Horses with insulin dysregulation in horses (ID) have hyperinsulinemia, or high circulating insulin levels. Traditionally, horses with ID are fed high-protein ration balancers to complement the low-nonstructural-carbohydrate (NSC) grass hay that makes up the bulk of their diets. However, recent evidence in both human and equine studies calls this practice into question.

Allison Palmer, DVM, an equine internal medicine resident at The Ohio State University, in Columbus, reviewed available literature on the influence of dietary protein on equine insulin levels after a high-protein ration balancer meal during her presentation at the 2023 American Association of Equine Practitioners Convention, held Nov. 29-Dec. 3 in San Diego, California.

“Dietary management is the most important ‘treatment’ for equine ID, as it encourages weight loss when necessary and minimizes postprandial hyperinsulinemia (increased insulin levels following a meal),” said Palmer.

Often, owners manipulate the NSC content of horses’ diets using strategies such as soaking hay prior to feeding. “Soaking hay decreases the NSC content, but it also leaches other important water-soluble nutritional components and creates deficiencies over time,” said Palmer. “Therefore, ration balancers are usually fed in order to supplement protein, vitamins, and minerals.”

What do we Know About Protein and Horses With ID?

The dietary protein requirement for an adult horse in light work is estimated to be around 0.76 kg/day with 33 g/day of the essential amino acid lysine. Protein quality and quantity vary with forage type and maturity.

“Vegetative legume forages, fertilized cool-season grasses, and soybean meal are all examples of sources of high-quality protein in equine diets,” Palmer noted. Currently, researchers do not believe that horses with ID have different protein requirements than healthy horses, and protein deficiencies in horses are rare.

Palmer added, “It is important to note that there are no known benefits of regularly exceeding daily protein requirements.” She noted that researchers have shown that high-protein diets in humans can result in significantly enhanced insulin secretion over time, independent of weight loss. This is opposite of the goal for equine patients with ID because hyperinsulinemia is the primary risk factor for laminitis in horses.

In horses with ID, high-protein meals have been associated with exacerbated insulin responses. So, while NSCs are a principal driver of hyperinsulinemia, evidence is accumulating to support the idea that protein also affects insulin levels. Specifically, researchers have shown a ninefold greater insulin response in ID horses following a high-protein meal compared to normal horses. “This is strongly suggestive that high-protein ration balancers may exacerbate insulin responses,” said Palmer.

How to Know if a Feed Product Will Cause a Spike in a Horse’s Insulin

To further assess the effect of a specific feed product on insulin responses, veterinarians can perform a feed trial to assess the degree of postprandial hyperinsulinemia associated with that meal. Palmer said the test is conceptually like an oral sugar test, which is widely performed in clinical practice to diagnose horses with ID.

“Feed the horse one flake of grass hay the night before the test, then collect a baseline blood sample (in the morning),” said Palmer. “Feed the dietary component of interest, the ration balancer for example, then draw two additional blood samples 60 and 90 minutes later and measure insulin values.” If insulin exceeds 45 mIU/L at either time point or if the veterinarian observes more than a 30-40 mIU/L increase in insulin from baseline, that aspect of the horse’s diet should be scrutinized or possibly eliminated (especially in patients whose ID is poorly controlled).

“This type of test can help evaluate how individual horses respond to specific feed products. This will ultimately help guide feeding practices for our ID horses,” Palmer said.

Take-Home Message

Nutritional intervention is the most important part of managing ID. However, veterinarians should reevaluate the role of protein because high-protein meals have been associated with undesirable effects on insulin and glucose dynamics.

“Lower protein might be a better option to decrease the risk of exacerbating hyperinsulinemia,” said Palmer. “There are already commercially available products that do have lower protein (12-15%) and lower NSC content appropriate for ID horses.”