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Insulin Resistance Explained
By Richard Vialls DEP, MEPA(UK)
Anyone who's trawled the internet on the subject of laminitis will have come across the term 'insulin resistance'. This article briefly explains what the term means.
Blood sugar (more accurately blood glucose, glucose being a simple sugar) needs to stay within quite tight bounds for a horse (or a human for that matter) to be healthy. If it goes high (hyperglycemia) or low (hypoglycemia) then the horse (or human) becomes ill (as any diabetic human will tell you). The symptoms of both hyperglycemia and hypoglycemia can be very serious, so it's important that the body regulates blood glucose levels correctly.
When a horse eats food containing carbohydrates, those carbohydrates are broken down gradually into glucose in the gut and the glucose then passes into the blood stream. That results in a rise in blood glucose. The body needs to bring blood glucose down, so the pancreas responds by releasing insulin into the blood. In a healthy horse, the insulin tells muscle and fat cells to absorb and store the excess glucose. When the blood glucose level reaches normal, the pancreas stops producing insulin again.
In insulin resistance, the muscle/fat cells become 'resistant' or less sensitive to insulin so it takes a larger amount of insulin to get the same effect in terms of reducing blood glucose. The result is that the pancreas needs to produce a higher level of insulin for a given increase in blood glucose. The body's control mechanisms still work, so the blood glucose levels will still stay within the normal range, but whenever the horse eats anything that can be converted to glucose, the insulin levels in the blood go up higher than normal.
One theory is that it is these increased levels of insulin in the blood after eating that are responsible for laminitis (although there are other, competing theories). If this theory is correct, you'd expect to see a higher risk of laminitis in horses with higher degrees of insulin resistance. This does seem to be the case, at least in the majority of horses studied (there may be other mechanisms that cause laminitis in a smaller subset of horses). You'd also expect the highest risk of laminitis to be just after the horse has eaten something that is easily converted to glucose (e.g. sweet foods such as molasses, starchy foods that are easily converted to sugars such as grains, or the sugar rich grass that is produced during the spring flush of growth). Again this does seem to be the case, providing some support for role of insulin resistance in laminitis.
Insulin resistance tends to cause a horse to put on weight more easily, so insulin resistant horses are more likely to be overweight and are more difficult to slim. Insulin resistance also tends to cause an abnormal distribution of fat in the body. In horses, there tends to be more fat put down in the crest of the neck. The crest and the supra-orbital fossa (the depressions just above the eyes) also tend to fill with fluid (oedema) when a horse is at highest risk of a laminitis attack. If the level of fat in these areas is higher than would be expected given the general weight of the horse, then this is a good indicator of the presence of insulin resistance. In the most serious cases of insulin resistance, the horse may have random lumps of extra fat all over the body, but this is rare. Insulin resistant horses also tend to put on more weight in the tummy area and are often bloated (although this may not be a direct consequence of the insulin resistance).
A typical cresty neck in an insulin resistant horse
Typical supra-orbital oedema in an insulin resistant horse
It is possible to test for insulin resistance. The gold standard is a test called the hyperinsulinemic euglycemic clamp. This is a complex and expensive test that is only really used for research purposes. More simple blood tests can be performed by your vet, but there's some argument about how effective these simple tests are. New approaches to interpreting tests are being researched, but there is still some risk that such blood tests provide false positives and false negatives. I tend to take the line that if a horse shows signs of laminitis, then insulin resistance should be considered to be likely (even if blood tests come back negative or inconclusive). As such, I would manage the diet of any horse suspected of insulin resistance (either showing physical signs such as a cresty neck, or showing signs of laminitis) on the assumption that insulin resistance is likely. It is safer to gradually increase access to 'high risk' foods such as grass while carefully monitoring for signs of problems (e.g. raised digital pulses), even if blood tests prove negative or inconclusive.
© 2007 Richard Vialls