Q: I was rather surprised upon reading “What to Do About Enteroliths” (EQUUS 481) to see that adding apple cider vinegar to a horse’s feed is suggested as a possible solution to preventing enteroliths. How does this increase acidity in a horse’s gut?
Recently, while researching the benefits of apple cider vinegar, I was surprised to learn that it leaves an alkaline residue when it is digested. I also read that, when apple cider vinegar is digested, the gut does not have to release as much acid to absorb it, and so a more acidic pH is not achieved.
Since, in addition to swallowing foreign objects, risk factors for enteroliths include a diet excessively rich in minerals and a gut pH that is too alkaline, how is apple cider vinegar a solution?
Miriam G. Cunningham
A: These are excellent questions on a topic that is not well understood. As the article you referenced described, enteroliths are rocklike concretions that can form in the gut of horses with certain risk factors, including the swallowing of foreign objects, a mineral-rich diet and a higher (more alkaline) gut pH.
You are correct that apple cider vinegar is believed to have alkalinizing effects “in the body.” However, it is a highly acidic substance with a pH between 3.1 to 5. (A neutral pH is 7, and more basic, or alkaline, pH values are higher, from 7 to 14.) Perhaps the alkalization is occurring after metabolism and absorption.
The recommendation to feed a horse apple cider vinegar in an effort to reduce the risk of enteroliths is based on two studies that directly measured the effect of this practice on the acidity of the gut contents.
In the first, researchers from Cornell University reported that the addition of 112 ml (3.8 ounces) of apple cider vinegar to grain fed twice daily to ponies caused an increase in acidity in the lower gastrointestinal tract and that this drop in pH “seemed to prevent enterolith formation.” They also noted that apple cider vinegar seemed to work better than distilled vinegar.
However, these results must be viewed cautiously because they were not published in a peer-reviewed publication but only reported at a symposium---specifically, the 1989 Proceedings of the Equine Nutrition and Physiology Symposium. In addition, the feeding of grain alone will result in some degree of acidification of the lower gastrointestinal tract.
The second study was one of my own. Six horses---three known “stone formers” and three with no history of enteroliths---were fitted with fistulas0 in their right dorsal colons. This gave us easy, continuous access to the contents of their colons for sampling. Twice daily, I fed each horse either grain with eight ounces of apple cider vinegar or grain with no vinegar. The horses who consumed the apple cider vinegar had a small, but statistically significant, drop in gut pH compared to the horses who ate grain alone.
Neither of these studies tried to determine exactly how apple cider vinegar acidifies the equine gut, so we can’t answer that part of your question. We also don’t know for sure whether this extra acidity in the gut has any effect on the formation of enteroliths. But we are certain that feeding apple cider vinegar to horses does acidify the contents of the gut to some extent.
Diana Hassel, DVM, PhD,
Colorado State University
Fort Collins, Colorado
This month's expert: Diana Hassel, DVM, PhD, is an Associate Professor of Equine Emergency Surgery and Critical Care and the Equine Section Chief at the College of Veterinary Medicine & Biomedical Sciences at Colorado State University. Her areas of research include equine colic, endotoxemia, and the equine gastrointestinal microbiome. She is
a diplomate of both the American
College of Veterinary Surgeons and
the American College of Veterinary Emergency & Critical Care.
This article was originally published in EQUUS 486, March 2018