Managing Equine Cushing's Disease
Certain feeding strategies can help your horse weather equine Cushing's disease.
Your horse has equine Cushing's disease, for which there's no cure. You've consulted your veterinarian for advice and have a good handle on routine care and medical management, but you might still be missing a key component to his care: his diet. Nutritional management can make a measurable difference in how your Cushing's horse thrives.
First, Some Background ...
To understand the science behind nutritional management of Cushing's, it's important to first grasp the condition's essential mechanisms. Cushing's is more correctly referred to as pituitary pars intermedia dysfunction, or PPID--a condition of the pituitary gland, which is located at the base of the brain. In the healthy brain, the hypothalamus produces a neurotransmitter called dopamine, which regulates the pituitary gland's production of adrenocorticotropic hormone (ACTH). A tumor in or hypertrophy (enlargement) of the pituitary gland exerts pressure on the hypothalamus, causing a reduction in dopamine secretion; consequently, the pituitary pars intermedia cannot stop releasing ACTH.
Since ACTH stimulates the adrenal gland to produce the hormone cortisol (which at high levels elevates blood sugar), uncontrolled ACTH release can cause tissues to become insulin resistant (IR), or unable to control blood sugar levels with normal amounts of the hormone insulin. Thus, insulin levels rise in the bloodstream. The result is often laminitis--inflammation of the sensitive laminae that connect the horse's hoof to the coffin bone--but the exact mechanism is still unclear.
As horses age, obesity and insulin resistance might contribute to PPID development. But this condition also can occur in younger horses, leaving us to wonder if we may be at fault in the way we manage our horses. We know that many years of high sugar/starch diets and physical stressors ,such as gastric ulcers, vaccinations, and pain, lead to an imbalanced hormonal response. Such stressors initiate the downward cascade of elevated cortisol secretion, insulin resistance, and laminitis.
The condition can affect any breed. "Many horses are kept in high-stress situations that contribute to the cortisol release and adrenal stimulation," explains Joyce Harman, DVM, a practitioner in Flint Hill, Va. "If it is possible to decrease the environmental stress, the horse will benefit greatly. Many of these horses may be past their high-stress years, and the current owner may not be showing heavily or keeping the horse in the high-stress environment, yet (the horses) still have Cushing's. These horses are experiencing (effects from) the previous lifetime of stress. Cushing's is the result of many years and many factors."
Horses with PPID usually become either overweight or underweight. They generally exhibit regional adiposity (seen as fat deposits along the shoulders and tailhead, a cresty neck, etc.) resulting from ¬insulin resistance. Thus, reducing circulating insulin levels is key to managing the diet and, consequently, the condition.
Since the body releases insulin in response to elevated blood glucose levels, avoid feeds that are high in sugar and starch, measured by feed companies as water soluble carbohydrates (WSC), ethanol soluble carbohydrates (ESC), and starch.
To evaluate the total amount of sugar and starch in a diet, calculate the percent of nonstructural carbohydrates (% NSC) from the above values: % NSC = % WSC + % starch. While a full discussion of these measurements is beyond the scope of this article, definitions can be summarized as:
- WSC are comprised of simple sugars and fructan (chains of fructose molecules)
- ESC are mainly simple sugars
- Starch is made up of a long chain of sugar molecules
Blood glucose levels and, hence, insulin levels, tend to rise when horses consume feeds high in ESC and starch. Historically, researchers have thought that hindgut microbial fermentation of fructan increases laminitis risk. However, fructose molecules causing a gradually increased insulin response might actually be the mechanism responsible. Kathryn Watts, BS, plant scientist for Rocky Mountain Research and Consulting, in Center, Colo., explains that "while fructose may not cause an immediate insulin response like glucose, preliminary findings show that fructose may exacerbate IR in horses, possibly via a toxic effect on the liver. It's similar to the implication by human researchers that the increased use of high fructose corn syrup may be correlated with the epidemic of obesity and diabetes in humans. Fructan may be hydrolyzed (via a reaction with water) or fermented to fructose within the equine digestive tract, providing a mechanism for previously noted concerns with the sugar fructose."
Consequently, it is best to have your hay analyzed so you know what your Cushing's horse is eating. Look for forage that has an NSC value of less than 12%, and preferably less than 10%. If your hay's value is higher than this, soaking it for 30 to 60 minutes will remove some of the sugar (although remember to discard the water).
The above left table compares the average % NSC (dry matter basis) of common feeds (adapted from Equi-Analytical Labs' data and Watts' work). These levels can vary depending on the conditions under which the forage was cut, including nighttime temperatures, sun exposure, rainfall, and even fertilization with nitrogen.
Cereal grains such as oats, corn, and barley are very high in starch. Notice that among the different types of hay, alfalfa averages the lowest % NSC, making it an excellent choice for the insulin-resistant horse (but test the batch before feeding). Grain hay, such as oat and barley hay, and rice and wheat bran might have relatively high NSC levels, making them inappropriate to feed to an IR horse unless they test within a safe range. Unsweetened beet pulp is an excellent feed with only 12.3% NSC.
Depending on the time of day, it might be safe to allow a PPID-affected horse to graze--the key is whether the pasture plants have been stressed. Cold, drought, lack of proper fertilization, and overgrazing all can significantly increase grasses' NSC content. When the sun is out sugar and starch levels increase (via photosynthesis) in grasses, with levels peaking in the late afternoon on a sunny day. The grass uses this fuel for growth during the dark hours, and by morning the levels are at their lowest if conditions have been good for growth. So when nighttime temperatures are mild (above 40°F), the safest time for a horse to graze begins approximately four hours after sunset and before 10 a.m. the next day. However, when nighttime temperatures drop below 40° for more than five hours, followed by a sunny day, plant stress increases as does NSC content, and owners should be cautious about allowing PPID-affected horses to graze.
Seasons affect NSC content as well. Spring and fall grasses tend to contain more sugars and starch because of the colder night temperatures. Even brown grass often presents some green at the base, which is high in sugar and starch. If it hasn't rained in a while your grass will look dried out, but be careful: Dry grass can actually have higher NSC levels than long, lush-looking grass.
Owners often use grazing muzzles to limit pasture intake, but if a grazing muzzle is frustrating to your horse, limit its use to no more than three hours. Restricted chewing time can also be stressful (saliva production is neccesary to neutralize the constant flow of acid in the stomach, and less chewing means reduced saliva amounts).1
Grass Hay 24/7
If your Cushing's horse is overweight, avoid restricting his diet entirely. You can remove concentrates, but restricting hay is the worst thing you can do, as hunger will stress your horse. Plus, elevated ¬insulin levels prevent your horse from burning body fat, so you defeat your purpose. Instead, provide free-choice low % NSC hay.
Underweight Cushingoid horses, however, need more calories than hay can provide. In this case consider a low-starch feed, such as beet pulp (molasses-free), or a commercial feed with a low NSC level (again, not exceeding 12%). Additional fat such as rice bran oil, flaxseed meal/oil, or chia seeds are safe calorie sources. Rice bran (not the oil), though high in fat, is not the best calorie source for these horses since its NSC level is approximately 24%.
Magnesium and chromium might help reduce the typical cresty neck and fat pads by reducing insulin levels in the bloodstream. Many horses do not consume enough magnesium; marginal deficiencies can be easily corrected by adding some (e.g., 5,000 mg per 250 lbs of body weight) to the diet, note researchers.
Research on chromium supplementation is less conclusive. Involved in carbohydrate and fat metabolism, it might work with insulin to increase body cells' glucose uptake and, thereby, reduce the need to produce extra insulin. According to Bonnie R. Rush, DVM, MS, Dipl. ACVIM, of Kansas State University's College of Veterinary Medicine, "Dietary supplementation with chromium (5-10 mg/day) will improve peripheral insulin function and glucose tolerance. Serum glucose and insulin concentrations may be reduced, potentially to normal values, weeks to months after initiation of exercise, dietary management, and dietary chromium supplementation."
Researchers at the University of Florida concur: "Supplementation with chromium tripicolinate has been shown to increase glucose uptake during a glucose tolerance test in normal yearlings." Furthermore, recent study results from Colorado State University revealed improved insulin sensitivity when mares received a supplement containing chromium. Conversely, authors of studies at the University of Tennessee have not found chromium supplementation to alter resting insulin levels or insulin sensitivity in laminitic obese horses.
Overall, because horses' maximum tolerable chromium level is high (300 ppm), it might be worthwhile considering supplementation for insulin-resistant horses.
Omega-3 fatty acids help the body regulate insulin levels. Add flaxseed (meal or oil) or chia seeds (¼ cup of meal or chia seeds, or 1 tablespoon oil, per 400 lbs of body weight) to provide the essential omega-3 fatty acid called alpha linolenic acid.
Joint supplements containing glucosamine have led to a rise in insulin in humans taking high doses. Evidently, glucosamine confuses the tissue cells into thinking they have enough glucose. So, glucose from other sources cannot enter the cells. The result is increased blood glucose, not from glucosamine, but from the diet in general, leading to elevated insulin.
While we don't know if the same thing happens in horses, if your horse has Cushing's you might consider using a joint supplement that doesn't contain glucosamine.
Iron In human-based studies, excessive iron in the diet has exacerbated insulin resistance. This might also be the case for horses, so until equine research catches up, you might avoid iron-containing feeds and supplements. Your horse needs this mineral, but forage supplies plenty naturally.
Veterinarians report that Cushing's disease is becoming more prevalent. One reason is horses are living long enough for the disorder to manifest, but increased stress, high-starch and high-concentrate diets, and equine obesity might also have contributed to this trend. By managing our horses' diets, we not only can manage this disorder and prolong our horses' lives but also help reduce overall disease incidence.
1. Andrews, FM, 2003. Equine gastric ulcer syndrome. American Association of Equine Practitioners.
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