We see a large number of horses in our practice that are overweight and although some of these horses are overfed, most have a syndrome called equine metabolic syndrome. Horses with equine metabolic syndrome were in the past called easy keepers because they gained weight and stayed fat with very little feed. At that point no one knew why this occurred but now we know that these horses are resistant to the effects of insulin. Because of this resistance, horses increase the production of insulin, and increased insulin is why these horses develop laminitis and founder.
Insulin levels can be measured and if they are increased, treatment is initiated by decreasing non-structural carbohydrates in the ration. Horses should be fed hay. A 1,000-pound horse should receive about 15 pounds per day. Ideally, the hay should be tested to make sure it is less than 10% nonstructural carbohydrates. Soaking hay in water can decrease the carbohydrates, and that, is a good idea but we recommend testing the hay after it is soaked to see if it is really below the 10% level. If not, it should not be fed to horses with equine metabolic syndrome.
These horses should also be fed either a vitamin mineral supplement that contains vitamin E or a low-carbohydrate feed designed for horses with equine metabolic syndrome. Because a horse's major source of carbohydrates is pasture, most of these horses will need very restricted time in the pasture or have pasture eliminated altogether. And since it takes a while for horses to lose weight and we are concerned about horses foundering before they lose weight, giving horses a thyroid supplement can increase insulin sensitivity and aid in weight loss.