Carbohydrates are a complex and often misunderstood group of macronutrients in the horse’s diet. The term is often used to describe only carbohydrates that can be broken down by enzymes in the horse’s small intestine, but fibres – indigestible by gut enzymes – are also carbohydrates. Carbohydrates are the main source of energy in the horse’s diet.
Carbohydrates can be divided into groups according to their appearance in plants, or how they are dealt with in the horse’s gut. In plants, carbohydrates broadly appear as simple sugars, storage substances e.g. starch and fructans, and structural substances e.g. hemicellulose and cellulose (fibre). In the horse’s gut, carbohydrates can be divided into two groups – those that can be broken down (hydrolysed) ready for digestion in the small intestine, and those that cannot, and instead are fermented by beneficial bacteria into volatile fatty acids (which can then be used by the horse).
The main source of structural carbs (fibre) in horse diets is fibrous forage such as grass, hay and haylage. Grass is the main source of fructans, which can vary widely in amount from 5 to 40% dry matter. Grass is also the main source of sugars in most horse diets. Cereal grains are the main source of starch. Soluble fibres including pectin are found in large amounts is sugar beet pulp.
A variety of laboratory analysis techniques divide carbohydrates into different fractions:
- ESC (ethanol soluble carbs): simple sugars e.g. glucose and fructose, disaccharides e.g. sucrose and maltose, and some oligosaccharides
- WSC (water soluble carbs): ESC plus starch and fructans
- NSP (non-starch polysaccharides): soluble fibre including pectin and gums, and insoluble fibre including hemicellulose and cellulose (structural carbs)
- NDF (neutral detergent fibre): hemicellulose and cellulose
- ADF (acid detergent fibre): cellulose and lignin
All carbs are made up of subunits of sugars joined together, but the reason some can be digested via gut enzymes and some cannot is down to the type of bonds that link these sugars. Carbs that cannot be broken down by gut enzymes contain bonds that are only breakable via bacterial fermentation. The volatile fatty acids (VFA) yielded by bacterial fermentation of fibre are a useful source of energy for horses; hence why horses can live on poor quality forage alone.
Horses have evolved to live on plenty of fibrous forage and limited starch intake, and their health depends on both. Starch is efficiently digested in the small intestine but only in limited quantities (believed to be about 1 g starch per kg bodyweight per meal) – any more and the digestive capacity is overwhelmed leading to starch flowing into the hindgut where it causes disturbance of the beneficial bacteria that reside there. Too little fibre intake and the beneficial bacteria that break it down will suffer, leading to gut disturbance and poor general health of the horse.
Fructans, despite being part of the water soluble carbohydrate fraction and not strictly a fibre, cannot be broken down by gut enzymes and instead are fermented by bacteria. Fermentation can be very rapid and large amounts can cause an acidosis that may result in laminitis.
Simple sugars and disaccharides are readily and efficiently digested and absorbed in the small intestine and do not cause gut disturbance when overfed. They do, however, when fed in substantial quantities, cause a marked glycaemic response which may cause insulin resistance and potentially increase the risk of laminitis.
Carbohydrates are a diverse group of macronutrients that are digested and absorbed by the horse in a variety of ways and careful consideration of the amounts of various types is necessary to ensure a balanced diet.