Risks of feeding a high starch diet to horses laid bare in study

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Horses are often fed high amounts of starch in their diets despite the well-established benefits of a fibre-based diet to promote gut health and animal welfare, researchers noted.
Horses are often fed high amounts of starch in their diets despite the well-established benefits of a fibre-based diet to promote gut health and animal welfare, researchers noted.

The risks associated with feeding horses a diet high in starch have been highlighted in fresh research in Italy, with researchers pondering the potentially damaging role of valeric acid in the gut.

Researchers from the University of Turin study found that a high-starch diet caused significant changes in the horse gut environment.

“A high starch diet has a profound effect on the horse’s gut environment in terms of dry matter, volatile fatty acid production and particle sizes,” Federica Raspa and her fellow researchers reported in the journal BMC Veterinary Research.

“We observed an increase in the dry matter content in the right dorsal colon, as well as reduced particle sizes and an increase in the production of valeric acid in all the gut compartments studied.

“High-starch diets should be avoided in favour of fibre-based diets with the goal of safeguarding gut health in horses,” the study team concluded.

Horses are often fed high amounts of starch in their diets despite the well-established benefits of a fibre-based diet to promote gut health and animal welfare, the researchers noted.

They set out to compare the effects of two different diets – one based on high amounts of starch versus one based on high amounts of fiber – on specific parameters of the gut environment across different parts of the intestine in the equine digestive tract.

They focused on the amounts of dry matter, organic matter and ash content, as well as particle size distribution and volatile fatty acids.

Their work involved 19 Bardigiano horses destined for slaughter. Nine were fed with high amounts of starch, in a diet comprising 43% hay and 57% starch-rich pelleted feed. The other 10 were fed high amounts of fibre, in a diet comprising 70% hay and 30% of a fibre-rich pelleted feed.

Horses fed the high-starch diet had a higher dry matter content in their right dorsal colon, the authors reported. They also showed a higher organic matter and ash content in the sternal flexure, pelvic flexure, right dorsal colon and rectum.

The high-starch horses also had a higher proportion of particles retained through an 8mm sieve and a higher proportion of particles that washed through the finest sieve (less than 1mm).

The total amounts of volatile fatty acids, as well as valeric acid, were found to be significantly higher in horses fed on the starch-rich diet when compared to those on the high-fibre diet. Indeed, valeric acid represented around 40% of the total volatile fatty acids produced in the hindgut of the high-starch group.

Valeric acid is produced by gut bacteria from undigested carbohydrates. It has a high lipid solubility and is able to penetrate the digestive lining. Previous research suggests it is may contribute to the development of gastric ulcers in horses.

“This effect may also be of significance in the hindgut, where inflammation processes have often been associated with high-starch diets,” they said.

Valeric acid was not detected at all in the gut of the horses fed a high-fibre diet.

The  researchers said their results confirm the notion that diet composition, and thus feeding management practices, are able to influence the gut environment and its function.

“Importantly, the observed alterations were primarily localized in the sites that colic surgeries frequency reveal to be associated with digestive problems.”

Considering the results, a fibre-based diet should be promoted in horses to safeguard gut health, they said.

Discussing their findings, the authors said high levels of volatile fatty acids found in the horses on a starch-rich diet may increase the risk of digestive disturbances.

They said it would appear that particle size in the gut is not only influenced by chewing and the condition of the teeth, but by the amounts of starch in the diet.

“It is well known that high amounts of undigested starch are responsible for alterations or shifts in microbiome composition, which lead to a reduction in the activity of fibrolytic microorganisms and, as a consequence, a reduction in the fermentation capacity of the fibre.

“This aspect seems particularly important if we consider that the adequate digestion of fibre is believed to be crucial for reducing particle retention in the intestine, the occurrence of which increases the risk of large colon impaction.”

The study team comprised Raspa, Maria Teresa Capucchio, Elena Colombino, Domenico Bergero, Claudio Forte, Martina Greppi and Emanuela Valle, all with the University of Turin; Ingrid Vervuert, with Leipzig University in Germany; Laura Cavallarin, Marzia Giribaldi and Sara Antoniazzi, with the Institute of Sciences of Food Production in Grugliasco, Italy; Damiano Cavallini, with the University of Bologna; and Ermenegildo Valvassori, with the Public Veterinary Service in Chieri, Italy.

Raspa, F., Vervuert, I., Capucchio, M.T. et al. A high-starch vs. high-fibre diet: effects on the gut environment of the different intestinal compartments of the horse digestive tract. BMC Vet Res 18, 187 (2022). https://doi.org/10.1186/s12917-022-03289-2

The study, published under a Creative Commons License, can be read here

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