Does Sugar Make Horses Anxious or Hyper?

How diets high in starch can impact hindgut function and performance, potentially leading to problems with the ‘gut-brain axis.’
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Does Sugar Make Horses Anxious or Hyper?
Elevated glucose levels, observed in horses consuming “high-starch” diets, are associated with increased dopamine production, which can lead to elevated awareness or hyperexcitability. |
When I was a graduate student at Virginia Tech, I was approached by my advisor regarding potential research projects. The one that most piqued my interest was evaluating fat-supplemented diets and if they influenced behavior. Specifically, were horses more or less active/reactive when on diets where calories from grains were replaced with calories from fats? The university faculty had heard anecdotal evidence from riders that their horses were “calmer” when on diets containing fat as an energy source. Through studies on both adult horses and weanlings, the researchers found results supporting this theory (Holland, Kronfeld and Meacham, 1996; Holland et. al, 1996).

Researchers on more recent studies have also evaluated diet’s effect on behavior. In a 2019 study Bulmer and colleagues reported a change in the hindgut (the cecum and large colon, or large intestine) microbiota of ponies fed a high-starch diet. The ponies were also more alert, nervous, and reactive in novel situations, and their heart rates were elevated when compared to ponies on high-fiber diets. Destrez and co-workers found similar results in 2015; horses fed high-starch diets presented with intestinal discomfort and displayed negative behaviors and elevated stress when compared to those on low-starch diets.

There are a few theories on why diets higher in fiber and/or fat and lower in more digestible carbohydrates might influence behavior. Horses on diets high in more digestible carbohydrates are prone to ulcers and hindgut acidosis. Hindgut acidosis occurs when, instead of being digested in the small intestine, starches reach the hindgut and are fermented by microbes. Signs of this condition include poor performance, poor attitude, and mild colic.

Diets high in starch can also negatively affect the microbial population found in the hindgut, which will negatively affect hindgut function and performance. This can lead to problems with the “gut-brain axis.” The gastrointestinal tract releases around 20 different hormones, including several neurotransmitters. Disruptions in the release of these hormones can cause “negative” behaviors, including hyperexcitability and irritability.

Glucose is a sugar that easily crosses the blood-brain barrier. Elevated glucose levels, observed in horses consuming “high-starch” diets, are associated with increased dopamine production. Elevated dopamine can lead to elevated awareness or hyperexcitability. Horses on diets higher in fiber and/or fat show more consistent and lower blood glucose levels.

Research is ongoing on how diet might affect behavior and why. One should remember that dietary changes alone, without consideration of training and management, will not “cure” the anxious horse. It is just one part of the equation. For some equine athletes, such as Thoroughbred racehorses that must rapidly replace glycogen stores in skeletal muscle, a higher-starch diet is beneficial.

Decisions to transition to a diet lower in “sugar” should be made after consulting with your veterinarian and qualified equine nutritionist. Your feed company can be an excellent resource, as it will have an equine nutritionist on staff who can answer your questions. You should also consult reputable resources for additional information.

It is important to make sure you are meeting, and not exceeding, your horse’s caloric requirements. So, adding fat to the diet will require a decrease in the soluble carbohydrate sources. This is not as easy as just replacing on a pound of feed basis, since fats have more than two times the calories of carbohydrates per pound. Dietary transitions should also be made slowly, ideally over about two weeks, so the digestive system has time to adapt. Diarrhea and odd-looking feces are indications the transition is being made too quickly.


Holland, J.L., D.S. Kronfeld and T.N. Meacham. 1996. Behavior of horses is affected by soy lecithin and corn oil in the diet. J. Anim. Sci. 74:1252-1255.

Holland, J.L., D.S. Kronfeld, R.M. Hoffman, K.M. Greiwe-Crandell, T.L. Boyd, W.L. Cooper and P.A. Harris. 1996. Weaning stress is affected by nutrition and weaning methods. Pferdeheilkunde 12(3):257-260.

Bulmer, L.S., Murray, J., Burns, N.M. et al. 2019. High-starch diets alter equine faecal microbiota and increase behavioural reactivity. Sci Rep 918621

Destrez, A., Grimm, P., Cezilly, F., and Juilland, V. 2015. Changes of the hindgut microbiota due to high-starch diet can be associated with behavioral stress response in horses. Physiol & Behav. 149:159-164.


Written by:

Janice L. Holland, PhD, is an Associate Professor and Director of Equine Studies at Wilson College in Chambersburg, Pennsylvania. A graduate of both Penn State and Virginia Tech, her equine interests include nutrition and behavior, as well as amateur photography. When not involved in horse activities she enjoys spending time outdoors enjoying nature.

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