From a plant perspective, carbohydrates fall into three categories: Simple sugars active in plant intermediary metabolism; storage compounds such as sucrose, starch and fructans; and structural carbohydrates such as pectin, cellulose and hemicelluloses. For the horse, however, it is more appropriate to classify carbohydrates by where and how quickly they are digested and absorbed.
Carbohydrates can be digested and absorbed as monosaccharides, simple sugars (primarily glucose and fructose), in the small intestine, or they can be fermented in the large intestine to produce volatile fatty acids (VFA) or lactic acid. The rate of fermentation and types of end products produced are quite variable and can have significant effects on the health and well-being of the horse.
Starch is the predominant carbohydrate fraction in cereal grains. Although all starch is made up of glucose chains, the starch molecule is constructed differently in various types of grain. These differences in the architecture of individual starches have a large impact on how well they are digested in the horse’s small intestine. Of the grains most commonly fed to horses, oats contain the most digestible form of starch, followed by sorghum, corn and barley. Processing of grains can have a huge effect of pre-cecal starch digestibility and glycemic index. This is particularly true for barley, sorghum and corn.
The quantity of blood glucose produced in response to a meal is a useful measure of a feed’s hydrolysable carbohydrate content. The glycemic index of a feed indicates the rate of carbohydrate absorption after the horse ingests that feed. For the purpose of comparison, oats are assigned a glycemic index of 100. Steam-flaked corn has a higher glycemic index of 144, as does sweet feed at 129, indicating that more glucose is derived from their pre-cecal digestion than from oats. Feeds with lower glycemic indices include cracked corn (90), rice bran (47), alfalfa hay (46) and bluestem grass hay (23).
Rapid fermentation can produce lactic acid, which may lead to a cascade of events culminating in laminitis. Undigested starch from cereal grains and fructans from pasture are the most likely compounds contributing to lactic acidosis in the hindgut. Slowly fermentable carbohydrates from the plant cell wall are absolutely essential to maintain a healthy microbial environment in the hindgut, and provide a major source of digestible energy for many horses.
Obesity is a growing problem in the horse population and is an important risk factor for laminitis through the association with insulin resistance and hyperinsulinemia. Excessive caloric intake can lead to the development of obesity and other clinical signs in horses with equine metabolic syndrome and pituitary pars intermedia dysfunction. Owners should monitor body condition of their horses and should not let them become obese. In general, a horse whose ribs are easily seen may be too thin; a horse whose ribs can’t be felt by pressing the fingertips over the horse’s barrel is probably too fat; and a horse whose ribs can’t be easily seen but can easily be felt with fingertip pressure is at a reasonable weight.