Understanding Insulin Resistance

What Is IR? Cells in the body can take up glucose, their most critical fuel, by a variety of mechanisms, but the two greatest consumers of glucose —fat cells and muscle cells—are highly dependent on insulin to take up glucose. With insulin resistance, as the name implies, these tissues are not as sensitive to the effects of insulin as in normal horses/ponies, with the result that it takes larger amounts of insulin to do the job of moving glucose out of the blood and into the cells. It’s somewhat similar to the situation of “pre-diabetic” in humans, although horses rarely progress to diabetes. How Big a Problem is This? The 1998 Equine Study performed by the National Animal Health Monitoring System reported that over a one-year study period 13% of the equine premises surveyed reported having a horse with laminitis. Even if only half of those were related to insulin resistance, that would amount to approximately 540,000 horses per year with IR severe enough to cause laminitis. High-Risk Breeds Insulin resistance caused by Cushing’s disease has no breed pattern, but non-Cushing’s-related IR apparently does. Ponies are the highest risk, with breeds remaining most true to their ancestors, such as Shetlands, being at highest risk. Tough, hardy breeds like Icelandics and Mustangs, are also at risk, as are Morgans, Arabians, and often gaited horses.

The first time we discussed insulin resistance was in 1999, when we did a field trial with magnesium supplementation of chronically laminitic and cresty horses and ponies. We wanted to see if there might be any truth to the “folk lore” that magnesium can help with laminitis.

What we saw were amazing, obvious changes in many of the supplemented animals, both in terms of foot comfort and obesity/abnormal fat deposits. These results caused our veterinary editor, Dr. Eleanor Kellon, to wonder if many of the cases of chronic laminitis might be linked to insulin resistance (IR), since magnesium is well documented to often be deficient in people with IR and supplementing it is beneficial to them.

The link between IR and laminitis in ponies was first made back in the 1970s, by Dr. J.R. Coffman. At the time, it was only documented in ponies but even back then Coffman suspected Morgans might also be involved.

For the most part though, vets believed IR in horses was only seen as a complication of Cushing’s disease. In 2002, Dr. Philip Johnson coined the term “equine metabolic syndrome” to describe easy weight gain and high-laminitis risk in horses that were insulin resistant without having a pituitary tumor.

Metabolic syndrome is the term given to humans with IR, a pre-diabetic state that is also associated with changes in circulating levels of cholesterol and triglycerides. This idea met with considerable resistance among veterinarians, and the concept that IR could exist without Cushing’s disease was slow to make its way to practitioners or be accepted by researchers. But 2006 was a pivotal year for research into IR, with some who had been the most vocal critics doing a complete about face and not only using the term metabolic syndrome but showing that IR was indeed a risk factor for laminitis, as Coffman had also shown back in the 1970s by documenting significant differences in insulin sensitivity in ponies that had a history of laminitis versus those that did not.

Coffman had also reported on lipid/fat abnormalities in ponies with IR, and noted, as have many others, that IR is a component of the often fatal hyperlipidemia syndrome in ponies and miniature horses or donkeys. Changes in fat metabolism aren’t as dramatic in full-size horses, but Dr. Nicholas Frank, at the University of Tennessee, reported in a study published this year that obese insulin-resistant full-sized horses do indeed have abnormal levels of fats and cholesterol in their blood.

Obesity And IR
The role of obesity in insulin resistance has not been completely sorted out. The prototype insulin-resistant horse or pony is a very easy keeper, overweight, with abnormal fat deposits like a prominent fatty crest. It’s also true that even a normally insulin-sensitive breed, like the Thoroughbred, will show a decline in insulin sensitivity if they become obese, but it’s not entirely clear whether it is the obesity per se that causes it, or the high grain and often high fat diets they must be fed to make them overweight. In any case, a poll of 605 owners of insulin-resistant horses and ponies, members of the 4,460 member Yahoo Equine Cushing’s and Insulin Resistance Group (http://pets.groups.yahoo.com/group/EquineCushings/) showed that the most consistently found symptom, in 29% of the cases, was known or suspected laminitis/foot pain. This was followed by abnormal fat deposits in 23%, while only 18% responded that the horse was actually overweight at the time of diagnosis. The bottom line is that while most insulin-resistant horses are at least recognized to gain weight easily, preventing excessive weight gain is not enough to prevent insulin resistance or laminitis. Furthermore, not every overweight horse is insulin resistant, at least not to the degree that can be easily detected on screening tests.

Exercise, on the other hand, is a potent tool for helping horses with insulin resistance. Regular exercise, even a single bout of exercise, significantly improves insulin resistance and glucose handling. There have been many cases of Arabians who were lean, kept on pasture and fed grain when they were in endurance training, but begin to gain weight rapidly, even develop laminitis, when exercise is stopped for even a few weeks.

Diagnosis
Researchers use a variety of complicated tests involving oral or intravenous glucose administration, intravenous insulin, and multiple blood samples to investigate and confirm insulin resistance. However, at the simplest level, all that is necessary is to document higher-than-normal levels of insulin in a horse that has not been recently fed a high simple carbohydrate meal. Complete fasting should not be done, since insulin resistance is a “dynamic” condition, an abnormal response to feeding.

On the day of testing, the horse should be fed nothing but with low- NSC (nonstructural carbohydrate) or well-soaked hay, if the NSC is not known. A test for glucose should be drawn at the same time. Although glucose is rarely elevated above normal in IR horses, the ratio of insulin and glucose to each other is abnormal. In other words, you will find the horse has produced more units of insulin per units of blood sugar than a normal horse.

Researchers have developed some logarithmic equations to describe insulin resistance in horses, but an easier approach widely used in human IR, and found to be just as sensitive as the more complicated equations in people, is the G:I ratio, glucose divided by insulin. The details have yet to be worked out in horses for these numbers, so we can only give you guidelines. There also may be differences by breed, just like there are differences by race in people. That said, when glucose is within normal ranges G:I ratios below 4.5:1 likely represent uncontrolled insulin resistance; 4.5:1 to 9:1 compensated insulin resistance, and 10:1 or greater is normal insulin sensitivity.

Treatment
The best news about insulin resistance is that once you know what you’re dealing with you can control it. If the horse is over 13 years old, she should also be screened for Cushing’s, since diet alone may not be able to take care of the problem without treatment of the underlying high-cortisol output. If you’re fortunate enough to not be dealing with laminitis, daily exercise is the place to start. An absolute minimum of 30 minutes/day, mostly trotting and cantering (if the horse is fit enough to tolerate this) should be done. If you can’t ride, longe the horse or find someone to do it for you.

If you can get enough consistent exercise into the horse, you may not have to make any other changes, but it’s wise to eliminate or greatly reduce any concentrated carbohydrate sources until the horse is a normal weight, has lost any abnormal fat deposits and is fit. The lower the NSC of the diet, the more liberally you will be able to feed your horse. When exercising, free-choice low-NSC hay with appropriate supplementation usually works out well.

If exercise is absolutely out, strict diet control is essential.  The cornerstone of treatment is a low NSC diet, “low” meaning 10% or lower for most horses. Since insulin resistance in all other species is associated with oxidative stress and some key mineral deficiencies, as well as overabsorption of iron, which worsens IR and inflammatory tendencies, careful attention to mineral intakes is also important. The recommendations below were developed by Dr. Eleanor Kellon and published in the February 2004 issue of Compendium on Continuing Education for the Practicing Veterinarian. They will also appear in the soon-to-be-released update of the textbook Equine Podiatry.

Recommendations
• Base diet: Grass or alfalfa/grass mix hay (Note: Some laminitic horses are sensitive to alfalfa), with NSC 10% or lower.

• Molasses-free beet pulp as a carrier for supplements. (Note: Some brands do not list molasses on the bag, but contain molasses anyway. Beet pulp should always be thoroughly rinsed before soaking, until the rinse water runs clear. This removes both surface molasses and any contaminating soil, which is high in iron.)

• 2 to 6 oz./day of  ground stabilized flaxseed for essential fatty acids. Its ratio of omega-3 to omega-6 most similar to grass, and high in anti-inflammatory omega-3 fatty acids.

• 2000 IU of vitamin E.

• Intake of all minerals a minimum of 150% of current NRC (National Research Council) requirements with the following ratios:

• Calcium:Phosphorus:Magnesium 2:1:1.

• Iron: Copper 4:1 (maximum 10:1 if the horse is not iron overloaded).

• Copper:Zinc:Magnesium 1:3:2.5.

Start by feeding low-NSC hay at 2% of the horse’s ideal body weight, or 1.5% of the horse’s current body weight, whichever is higher. Check weight with a tape weekly and adjust hay as needed. Horses that are highly immobile because of pain may need to be fed slightly less. Be careful, though, as starvation diets may cause more rapid weight loss but will not get rid of abnormal fat collections and may make the horse more insulin-resistant.

There are currently only a few commercially available feeds that we’re sure are both low enough in NSC and correctly mineral balanced. These are:

Ontario Dehy Balanced Timothy cubes* (www.ontariodehy.com, 877-289-3349): 10% or less NSC, minerals added as dictated by analysis of the hay in the pellets. Contain both hay and beet pulp. Can be used as a grain replacer or as a complete feed. All you need to add is ground stabilized flax and vitamin E. Feed with 1 to 2 oz/day of iodized table salt. Widely available in the United States. Hays for every batch are analyzed.

Sterett Brothers Low Carb Complete Feed*  (www.sterettbroshayandfeed.com, 425-822-9011): Pelleted hay with less than 10% NSC. Sold with separate balancing minerals in a flax base. Grain replacer or complete feed. Each batch of pellets is analyzed. Add only vitamin E and iodized salt. Availability limited to the West Coast.

Triple Crown Safe Starch* (www.triplecrownfeed.com, 800-267-7198): Chopped low-NSC hay with pelletized vitamins and minerals. Grain replacer or complete feed. Some additional ground stabilized flax and salt round out the diet. Check availability in your area.

*Editor’s Note: Dr. Eleanor Kellon was consulted by Ontario Dehy and Sterett Brothers to do the mineral formulations for those feeds and invited to comment on the Triple Crown Feed's formula.

Spillers Happy Hoof (www.spillersseminole.com, 800-683-1881): A blend of chopped hays and other low NSC ingredients that now contains no molasses and has safe NSC levels. Magnesium, zinc and vitamin E are lower than we would like to see, though, and iron and manganese are not specified/guaranteed. Also need to add flax and salt. Limited to the south.

Poulin Carb Safe (www.poulingrain.com, 800-334-7791): This is another low-NSC option, based on hays, beet pulp and flax. Again, mineral guarantees are incomplete, but vitamin E and disclosed minerals are good. It’s available is mainly in the New England region.

Otherwise, there are a host of feeds being advertised as “safe” or “low” carbohydrate that contain combined sugar and starch levels ranging from the teens to the 20 or 30+%. This level is lower than a straight grain mix usually not low enough for an insulin-resistant horse.

There are also many manufacturers using the term NSC when what they are really talking about is starch, not both sugar and starch. Be wary of any feed claiming to be safe that contains molasses, significant amounts of rice bran, grains or grain products (except distiller's or brewer's grains), or flours.

Restrictions
No grain, carrots, apples, or horse treats. A few sunflower seeds or alfalfa pellets can be used.

Hay or a hay-based complete feed is the vast bulk of the diet. To know the hay is safe, and how much of each mineral the horse requires, you need to have a hay analysis. To make this realistic, obviously you would need to buy your hay in bulk. If this isn’t the case, you have a few options:

1) Talk to your hay dealer to see if it would be possible to have a several-month supply of hay earmarked for you only, so that testing is a reasonable option.

2) Locate a nearby farmer who will rent you loft space (usually quite reasonable) so that you can purchase a large load and store it.

3) Convert some unused space to hay storage (weather-tight outbuildings, empty stalls, etc.).

4) Convert the horse to 100% diet of a known safe feed.

If worse comes to worst, soak all your horse's hay and research the average mineral profiles for the hay type and the region where it was grown. The state university’s agriculture department is the place to start.

Bottom Line
Getting started on the diet changes is the hardest part of managing a horse who is diagnosed as truly insulin-resistant. Like any other management change, once you get rolling it will become routine.

The important thing to remember is that halfway measures will get halfway results. This is a full lifestyle change for these horses. Most people only become committed to properly caring for IR horses after they have run into problems with laminitis, but if they  were aware of how to deal with it and instituted a correct diet and exercise program, the suffering could have been avoided.

The extra effort to really do it right is well worth it. We’ve seen horses that have been struggling with weight problems and repeated episodes of laminitis for many years that have been able to trim down and be put back into active use with the correct diet, management and hoof care.