April 19, 2023 — Lexington, KY — Properly feeding horses with insulin dysregulation (ID), a key component of equine metabolic syndrome (EMS), has always been an important part of keeping them healthy. But designing the ideal diet has relied heavily on anecdotal evidence and, to a certain extent, trial and error.

Two recently published studies are shedding new light on how affected horses’ bodies respond to feedstuffs. And this, the researchers say, will ultimately help fine-tune feeding recommendations for horses and ponies with ID. Amanda Adams, PhD, an associate professor at the University of Kentucky (UK) Gluck Equine Research Center—along with then-PhD candidate Erica Macon and the team at the UK Aged Horse Research Center—led the international effort to complete the studies, with support from MARS Horsecare™ and the Waltham Petcare Science Institute.

They collaborated closely with Patricia Harris, MA, PhD, Dipl. ECVCN, VetMB, MRCVS, director of science at MARS Horsecare™ and head of the WALTHAM™ Equine Studies Group, in Leicestershire, United Kingdom, and Simon Bailey, PhD, Dipl. ECVPT, FRCVS, professor of veterinary biosciences and head of the Department of Veterinary Biosciences at the University of Melbourne Veterinary School, in Victoria, Australia.

Current Challenges

“These studies really came from the challenges we all faced in managing individual ID animals, as well as for me in particular with the herd of ID horses at UK, coupled with the questions we received from countless horse owners asking the same questions: What and how do I feed my ID horse?” says Adams.

Although horses with ID are often overweight or obese, the disease can affect horses of healthy weights, as well. Similarly, not all obese or overweight animals are ID, either. As such, blood testing is absolutely key to determining whether a horse has ID. In affected horses, diagnostic tests will reveal high basal insulin concentrations in their bloodstreams and/or abnormal responses in the bloodstream after consuming starch and sugar.

Both of these issues put equids at an increased risk of laminitis.

But, Bailey says, “keeping the insulin response to a moderate level after feeding in ID animals will significantly help to reduce their risk of laminitis.”

Adams says that, to achieve this, her approach—and what she’s always recommended to owners—has been to “put them on a diet low in NSCs (starch and water-soluble carbohydrates, collectively known as non-structural carbohydrates).”

But she always came back to the same question: What does a “low-NSC diet” really mean, as most of the research on insulin response to diet has been conducted in non-ID animals.

“It was always frustrating that, while anecdotal evidence largely supported the recommendations we’ve been making, we didn’t yet have any scientific data or research to back them up,” she says.

Meanwhile, Macon developed an interest in metabolic horses while she was completing her master’s research, which focused on differences in circulating protein concentrations between ID and non-ID horses. In hopes of continuing this path, she sought Adams in hopes of exploring whether protein could be a significant driver of ID horses’ insulin responses.

“We decided it was finally time to start finding some answers,” Adams says.

What the Studies Showed

First, the researchers compared how ID and non-ID horses (all weighing about 500 kilograms, or 1,100 pounds) responded to consuming one meal (approximately 600 grams, or 1.3 pounds) of four diets: three different restricted NSC feeds with low, moderate, and high protein levels (to evaluate at the potential role of protein in insulin responses) plus cracked corn with molasses.

In this study, protein content did not have an impact on how horses responded to eating a meal, the researchers found. As the team expected, the non-ID horses’ insulin levels remained well within normal ranges after they’d eaten any of the feeds.

“But, we were somewhat surprised at how quickly and how significantly the ID horses’ insulin levels increased—especially in some individuals after eating one relatively small meal of any of the feeds—compared to the non-ID horses,” says Adams.

Based on those findings, the team wanted to confirm that the ID horses’ increased insulin levels weren’t just a response to eating, Adams says, since this hadn’t been studied previously. They followed up by evaluating how non-ID and severely ID horses responded to a variety of feedstuffs all fed at a rate of around 500g (about 1.1 pounds) for a 500-kilogram horse. The diets included a high-protein restricted NSC ration balancer, cracked corn with molasses, steam-flaked corn with molasses, dehulled oats, and a custom very low-NSC feed produced specifically for the study by BUCKEYE™ Nutrition.

They found that the ID horses’ insulin responses after consuming the low-NSC feed were significantly lower compared to when they ate the other diets, “which we were very relieved to see,” says Adams.

“If the feed they consumed had a low enough NSC level, they didn’t respond to the same degree; their responses looked more similar to non-ID horses’ responses although they may have started from a higher starting point,” she says. “This confirmed it wasn’t just the act of eating that induces an insulin response.

“Of course,” she adds, “that begged the question: What’s the threshold for ‘low enough’?”

To find out, the team evaluated how ID horses’ bodies responded to eight feeds with a similar base but varying NSC levels.

“I spent an entire year sitting at the lab bench with a coffee grinder,” Macon says. “I would make a new diet every week using mixtures of different feedstuffs to get varying amounts of NSC, and then send off to the lab. Eventually we settled on adding specific amounts of pure starch and sugar to the base diet so that I could get the levels right.”

Once the horses consumed the different diets, the team sent the blood samples off to evaluate the insulin responses.

“Every time I got a new set of results back, I had my fingers, toes, and eyes crossed that they would be within the range I needed,” she says. “One day, they were!”

They found that the threshold appeared to be less than 0.1 g NSC/kg bwt/meal. Above that, horses’ insulin levels were more likely to increase significantly.

“There is great variation in how individuals respond,” Harris notes, “and some horses and ponies will have a different threshold. Of course, any such threshold will depend on how low an insulin response is actually required as none of the animals in our studies developed any clinical issues.”

Further research is needed, she adds.

“These studies are therefore the first in a series that we hope will help put science behind the way we feed ID horses,” Adams says.

She and her colleagues are currently working on additional research to answer other common questions, such as how ID horses respond to small amounts of higher NSC containing feedstuffs as well as larger amounts of lower NSC providing products.

The research team is also conducting similar studies using hay pellets and long-stem forage to better understand how ID horses respond to help develop threshold guidelines. All of which will improve how owners around the world can keep their ID horses healthy and happy.

“This work is important in giving us further evidence to provide more effective advice,” Bailey says.

Adams adds, “The ultimate goal is to manage these horses on an appropriately low-NSC diet (both forage and complementary feed), but we still have to determine what that means and how low of a NSC is needed under different clinical situations so that horses don’t have inappropriate insulin response.”

Feeding for Now

For now, owners of affected horses can keep them on the right path by having the foundation of their diet be a low-NSC (ideally less than 10% NSC on a dry matter basis) forage diet, she says, and many horses can be managed well with just such a forage and a forage balancer. Harris recommends that owners of severely ID horses divide a low-NSC ration balancer into several meals a day.

“As the dietary insulin response is variable, and we do not know all the nutritional triggers, it is advisable to monitor an individual horse’s or pony’s response to their specific diet (both feed and forage) if it is considered essential to induce only a low insulin response,” Harris emphasizes.

As such, Adams adds, “it is important that you work closely with your vet (who, in turn, will work closely with their diagnostic laboratory) and your nutritional advisor.”