Insulin resistance [IR], which is a failure of insulin responsive cells to take up glucose under normal levels of circulating insulin, affects an estimated 12 to 15% of horses. Insulin resistance is typically (but not always) associated with easy weight gain. Abnormal fat deposits are usually evident. The most important consequence, however, is a risk of developing laminitis.
In a well-intentioned effort to protect horses from laminitis caused by uncontrolled IR, many things have been proposed as causing IR. These include feeding grain, improved strains of pasture grasses, insufficient exercise, obesity and a host of proposed environmental factors including pesticide and herbicide exposure.
Arguing against the idea there are external causes is the observation that there are very clear breed risk factors for IR, with it being extremely rare to nonexistent in some, like Thoroughbreds, but very common in others, such as Arabians. Furthermore, the Virginia Polytechnic large long term study of IR in a herd of mixed breed ponies also found strong evidence of a genetic component when they did pedigree analysis.
Most recently, a study just released in the Journal of Animal Science searched the genome of the Arabian horse and identified a genetic risk locus where markers correlated with laminitis, high blood insulin, abnormal indirect measures of glucose metabolism and potential obesity. The findings were strong and confirm that IR has a genetic cause.
It is also true that feeding an inappropriate diet, no exercise and letting the horse get obese will indeed worsen IR. So, if these are factors that need to be watched closely what difference does it make whether they actually cause it or not? It makes a big difference.
If someone has a life-threatening strawberry or peanut allergy, they must avoid those things at all costs but can otherwise live a perfectly normal life if they accept and respect that restriction. The allergy does not go away, it simply is managed by avoiding the dangerous trigger. The same is true for an insulin resistant horse.
On the individual level, if a horse diagnosed as insulin resistant is put on an appropriate diet, exercised and loses weight, eventually resulting in blood insulin levels returning to a normal range, that horse is not cured of insulin resistance. The underlying genetics are still there. Failure to appreciate that leads people to do things like turn the horse out on unsafe pastures that put them at risk of deteriorating and developing laminitis.
Buying into the idea that IR can be caused by diet has also led to an epidemic of concerned owners becoming positively paranoid about feeding any starch and the simple carbohydrate levels in pastures. While it is certainly true that many horses do not need supplemental grain (or fat calories), it is not a metabolic poison that must always be avoided. Similarly, the vast majority of horses are not at risk of laminitis from pasture levels of simple carbohydrates. Despite this, grass “sugar” tends to be blamed for a host of hoof quality issues, hoof tenderness and even thrush when the real cause(s) has nothing to do with sugar.
It is important to be diligent about correctly identifying and managing horses with IR as early as possible for the best outcomes. It is equally important to realize this is a lifetime commitment. At the same time, remember that the vast majority of horses do not have this issue and there is no reason to be unduly restrictive with their turnout or diet and certainly no need to buy into a long list of supplements to treat or prevent a condition they do not have.