Insulin resistance refers to the inability of insulin to exert its physiologic effect of facilitating the movement of glucose from the blood into insulin-sensitive tissues, such as muscle and adipose (fat) tissue. Because glucose isn’t cleared from the blood it stays elevated, and stimulates the pancreas to release more insulin, resulting in hyperglycemia (high blood glucose) and hyperinsulinemia (high blood insulin), respectively.
Insulin resistance is difficult to diagnose in a clinical setting, therefore the scientific community is using the term insulin dysregulation (ID) to describe abnormal glucose and insulin dynamics. ID describes an excessive amount of insulin released with carbohydrate consumption or administration, basal hyperinsulinemia and/or true tissue insulin resistance. ID is a concern as it is associated with an increased risk of laminitis, and is a component of Equine Metabolic Syndrome.
Insulin resistance can develop over time, when horses consume regular meals that are high in soluble carbohydrates. It is believed that the repeated rises in blood glucose following high sugar meals and the resultant peaks in insulin down regulate the tissue’s responses to insulin. Many horses that develop insulin resistance are overweight, and research has shown that adipose (fat) tissue produces inflammatory proteins that can also affect insulin signaling. Horses that have Cushing’s Disease (PPID) may also develop insulin resistance. This is because Cushing’s affects horses that have increased cortisol production – a hormone that essentially counteracts the effects of insulin in part by promoting glucose production and release by the liver. Cortisol also promotes visceral fat development, which can further impair insulin function.
Insulin dysregulation can be diagnosed a few different ways. If a horse has basal (resting) insulin concentrations in their blood higher than 20 μU/ml, there is some degree of ID that warrants further testing. Ideally, this blood sample is taken when the horse has not had any grain or pasture, but just dry hay for several hours prior to sampling. Additional testing would confirm an exaggerated insulin response to carbohydrate, either through a high grain meal or oral glucose administration. True tissue insulin resistance is diagnosed with more complicated tests, such as a frequently sampled intravenous glucose tolerance test, or a euglycemic-hyperinsulinemic clamp.
Insulin dysregulation is highly linked to risk of laminitis. Research has shown that highly elevated insulin concentrations (~1000 μU/ml) in the blood can directly cause laminitis. It is possible for horses and ponies to have basal insulin concentrations upwards of 100 μU/ml, and these can increase further with soluble carbohydrate consumption, either from grain or rich pasture. Therefore, it is of interest to feed and manage horses to improve insulin signaling.
Because many horses with ID are also obese, feeding to promote weight loss is a good step toward improving insulin sensitivity. General recommendations are to feed horses at 1.25% of their body weight as dry forage (hay), balanced with protein, vitamins and minerals. It is possible that soaking the hay (~30 minutes in water, drain and throw away the excess water) to leech out any excessive sugar (and thus calories) will further help weight loss. A hay analysis will let you know how much protein your horse is already getting from the hay, and how much more he might need to meet his requirements. A good concentrated source of protein is soybean meal; a horse might only need 200g of soybean meal to top off protein requirements. A concentrated vitamin-mineral supplement is ideal for horses looking to lose weight, ideally one that is fed a rate of less than 100 grams total. For weight loss, feeding only a few hundred grams of vitamins/minerals and protein is better than even feeding upwards of 500 grams (~1 lb) of a ration balancer. Working with your hay analysis an equine nutritionist can recommend how many grams of protein and minerals, and units of vitamins, your horse needs.
For horses of good weight that are insulin resistant, calorie source becomes critical. These horses should also be fed low-sugar hay – either hay that has been tested to have low sugar (low non-structural carbohydrate), and/or hay that has been soaked. If these horses need additional calories to maintain weight, more hay should be offered, perhaps along with other high-fibre types of feeds, such as beet pulp (without molasses!) or rice bran. Adding in additional oil can also provide calories while being low in sugar, and total fat intake is still lower than what we might see in human diets. ID horses should not be fed typical grain mixes that are high in corn, as corn is very high in starch. Starch is problematic because it is digested to and absorbed as glucose, thus increasing blood glucose and blood insulin and exacerbating the problem. Oats and barley have more moderate levels of starch, but intake should still be limited.
Treats may also need to be limited to horses that have insulin signaling issues. Carrots are probably the best option because they are mostly water, and have very little sugar (5 g/100 g). Apples also have low sugar (10g /100 g) but a bigger serving size (~200 gram apple – so about 20 g of sugar), while peppermints are almost all sugar (70g/100 g). If mints are beloved by your horse, you might only feed one or two (so total of maybe 13g of sugar). The bigger problems are the horse treats that are full of grains and often also molasses. Those should be avoided for ID horses.
Another key factor in managing ID is exercise. Exercise has been shown to increase insulin sensitivity in horses, regardless of the horse’s level of adiposity. My own work at North Carolina State University showed that when horses achieved weight loss through either dietary restriction or through exercise conditioning, only the exercise group showed improvements in glucose metabolism. Therefore, regardless if your horse is overweight or not, exercise will help improve insulin signaling.