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Equine Metabolic Syndrome
A well recognized syndrome of obesity, abnormal fat deposition, insulin resistance and increased incidence of laminitis has long existed in horses much younger than those typically diagnosed with equine Cushing’s disease. Formerly referred to by some as hypothyroidism (thyroid dysfunction in horses is considered to be a subset of other syndromes, not a disease entity of its own), this disorder has recently been renamed Equine Metabolic Syndrome. These horses typically cannot be diagnosed with, nor do they respond to treatment for Cushing’s disease.
The term metabolic syndrome was borrowed from human medicine, noting the similarities between affected horses and human metabolic syndrome, which is characterized by:
1. Obesity, especially accumulation of fat within the abdomen
2. Elevated blood triglycerides and decreased high density lipids (good cholesterol)
3. Insulin resistance and elevated blood glucose
4. High blood pressure
Although differences exist between the human and equine syndromes, obesity appears to be the common thread in both horses and people with metabolic syndrome. Fat cells were previously considered inert substances that functioned simply as a form of energy storage. However body fat, especially that stored in the abdomen, contains cells that are very active metabolically and hormonally. When present in excessive amounts, fat cells can trigger insulin resistance and elevated blood sugar, predisposing horses to laminitis.
Metabolic syndrome is often recognized when laminitis develops in a fat horse lacking other founder triggers. Observance of obesity, often with abnormal fat deposits in the neck, rump, tail-head and sheath is cause for suspicion. Additional testing of insulin, glucose, and thyroid hormone can be helpful in establishing a diagnosis.
The cornerstone of treatment for metabolic syndrome is weight loss via diet and if possible, increased exercise. There must be strict limitation of soluble carbohydrates in the diet. A grass hay only diet should be maintained until the horse reaches its ideal weight. Sources of soluble carbohydrates such as fresh grass, grain, sweet feed, apples, carrots, etc. must be completely eliminated to resolve insulin resistance.
Treatment with levothyroxine may be used to facilitate weight loss, especially for laminitic horses that cannot exercise. Other supplements such as chromium, magnesium and cinnamon have shown to be beneficial in human metabolic syndrome; however their effects have not been evaluated in horses, when fed at recommended doses, they are unlikely to be harmful.