Why Does Starch Make Horses Reactive?

Owners often supplement their horses’ diets with high-starch cereal grains to meet their nutrient and caloric requirements. Study results have shown, however, that starch can make horses more reactive. The question that remains: Why?

Louise Bulmer, a PhD student studying under Peter Hastie, BSc, MSc, PhD, and Prof. Jo-Anne Murray, PhD, MSc, PgDip, PgCert, BSc (Hons), BHSII, RNutr, PFHEA, at the University of Glasgow, in Scotland, took a closer look at the gut-brain axis to try to find an answer. She shared her findings at the 2019 Equine Science Society Symposium, held June 3-6 in Asheville, North Carolina.

The gut-brain axis is the bidirectional communication system between the gastrointestinal (GI) system and the central nervous system (CNS). Changes in gut microbes can affect CNS function and vice versa via known pathways.

“Therefore, the aim of this study was to develop a method to measure dopamine and serotonin receptor (two neurotransmitters present in the gut and the brain) densities in equine gut and brain tissues to investigate the mechanisms behind increased behavioral reactivity,” Bulmer said.

In doing so, she split 10 unhandled Welsh Ponies into two groups of five and fed them either a high-starch or high-fiber diet for 14 days.

“Behavioral test results found that the ponies were more reactive and startled on the high-starch diet,” Bulmer noted.

Then, she collected post-mortem samples from each pony’s digesta (food in the digestive tract), gut tissues, and brain tissues.

Bulmer found that the high-starch diet group had:

  • Lower hindgut bacteria diversity and richness than the high-fiber diet group;
  • Significant differences in microbiota in the ventral (front), dorsal (back), and small colon regions of the hindgut;
  • Decreased fibrolytic bacteria, which are important for fiber digestion; and
  • Increased Streptococcus bacteria, which have previously been found to proliferate prior to laminitis development.

Then she looked at dopamine and serotonin levels and found a significant difference in dopamine receptors in the brain related to diet and brain region.

“The gut-brain axis pathway investigated here could be one of several mechanisms behind the increased behavioral reactivity associated with high-starch diets,” she said. “Microbiota changes related to diet could be a key factor.”

Her results can help pave the way for more research into the mechanisms behind diet-related behavior changes in horses and understanding gut-brain axis pathways.