Feeding Horses With Endocrine Disorders

Nutritional management revolves around weight loss and low-sugar and -starch diets

The horse’s body functions like a harmonious orchestra. Think of the endocrine system, which includes organs such as the hypothalamus, pituitary gland, and thyroid gland, as that orchestra’s conductors. By secreting signaling chemicals called hormones (your hypothetical batons) into the bloodstream, they can direct the entire orchestra or just individual musicians—specific organs—to perform certain tasks. The results? A pitch-perfect symphony.

Now, think of endocrine disorders as resulting from misdirected musicians. When organs and glands over- or under-secrete hormones or when tumors affect an organ, cacophony happens. The two most common equine endocrine disorders are pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease) and equine metabolic syndrome (EMS).

What is nutrition’s role in the development and management of these disorders and, more importantly, in reducing a horse’s risk of developing secondary diseases such as laminitis? Let’s take a look.

Similarly Different Disorders

As the name suggests, PPID involves the horse’s pituitary gland. The condition affects adult horses of all ages, but statistical data reveal that the odds of developing clinical signs associated with PPID increase by approximately 20% per year in equids older than 15. In affected horses, the pituitary gland becomes enlarged, releasing large amounts of adrenocorticotropic hormone (ACTH) and other pituitary hormones that signal the adrenal glands to produce cortisol (also known as the “stress” hormone). Problems arise as a result of this excessive hormone production, including recurrent episodes of the hoof disease laminitis; a long, curly hair coat; failure to shed out fully; loss of muscle mass over the back and hindquarters; and polyuria (excessive urination) and polydipsia (excessive thirst). Additionally, PPID exacerbates insulin regulation issues for genetically predisposed horses or ponies.

Equine metabolic syndrome occurs when endocrine and metabolic abnormalities combine, often resulting in laminitis. The condition is classified as an endocrine disorder because it affects blood concentrations of the hormone insulin, which the pancreas releases in response to a meal and functions in a normal horse to signal body cells to take up and store blood sugar for later use. Increased insulin release, decreased insulin clearance, and insulin resistance (IR, a decrease in tissue sensitivity to insulin) contribute to excessively high blood insulin concentrations (hyperinsulinemia). Veterinarians refer to these insulin problems collectively as insulin dysregulation (ID). The Equine Endocrinology Group (sites.tufts.edu/equineendogroup) defines EMS characteristics as ID, abnormal fat distribution—specifically on the crest of the neck and around the tailhead—resistance to weight loss (i.e., being an “easy-keeper”), and a history of or high risk for developing laminitis. In fact, researchers have been able to induce laminitis in apparently healthy ponies by infusing insulin and glucose intravenously.

Equine metabolic syndrome appears to have both an environmental and a genetic component. “Certain equines, such as ponies and Miniatures, have a higher genetic susceptibility to develop EMS,” says Nicholas Frank, DVM, PhD, Dipl. ACVIM, professor and department chair at Tufts University’s Cummings School of Veterinary Medicine, in North Grafton, Massachusetts.

Researchers at Virginia-Maryland College of Veterinary Medicine found EMS to be more common in pony breeds, Morgans, Paso Finos, Arabians, and Warmbloods. And in a 2016 study Danish researchers found that cold-blooded-type ponies, such as Shetlands and Welshes, were at an increased risk of developing laminitis. All these types of horses tend to be more metabolically efficient and can become obese starting at a young age.

Environmental factors such as grazing large lush pastures, grass changes due to weather, and overfeeding grain contribute to EMS development by raising blood insulin concentrations and inducing obesity and distribution of regional fat, especially on the crest of the neck.

Although PPID and EMS differ in their causes, many similarities exist between the two disorders.

Insulin dysregulation is a key component of EMS that also occurs in some, but not all, PPID cases. Middle-aged horses can have both disorders simultaneously, and researchers believe PPID further exacerbates ID in horses or ponies that already suffer from EMS and increases blood insulin levels. Many researchers also believe that chronically high insulin concentrations can cause laminitis.

Disease Characteristics Present in PPID? EMS?
Insulin dysregulation Some cases Yes
Laminitis Some cases Yes
Increased ACTH Yes No
Long, curly hair coat Yes No
Obesity Some cases Most cases
Muscle loss Yes No
Regional fat deposition Some cases Most cases
Grazing muzzle

Preventative Feeding Measures

Feeding strategies represent just one way to lower PPID and EMS horses’ risk of developing laminitis. Although the exact mechanisms have not yet been identified, we do know that hyperinsulinemia can and does lead to laminitis (Asplin et al., 2007). Therefore, feeding these horses involves managing hyperinsulinemia.

Obesity, consumption of high-sugar diets, inactivity, and PPID can all aggravate hyperinsulinemia. Let’s discuss some best practices for managing each of these risk factors nutritionally.

Obesity

Horses with a body condition score of 6 or above on the 9-point scale are considered obese. Encourage weight loss by reducing caloric intake and implementing an exercise program; your goal is for the horse to reach a body condition score of about 5. Even light exercise can help improve insulin sensitivity while also burning calories, but be sure to limit the exercise of horses suffering from laminitis. Reduce or remove all concentrates from the diet, and replace them with a vitamin and mineral supplement or ration balancer to ensure the diet continues to meet the horse’s nutrient needs. Turn the horse out with a grazing muzzle or in small paddocks of approximately one-third to one-half acre per horse to restrict grass intake.

Feed forage (ideally, hay) at 1.5% of body weight (BW) per day for 30 days, then reassess. If the horse hasn’t lost weight, reduce the hay gradually from 1.5% BW to 1.5% ideal BW. A more drastic reduction to 1.25% BW, followed by 1.0% BW, can help horses lose weight faster, but only go to these extreme measures with a veterinarian’s guidance.

Analyze hay for nutrient content to ensure that nonstructural carbohydrate (NSC; sugar plus starch) levels are below 10-12% on a dry matter basis. If feeding hay above 12% NSC, or if you’re not able to get your hay analyzed, soak hay in cold water no longer than 60 minutes to reduce NSC before feeding it (and be sure to toss the water!).

High-sugar diets

If you must feed your thin horse or hard keeper a concentrate to maintain his weight and body condition, look for one with moderately lower sugar and starch levels (listed as NSC) on the guaranteed analysis. An NSC value below 30% is typically moderate. If the feed tag doesn’t list NSC, look for high starch and sugar grains or additives on the ingredient list and steer clear of them.

“You basically want to avoid corn, oats, barley, and wheat as much as possible, as they all end up converting (from) starch to sugar,” says Jake Davidson, PAg, PAS, owner of Bar-D Agri-Ventures, in Manitoba, Canada.

General collective terms listed on a feed tag (such as “grain products” or “roughage products”) make it difficult for buyers to know what’s in there, especially in pelleted feed where it’s impossible to see and identify individual grains. If possible, Davidson suggests looking for a high-fiber concentrate with ingredients such as alfalfa meal, rice bran, beet pulp, and soy hulls.

“By weight, beet pulp and oats are about equal in energy, so feed the pulp and skip the starch,” he says.

Fat can be a valuable concentrated source of calories that does not drastically affect glucose and insulin dynamics. Look for a concentrate with at least 6% crude fat, or top-dress a fat supplement to a ration balancer pellet.

Just like you would with obese horses and ponies, have your hay analyzed to ensure NSC content is not too high.

Inactivity

Exercise might improve insulin sensitivity while burning calories to reduce weight. In 1992 Freestone et al. exercised hyperinsulinemic ponies and observed improved insulin sensitivities after just two weeks. In 2002 at the University of Kentucky, researchers exercised a group of obese mares at simply a light trot daily for a week and saw improvements in insulin sensitivity.

PPID

According to Nutrient Requirements of Horses (2007), calorie sources that do not contribute substantial ­quantities of glucose to the bloodstream, such as fat and fiber, appear to be essential for managing PPID horses. If your horse is losing weight or exercised regularly, your goal should be to provide him with the calories needed to maintain body condition or perform work without affecting glucose and insulin concentrations. Start with a diet high in good-quality forage with controlled pasture access. If the PPID-affected horse does not have insulin dysregulaton, you don’t need to make major dietary changes.

Equine metabolic syndrome

With EMS, Frank emphasizes identifying affected horses with a higher genetic risk of insulin dysregulation. “We assess the insulin status of a horse by replicating consumption of a concentrate meal using the oral sugar test,” he says. This involves a three- to eight-hour fasting period followed by oral administration of corn syrup and by blood sample collection at 60 and 90 minutes post-administration. This reveals how insulin responds to sugars versus simply evaluating resting insulin levels. Insulin status can vary from one horse to another; therefore, it’s crucial to determine each individual’s situation, and manage diet as described previously based on BCS.

Skip the Supplements

Many owners look for a supplement they can add to their horse’s diet to help manage insulin sensitivity.

“There are a few key ingredients that may have some benefit to insulin sensitivity, although evidence in horses is lacking,” says Shannon Pratt-Phillips, MSc, PhD, associate professor at North Carolina State University, in Raleigh.

Magnesium and chromium, as well as antioxidants such as vitamin C and E, might have some function counteracting insulin resistance and possibly affecting body weight. Owners have given other nontraditional ingredients as supplements, such as cinnamon, chasteberry, and grapeseed extract, to improve insulin sensitivity, but Pratt-Phillips says scientific evidence regarding their safety and efficacy is lacking and, so, does not support their use in equine rations.

The Road Ahead

In most recent studies involving endocrine disorders, researchers have focused on unveiling physiological pathways leading to laminitis development and potential medicinal support for IR horses. Specifically, several groups of researchers are studying how incretin hormones work. In the normal small intestine, the presence of sugars, amino acids, and fats stimulates intestinal cells to release incretin, in turn signaling the pancreas to release insulin.

In 2013 Frank and coauthor Lisa Tadros, DVM, PhD, theorized that high levels of incretin could signal the pancreas to release excessive insulin, resulting in hyperinsulinemia. In 2016 Chameroy et al. sought to determine if they could detect an incretin hormone called active glucagon-like peptide 1 (aGLP-1) during an oral sugar test in clinically normal and EMS horses. Although they did not see a signficant difference in aGLP-1 concentrations when they compared normal and EMS horses, likely due to individual horse variation, they did notice a trend in EMS horses toward higher percentage increases in aGLP-1. With further ­investigation, these researchers could confirm the importance of managing EMS with diet and, specifically, reducing blood sugar by lowering sugar and starch in the diet.

Take-Home Message

Detecting hyperinsulinemia early, either by performing an oral sugar test or identifying at-risk individuals, is key to managing horses with endocrine disorders and preventing laminitis. Scientists remain vigilant in their search for early, reliable clinical or laboratory criteria for those horses that do not display obvious signs of disease, but for whom we know breed tendencies and grazing on lush pasture can exacerbate hyperinsulinemia. Limiting access to lush pasture, feeding low-sugar and -starch rations, and, ideally, making sure horses are getting most of their calories from fiber, can help keep insulin sensitivity and laminitis risk in check.