This older mare was diagnosed with Cushing’s disease about 10 years ago, but her owners keep her insulin resistance at bay by closely monitoring her diet.

We’re familiar with diabetes in humans, but not many people know that horses can also have diabetes and blood-sugar problems. “Insulin resistance” is sometimes called “pre-Cushing’s” because it was felt that insulin-resistant horses are in the early stages of Cushing’s disease. You may also hear it called “equine metabolic syndrome,” because there are similarities with human insulin-resistant conditions. It has only recently been recognized that horses can be insulin resistant without having a pituitary tumor (Cushing’s disease).

What Is It?
Insulin is a hormone secreted by the pancreas. It signals the body’s cells to take in glucose, “blood sugar.” Every cell has a minimum requirement for glucose, which is the body’s major fuel. The skeletal muscles consume most of the glucose in your horse’s body.

Signs Suggestive of Insulin Resistance

Family history of easy weight gain Family history of laminitis Easy weight gain beginning soon after the horse stopped growing Gains or holds weight on much less feed than other horses Fatty crest on the neck (Note: This is never normal. It may very well be extremely common in some breeds, but it’s not normal.) Fat deposits at the base of the tail or elsewhere on the body, may have a dimpled appearance (like cellulite) Bulges above the eyes in the area that is normally hollow (supraorbital fossa) History of grass-related laminitis History of laminitis when pregnant

Insulin resistance (IR) is exactly like it sounds — the cells are resistant to the effects of insulin. To compensate for this, the pancreas has to put out an abnormally large amount of insulin to get the job done. If the pancreas is unable to produce enough insulin, the blood sugar rises abnormally high. At that point, insulin resistance becomes diabetes.

It’s very important to realize that an insulin-resistant metabolism is not a disease per se. It has allowed many of the very hardy breeds to survive under harsh conditions. It only becomes a problem when they are fed inappropriately.

How Do Horses Get It?
It’s been known for more than 30 years that many ponies are naturally insulin resistant to some degree. This gave them a survival advantage in the wild because they would gain weight very easily when food was available and use that fat reserve to sustain themselves when there was less to eat.

Some people are the same way, and this tendency for easy weight gain was dubbed “the thrifty gene” by some human researchers. Several genetic markers have been identified in people. No genetic work has been done in horses yet, but evidence is accumulating that the easy keeper breeds are more likely to be insulin resistant.

Overfeeding a horse to the point it becomes obese can also result in insulin resistance, although not all fat horses are insulin resistant. Cushing’s disease, a condition common in older horses, occurs when a tumor has grown in the pituitary gland in the brain and puts out large amounts of adrenocorticotropic hormone (ACTH). ACTH in turn makes the adrenal gland secrete the hormone cortisol. Cortisol blocks the action of insulin on cells, creating insulin resistance. Not all horses with Cushing’s disease are insulin resistant, at least initially, but that condition does put them at high risk.

IR Consequences
IR initially causes horses to be overweight, if not obviously obese. Many people equate a horse being “round” with being well cared for and healthy, but this is not the case. The extra weight puts unnecessary stress on your horse’s back, joints and heart.

IR also causes your horse’s body to react with exaggerated inflammatory responses. This combined with a tendency for blood vessels to spasm easily is thought to be why these horses are more prone to laminitis. In the advanced stages of IR, the horse actually begins to lose weight and muscle mass. His cells are “starving” for glucose that he is unable to get into them.

Ponies by their hardy nature are prone to insulin resistance, so limit their access to grain. Note the distribution of fat in the hindquarters, a characteristic often seen in insulin-resistant horses.

Feeding more grain only makes things worse. Fat doesn’t help either. In fact, his body may already be mobilizing large amounts of fat, even to the point that cholesterol and triglyceride levels are elevated and the liver becomes loaded with fat.

Diagnosis
Early insulin resistance can only be diagnosed for sure with specialized blood tests, such as tubing the horse with glucose to see the insulin response or intravenous infusions of glucose and insulin. Later cases can be detected simply by checking the horse’s insulin.

Don’t give your horse any grain on the day of testing, but grass hay is OK. A chemistry screen with glucose and triglycerides should be checked at the same time. If a horse is showing the signs of insulin resistance but insulin is still testing within the lab’s normal ranges, it’s wise to assume the horse does have IR and manage him accordingly.

Treatment
A variety of drugs are available for people with insulin resistance, but none come even close to having the effectiveness of exercise and diet control. Those drugs haven’t been tested in horses, but the good news is that exercise and diet also work extremely well.

NO-NO’s

This is the part that makes most owners cringe — the list of things they have to stop feeding their insulin-resistant horse. Just keep reminding yourself that it’s bothering you a lot more than the horse, who’s likely quite content with the generous hay he’s getting. The things on this list should be avoided entirely. Carrots — Believe it or not, they’re high in sugar. Apples — Peels are OK, but not the fruit itself. Other fruits — Small bits of plums, prunes or cherries are OK, otherwise no. Grain — No grain of any kind, including oats, corn, barley — whole or in a pellet. Check all your supplements too, to make sure they are not in a grain base. Horse treats made with these ingredients. Human food — no cookies, bread, doughnuts, etc. Pasture — Until the horse’s insulin is well within normal, and any laminitis resolved, grass must be avoided. The sugar level in live grasses varies widely and even over the space of a day’s time. There’s just no way to be sure the sugar will be low enough. Horses can still be turned out, but only with a taped-over muzzle that doesn’t let any grass get close to their lips. Once the horse is back to a normal weight and insulin level, he may be able to tolerate a little grazing during the low- risk months of July and August, but you will have to watch very carefully for any signs of foot soreness, weight gain or changes in the neck crest.

Hard, cresty necks are not a conformational breed characteristic, but can be a sign of insulin resistance. Some of the cold-blooded breeds metabolize food differently from hot-blooded horses and thus are more prone to the condition.

When the horse exercises, glucose is able to get into his muscles by pathways that do not require insulin. This effect lasts as long as 24 hours. By making sure the horse has formal exercise every day, you decrease the need for insulin. He needs 20 to 30 minutes of nonstop exercise to get this effect.

Diet is as important for these horses as it is for a diabetic person. Controlling the diet is the key to getting the horse to a normal weight and avoiding or treating laminitis. Cheating is like giving a chocolate bar to a diabetic child.

Owners usually have more trouble sticking with the diet than the horses do. Don’t feel sorry for your horse if you take away treats and grains. Think how much healthier he’s going to be. Above all, remember that nothing is worth the pain of laminitis.

The good thing about a diet is that your horse will have a lot to eat. These horses aren’t fat because of how much they eat; it’s what they eat. If you feed the horse the way he’s designed to be fed, he won’t be fat.

The cornerstone of the diet is a grass hay with low sugar and starch content. We don’t normally think of hays as having sugar or starch, but they do. The grain hays, like oat hay, are usually very high. So is Brome. Bermuda or a native prairie grass is usually low. Timothy and Orchardgrass hays vary widely. Alfalfa is lower in sugar and starch than many grass hays, but some horses are sensitive to it for unclear reasons and may develop laminitis. It’s best to limit alfalfa or avoid it entirely if there’s any suspicion the horse may not tolerate it well.

Look for a hay that’s likely to be safe, then have it tested to be sure the NSC (NonStructural Carbohydrates) level is low and mineral levels appropriate (see sidebars).

The good news is that once you have located a low NSC hay, the horse can be fed as much as 2% of his bodyweight/day (20 lbs./day for a 1,000 lb. horse). Weigh the horse, or use a weight tape, every two weeks. As weight drops, adjust the hay accordingly to provide 2% of his body weight until the horse has reached a healthy weight, with a body condition score between 5 and 6. (See January 2004 for a description of how to evaluate your horse’s body condition.)

Hay Testing

Hays should be tested for sugar, starch and mineral levels. Ideally, you want a hay with an NSC (NonStructural Carbohydrate), which is the total of sugar and starch combined, less than 10%. Some IR horses can tolerate 12% or even 15% NSC, but many can’t, so search for the lowest NSC hay you can find. Most hay-testing services don’t provide NSC testing, but you can get it done by sending your sample to either Litchfield Laboratories, www.litchlab.com, or Equi-Analytical, www.equi-analytical.com. Request the lab to test for sugar and starch first. If the level is low enough, tell them to go ahead with mineral testing as well: profile 603 from Equi-Analytical or the Equus Plus profile from Litchfield.

At this point, hay can usually be given free choice as long as the horse is being exercised regularly. Horses confined to small areas and not being worked (for example, if they are recovering from laminitis) may still need to have the hay restricted to an amount that doesn’t cause them to gain weight.

Another staple in the IR horse’s diet is plain beet pulp, no molasses added. Plain beet pulp is actually lower in sugar than most hays. You can substitute soaked beet pulp for any grain the horse may have been receiving and can use it as the carrier for needed vitamins and minerals. Because soaked beet pulp can hold as much as four times its original weight in water, it becomes a large and satisfying “meal.” Spice it up by adding 2 oz. each of rice bran and ground stabilized flax, and you have the IR diet. Salt should also be available at all times.

What to Expect
After you institute the necessary changes to the horse’s diet and lifestyle, improvements usually occur rapidly. On a low NSC diet, the horse can eat more and still lose weight down to a healthy level. The pain of active laminitis begins to let up within a very short period of time, days at the most.

Bottom Line
An insulin-resistant metabolism shouldn’t be considered a disease. It’s actually a gift that allowed your horse’s ancestors to get by on sparse diets. Everyone enjoys spoiling and treating their horse a bit, but it’s just not worth it if these habits are making him sick. Good things start to happen when you feed the horse like a horse. He’ll be trimmer, sounder, more energetic and healthier. PH*