would be reasonable to add glucosamine back into his daily regimen at his prior dose, while monitoring him closely for signs of insulin resistance and uncontrolled Cushing's disease, such as frequent urination, increased drinking, laminitis, etc. This might require more frequent checks of his serum insulin concentration. I would absolutely discuss this first with your regular veterinarian, who knows your horse and his lameness history.

On a side note, based on your description of your horse's signs (or lack thereof) and the past diagnostic workup for Cushing's disease, it is possible that your horse may have insulin resistance, but not Cushing's disease. What motivated you or your veterinarian to run the blood tests last year? Has a drug, such as pergolide or cyproheptadine, been prescribed?

The elevated insulin measurement suggests a state of insulin resistance is present. A one-time measurement of insulin can be helpful as a screening tool in horses with suspected endocrine disorders. To better address the question of insulin effectiveness in his body, I suggest talking to your veterinarian about performing a combined glucose-insulin test. This is a more precise way of evaluating a horse for insulin resistance.

Also, the cortisol rhythm test is an option for diagnosis of Cushing's disease, but it is less sensitive than several other tests. None of these tests is 100% accurate, but I prefer some other options, including the overnight dexamethasone suppression test, a plasma ACTH (adrenocorticotropic hormone) concentration measurement, or a combined TRH (thyrotropin-releasing hormone) stimulation/dexamethasone suppression test. Some good information can be found on the AAEP's Web site and, for your veterinarian, in past AAEP convention Proceedings.

The bottom line is--whether Cushing's disease is present or not--a diet of low nonstructural carbohydrate forage or feed is important to help insulin do its job. If you have not already discussed nutritional changes with your veterinarian, I would suggest doing so.

About the Author

Emily Graves, VMD, MS, Dipl. ACVIM

Emily Graves, VMD, MS, Dipl. ACVIM, is the founder and principal of Equine Consulting of the Rockies in Ft. Collins, Colo.

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