Does your client have an overweight horse? Chances are it is insulin resistant. Excess body fat leads to elevated insulin. Elevated insulin leads to more body fat storage, which leads to greater insulin resistance, and the vicious cycle continues. Even horses of normal weight can be insulin resistant, exhibited by regional fat deposits along the neck, shoulders, tailhead and back.

The opposite of insulin resistance is insulin sensitivity

Unlike when humans are “sensitive” to a food or ingredient — meaning intolerant or allergic — insulin sensitivity is a good thing. It simply means that the horse’s cells respond well to insulin and there is no longer a need for the pancreas to continually pump out high levels of this hormone. Storage of body fat normalizes, as well as blood sugar levels.

Putting the horse on a diet that restricts forage will not increase insulin sensitivity. The problem is that horses are designed to graze. They need to consistently chew to neutralize stomach acid (through saliva production, a natural antacid). Left with an empty stomach, the horse will likely develop ulcerations. But even more damaging is the hormonal stress response it will experience. This results in increased insulin, and ironically, the horse remains overweight because excess insulin tells the body to hold on to body fat. Furthermore, insulin is highly inflammatory, leading to pain and inflammation throughout the body, including increasing the propensity for developing laminitis. 

Consistently eating forage, on the other hand, matches what a horse would naturally do. It allows cortisol (stress hormone) secretion to calm down, making the cells more responsive (more sensitive) to insulin. Researchers at Louisiana State University¹ evaluated how eating affects insulin sensitivity; they determined that hay-deprived mares experienced a greater degree of insulin resistance and less insulin sensitivity than those mares who were fed hay ad libitum (free-choice). These results are consistent with a horse’s natural eating pattern. A healthy, insulin sensitive horse is a horse that will not easily gain weight when fed forage free-choice. A healthy horse will burn body fat and not store excessive amounts.

Reduce Concentrates But Not Forage

Calorie reduction, although important, should only be accomplished by reducing or even eliminating commercial feeds and cereal grains. Never reduce forage intake. While pasture grazing may not be an option for the overweight, insulin resistant horse during certain times of the day or seasons of the year, your client should always offer hay, day and night, 24 hours a day. Be sure to provide a vitamin/mineral supplement to fill in the gaps that exist with hay.

To make sure the hay is appropriate to feed free-choice, it is best to have it tested, so you know that it is low enough in sugar, starch, and calories.² To evaluate the testing report, look at the column labeled “Dry Matter.” Add the ESC (simple sugars) to the starch. This amount should not exceed 11%. The Digestible Energy (DE) is an indicator of calories and should not be more than 0.95 Mcals/lb (2.09 Mcals/kg) on a dry matter basis.

Allow The Horse To Self-Regulate

When forage is restricted, the horse perceives this as “winter is coming,” and its body adjusts by holding onto body fat. But if hay is always available, it will soon get the message that it can walk away, and the hay will be there when it returns. It is important, however, that the hay never runs out, not even for 10 minutes, to allow his brain to relax. It is amazing to watch — it starts to calm down his eating, eats more slowly, and eats less — eating only what its body needs. Insulin sensitivity increases, and body weight begins to normalize. With the help of exercise, the horse’s cells will respond to insulin even better and no longer store excess body fat.

Bottom Line

The closer one gets to a feeding environment that simulates a natural setting, the healthier the horse will be. Give the horse a chance to be a horse, and let him communicate how much forage it needs. The horse has the ability to regulate its intake to match just what it needs, resulting in a normalized hormonal response. It will be insulin sensitive, not resistant.

Juliet M. Getty, PhD, is an independent equine nutritionist. Her research-based approach optimizes equine health by aligning physiology and instincts with correct feeding and nutrition practices. Getty’s goal is to empower the horseperson with the confidence and knowledge to provide the best nutrition for his or her horse’s needs.

¹ Lestelle, LR., Earl, L.R., Thompson, Jr., D.L., Hebert, R.C., and Mitcham, P.B., 2011. Insulin-glucose dose response curves in insulin sensitive and insensitive mares and effect of overnight and long-term feeding regimen. Journal of Equine Veterinary Science, 31, 285-286.

² Equi-Analytical Labs – www.equi-analytical.com is a well-respected testing facility that specifically evaluates forages and feeds for horses.