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Hindgut acidosis is a condition in which the hindgut of the horse becomes excessively acidic. It is usually caused by too much starch in the horse’s diet, resulting in increased production of lactic acid in the lower intestinal tract.
When lactic acid levels rise, the result is a lower pH environment in the hindgut and disturbances to the microbial population.
This can also result in inflammation in the intestinal wall and decreased resistance to pathogens and toxins found in feed.
Hindgut acidosis can have meaningful consequences for your horse’s overall well-being, including decreased nutrient absorption and feed efficiency, increased risk of hindgut ulcers, and increased risk of digestive or immune complications.
Prevention and treatment of hindgut acidosis start with dietary management. Ensuring that your horse’s feeding program is designed to minimize starch overload is an important first step.
Eliminate or reduce feeding grain and concentrates. Give your horse access to high-quality forages at least 12 hours per day.
You may also want to consider feeding digestive support supplements such as probiotics, yeast, or digestive enzymes.
Proper Hindgut Function in Horses
The horse’s gastrointestinal tract can be broken down into two main sections: the foregut and the hindgut. [1]
The foregut is composed of the mouth, esophagus, stomach, and small intestine. It makes up approximately 30% of the overall digestive tract.
Digestion begins in the foregut. The small intestine contains enzymes that start to break down feed.
The small intestine absorbs simple sugars, starches, fatty acids, amino acids as well as vitamins and minerals. However, it does not have the ability to break down fibre or other complex carbohydrates which make up a large portion of a horse’s diet. [1][2]
Fibre, fructans, complex plant sugars, pectin, and beta-glucan are broken down in the hindgut through microbial fermentation. In human nutrition, all of the carbohydrates that cannot be digested in the small intestine are collectively called “fibre” and we will use that terminology in this article.
The hindgut consists of the cecum and colon and makes up approximately 60% of the digestive tract.
Healthy hindgut function is critical for a horse to absorb and utilize the nutrients found in the diet to meet daily requirements. [1][2][3]
Hindgut Fermentation
Horses are hindgut fermenters because fibre digestion by microbes occurs in the hindgut (cecum and colon).
The hindgut contains a complex ecosystem of bacteria, protozoa, yeast, and other microorganisms that break down fibre in the diet through fermentation. [3]
Hindgut fermentation breaks down fibre and some of the protein in the diet to produce:
- Energy: Mostly in the form of volatile fatty acids (acetate, butyrate and propionate) that can be absorbed and used by the horse. Lactate can also be absorbed and used as an energy source.
- Amino acids: The building blocks of proteins, these are mostly used within the microbial environment and not absorbed in significant quantities.
- B-vitamins: Water soluble b-vitamins such as biotin and thiamin. It is unclear how much of these vitamins produced by the hindgut can be absorbed.
What is Hindgut Acidosis?
The hindgut needs to maintain a relatively stable range of pH to support optimal digestion and nutrient absorption.
It is normal for the pH to change depending on the diet. However, if the hindgut environment becomes too acidic or too basic, it can interfere with the proper functioning of the beneficial bacterial species that populate the gut.
The ideal pH level in the hindgut for proper fibre fermentation is between 6.5 – 7.0. [6][7][8][9]
During the fermentation process, the pH drops below 6 (becoming more acidic) for a short period of time. This change in pH level makes fibre-digesting bacteria less effective but allows other microbes to work more effectively. [6][7][8]
However, long periods of low pH in the hindgut can be detrimental. Low pH is associated with excessive sugar or starch in the equine diets.
In extreme cases of “horse broke into the feed room” or experimental fructan overload, this causes fibre digesting bacteria to die off and allows Streptococci to proliferate causing the pH to drop below 6.
These microbes release toxins that can damage the junctions between cells of the hindgut and cause local and systemic inflammatory responses and laminitis. [6][7][8]
Signs of Acidosis in Horses
Horses with clinical hindgut acidosis are obviously ill with colic, fever, diarrhea and potentially laminitis. This occurs within the framework of horses getting unlimited ac