Does sugar make horses anxious or hyper? – Horse

When I was a graduate student at Virginia Tech, I was approached by my advisor about potential research projects. The one that piqued my interest the most was evaluating high-fat diets and whether they influenced behavior. Specifically, were horses more or less active/reactive when fed a diet where calories from grain were replaced with calories from fat? University faculty had heard anecdotal reports from riders that their horses were “calmer” when on diets that contained fat for energy. Through studies on adult horses and weaned horses, researchers have found results supporting this theory (Holland, Kronfeld and Meacham, 1996; Holland et. al, 1996).

Researchers in more recent studies have also assessed the effect of diet on behavior. In a 2019 study, Bulmer and colleagues reported a change in the microbiota of the hindgut (caecum and large colon, or large intestine) of ponies fed a high-starch diet. The ponies were also more alert, nervous and responsive in new situations, and their heart rate was elevated compared to ponies on a high fiber diet. Destrez and colleagues found similar results in 2015; horses fed high-starch diets exhibited intestinal discomfort and displayed negative behaviors and high stress compared to those fed low-starch diets.

There are a few theories as to why diets high in fiber and/or fat and low in more digestible carbohydrates might influence behavior. Horses fed a diet high in more digestible carbohydrates are prone to hindgut ulcers and acidosis. Small intestinal 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.

High-starch diets can also negatively affect the microbial population present 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 about 20 different hormones, including several neurotransmitters. Disturbances 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, seen in horses consuming “high starch” diets, are associated with increased dopamine production. High dopamine can lead to elevated consciousness or hyperexcitability. Horses on a high fiber and/or high fat diet have more consistent and lower blood sugar levels.

Research is ongoing on how diet might affect behavior and why. It should be remembered that dietary changes alone, without regard to training and management, will not “cure” the anxious horse. It’s only part of the equation. For some equine athletes, such as thoroughbred racehorses that need to quickly replace glycogen stores in skeletal muscle, a high-starch diet is beneficial.

The decision to switch to a low “sugar” diet should be made after consulting your veterinarian and qualified equine nutritionist. Your feed company can be a great resource, as they 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 ensure that you meet and do not exceed your horse’s calorie needs. Thus, adding fat to the diet will require decreasing sources of soluble carbohydrates. It’s not as simple as simply replacing on an animal food basis because fat contains more than twice as many calories as carbs per pound. Food transitions should also be done slowly, ideally over about two weeks, so the digestive system has time to adjust. Diarrhea and odd-looking feces are indications that the transition is happening too quickly.


Holland, JL, DS Kronfeld and TN Meacham. 1996. Horse behavior is affected by soy lecithin and corn oil in the diet. J.Anim. Science. 74:1252-1255.

Holland, JL, DS Kronfeld, RM Hoffman, KM Greiwe-Crandell, TL Boyd, WL Cooper and PA Harris. 1996. Weaning stress is affected by nutrition and weaning methods. Pferdeheilkunde 12(3):257-260.

Bulmer, LS, Murray, J., Burns, NM et al. 2019. High-Starch Diets Alter Equine Fecal Microbiota and Increase Behavioral Responsiveness. Scientific representative 9, 18621

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

Rachel J. Bradford