Winter Diarrhea in Horses

production by the pancreas. Insulin acts to facilitate the movement of the glucose out of the blood stream into body tissues.
Starch or sugar which resist either enzymatic digestion in the foregut or have been fed in an amount in excess of what the foregut can readily process move on through the digestive tract into the cecum and potentially into the large colon. There they are fermented by beneficial microbes into end products called volatile fatty acids or VFA for short. VFA can be absorbed through the gut wall and transported to the liver via the blood stream where they are utilized as energy by the horse.
Starches and sugars in the hindgut are fermented very rapidly and the VFA can accumulate. This can cause a significant change in the pH environment of the cecum. Many of the beneficial bacteria living in the hindgut are very pH sensitive and may not survive a significant increase in acidity brought on by a sudden introduction of readily fermentable carbohydrates into the hindgut. Sensitive microbial populations die off and this can be detrimental to the health of the horse. Sudden diet changes for horses especially those including starches and sugars can lead to colic laminitis andor colitis. 
Complex carbohydrates such as fibre in the form of cellulose and hemicellulose as well as complex sugars such as fructan polysaccharides called fructan resist enzymatic digestion in the foregut and as a result are available for fermentation by the microbes living in the hindgut. These complex carbohydrates are usually more slowly fermented by the hindgut microbes thereby reducing the risk of an accumulation of acidic VFA. Hay is a rich source of complex carbohydrates contributing fructan cellulose and hemicellulose to equine diets. Horses are well adapted to utilize these complex carbohydrates as a safe energy source through healthy
hindgut fermentation.