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.
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.