Oral Glucose Test in Horses: A Closer Look at Lipids

Veterinarians use the oral glucose test to help diagnose hyperinsulinemia based on high levels of circulating insulin. Hyperinsulinemia is an important finding in horses with insulin dysregulation and equine metabolic syndrome (EMS), and is a direct contributor to hyperinsulinemia-associated laminitis (HAL). In addition to measuring insulin levels, veterinary experts suggest that lipid (fat) responses during the oral glucose test should be evaluated.^*

Hyperinsulinemia results from an increased production of insulin, reduced response by tissues to insulin, or a combination of both. Tissue insulin resistance is believed to contribute to increased body condition score and cresty neck score often observed in horses with EMS.

“Horses can be hyperinsulinemic either with or without tissue insulin resistance. When tissue insulin resistance is also present, there may be differences in lipid metabolism and fat distribution compared to the horses exhibiting only hyperinsulinemia,” explained Ashley Fowler, Ph.D., a nutritionist for Kentucky Equine Research.

The veterinary research team set out to gain a better understanding of the role of lipids and lipid metabolism in horses with hyperinsulinemia with and without tissue insulin resistance. They recruited nine healthy control horses, seven horses with both hyperinsulinemia and tissue insulin resistance, and four horses with hyperinsulinemia but without tissue insulin resistance. All horses underwent a standard oral glucose test. In addition to measuring glucose and insulin levels, researchers measured two types of lipids—triglycerides and nonesterified fatty acids (NEFA).

As expected, triglycerides and NEFA were significantly lower in all three groups of horses following the oral glucose test. However, concentrations were significantly higher in horses with tissue insulin resistance than in horses with only hyperinsulinemia and the control group.

“In other words, the decrease in triglycerides and NEFA was less pronounced in horses with tissue insulin resistance,” Fowler said. “These results suggest that fat mobilization and accumulation vary between horses with different components of insulin dysregulation.”

This assertion is supported by the fact that horses with tissue insulin resistance had higher cresty neck scores compared to control and hyperinsulinemia-only horses in this study.

Thus, measuring lipids alongside insulin levels may help identify horses that have tissue insulin resistance in addition to hyperinsulinemia alone using the oral glucose test.

“It is possible that horses with different lipid responses could represent different subpopulations of insulin and glucose disorders, as happens in other species,” wrote the researchers.

This may suggest that horses with the combination of hyperinsulinemia and tissue insulin resistance might require a different approach to disease management.

Increasing skeletal muscle mass through exercise, if the horse is sound, can help improve insulin sensitivity and can also assist in reducing body fat. Dietary management, such as controlling forage intake and eliminating high calorie feeds, can also help reduce body fat in horses that are overweight.

“Despite having lower calorie requirements, the mineral and vitamin requirements are not decreased in horses requiring a weight-loss diet. Adding a vitamin-mineral supplement or ration balancer is essential to meet the horse’s requirements,” advised Fowler.

In some cases, particularly those with severe laminitis, the sodium-glucose cotransporter-2 inhibitors (SGLT2i) may be introduced to the treatment protocol.

^*Zemek, C.H.K, K.L. Kemp, and F-R. Bertin. 2024. Value of measuring markers of lipid metabolism in horses during an oral glucose test. Journal of Veterinary Internal Medicine:17199.