Feeding horses with pituitary pars intermedia dysfunction (PPID), also called Cushing's disease, can sometimes be difficult because these horses are often older, overweight or underweight, and may have insulin resistance, with or without recurrent laminitis. To make the best nutritional recommendations for horses with PPID, nutritionists must first consider whether the horse needs to lose or gain weight and whether insulin resistance is present.
Many horses affected with PPID are healthy and relatively asymptomatic other than excessive hair growth and retention, and a mild to moderate increase in both drinking and urination. Horses with PPID and a moderate body condition score without evidence of insulin resistance can essentially be fed as normal horses to maintain body condition and prevent obesity. The need for supplementary feed will depend upon the individual horse's metabolism and available pasture or hay. If extra energy is needed, feeds with moderate carbohydrate content, fed in small meals, are acceptable. Some horses may do well on just a balancer pellet rather than a traditional feed.
Obese horses with PPID should be managed to encourage weight loss by reducing caloric consumption, particularly reducing the intake of nonstructural carbohydrates (NSC). Grazing and hay intake need to be restricted to help with weight loss, and NSC content should be tested and the hay soaked if necessary to lower NSC levels. NSC of hay or other forage should be 12% or less. This plan is intended to also improve insulin sensitivity and reduce the risk of laminitis. A forage or fiber-based diet is ideal for obese horses with PPID to provide gut fill and supply energy in the form of volatile fatty acids. A low-intake balancer pellet is recommended to supply amino acids, minerals, and vitamins that may be missing from forage.
As PPID is primarily a disease of older horses and many are underweight as a consequence of dental problems, this needs to be considered in the design of a feed program. Nutritional recommendations for horses with PPID that need to gain weight are more difficult because the need for more calories must be balanced against the adverse effects of worsening insulin resistance. Pasture intake should be controlled to avoid sudden increases in carbohydrate intake that can trigger laminitis. Increased energy requirements can be met by feeding alfalfa (lucerne) hay or chaff, super-fibers such as beet pulp and soy hulls, or a low- to moderate-NSC feed. Feeds that are higher in fat (greater than 6%) are preferred as they are less reliant on carbohydrates for energy.
Extra fat can be added by top-dressing feed with 0.5 to 1 cup (125 to 250 ml) of vegetable oil twice daily. An alternative is to give about twice that amount of stabilized rice bran each day. Rice bran often contains added vitamin E and selenium, which are valuable contributors to antioxidant status. Because a high-fat diet may worsen glucose tolerance and moderate carbohydrate intake improves insulin response, it is advisable to use a combination of fats and carbohydrates along with additional forage to promote weight gain in thin horses with PPID. The addition of supplemental EPA & DHA, long-chain omega-3 fatty acids prevalent in marine-derived oils, has been shown to improve glucose tolerance in horses fed high-fat diets, therefore supplementing 1-2 ounces (30-60 mL) of a palatable fish oil per day may be beneficial to horses with PPID.