New research from Australia suggests that a horse’s breed ancestry rather than his body condition may account for his susceptibility to insulin resistance, as well as associated health problems such as laminitis.
Insulin is a hormone that facilitates the metabolism and storage of glucose, a sugar derived from carbohydrates that fuels the body’s cells. Typically, the pancreas releases insulin at a rate that corresponds with the amount of glucose in the horse’s bloodstream after a meal. Some horses, however, produce excess insulin in response to the ingestion of carbohydrates, which maximizes the uptake of glucose and minimizes the loss of fat reserves. Eventually, this overabundance of insulin causes cells to become resistant to the hormone’s effects, triggering the body to produce even more.
Elevated insulin levels, insulin resistance and increased risk of laminitis have all been linked to obesity, and horses with these characteristics are said to have “equine metabolic syndrome” (EMS). Yet there is some evidence that a horse’s weight may have less to do with his susceptibility to laminitis than other factors. Noting that ponies, Andalusians and other horses of certain breeds go on to develop equine metabolic syndrome and laminitis more frequently than the general population, Simon Bailey, BVMS, PhD, and fellow researchers at the University of Melbourne designed a study to determine if breed type alone might influence a horse’s insulin response.
For the study, the researchers used seven Andalusian-cross horses and eight mixed-breed ponies. “Unlike many other studies, the ponies in our study were not selected for being insulin resistant or showing any particular signs of EMS,” says Bailey. “They were just a typical sample of ponies of different breeds.” The researchers also used eight Standardbreds---a breed not known to develop laminitis often---for comparison.
The study horses were given an oral glucose tolerance test as well as an intravenous glucose tolerance test. “The IV test is used by researchers to calculate insulin sensitivity, because it can evaluate glucose uptake from the blood as a direct response to a known amount of insulin,” explains Bailey. “The oral glucose test, although it can’t give a value for insulin sensitivity, is actually more relevant to assessing metabolic syndrome and risk of laminitis because it shows how much insulin is produced by the pancreas when glucose enters through the digestive system.” Blood was collected frequently over a six-hour testing window, tested for insulin response and the data compared.
“We found that all the breeds had similar peak glucose levels after the challenges,” says Bailey. “However, the pancreatic insulin responses were significantly greater in the ponies and Andalusians. The increased insulin also took longer to be cleared from the bloodstream. In other words, the ponies and Andalusians were able to control their glucose levels effectively, but they had to produce a lot more insulin to achieve this control. This is insulin resistance. They appear to have an excessive pancreatic response when compared with the Standardbreds, who had to produce only a very small amount of insulin to cause their tissues to clear glucose from the bloodstream back to basal levels.
”This means that some horses may be prone to insulin resistance and associated laminitis even without outward signs of EMS. “Previously, it was thought that it may just be obesity that caused insulin resistance,” says Bailey. “This can be the case in humans. And in some non-obese animals in moderate body condition, we have found that there are underlying fundamental differences in insulin responses between horse types as well as between horses and ponies.”
Bailey adds that other types of horses are also probably at risk: “We certainly would expect many Morgans, Paso Finos and several other breeds to respond in a similar way as the Andalusians. Dr. Nick Frank in the United States has identified that EMS is more common in Morgan horses, Paso Finos, Arabians, Saddlebreds, Quarter Horses and Tennessee Walking Horses, as well as ponies. They could be really any animal that is considered to be an ‘easy keeper.’”
Careful management of these breeds, however, can mitigate the genetic risks, says Bailey. “If these animals are maintained in a moderate body condition through exercise and avoiding lush grass or too much grain in the diet, then it is likely that insulin levels will be kept in check and the risk of laminitis will be minimized.”
Reference: “Breed differences in insulin sensitivity and insulinemic responses to oral glucose in horses and ponies of moderate body condition score,” Domestic Animal Endocrinology, November 2013
This article first appeared in EQUUS issue #441.