Nutrition CPD September 2017

37

Understanding carbohydrates

By Anna Welch BVSc, BSc, MRCVS. Veterinary Nutrition Director, TopSpec

Carbohydrates play an essential role in a horse’s diet. When providing horse owners with nutritional advice, it’s essential to have an understanding of the different types of carbohydrate and how they are utilised. Getting the balance right will promote good health and performance, as well as relaxed behaviour. 

What are carbohydrates?

Carbohydrates are the main source of energy for a horse. Glucose is the simplest form of carbohydrate and is a building block for many others.

Glucose is a monosaccharide which consists of just one sugar unit (fig. 1). Two units linked together are called a disaccharide. Sucrose (glucose + fructose) is an example of a disaccharide and is the most common sugar found in plants. Another disaccharide is lactose (glucose + galactose) which is important for nursing foals but cannot be digested in mature horses. 

Oligosaccharides are made up of short chains of monosaccharide units. Mannan Oligosaccharides (MOS) are used in the diet of horses as an immune system stimulant and a prebiotic, promoting the normal cellulolytic (fibre-digesting) bacteria in the hindgut. 

Polysaccharides are long chains of sugar units which can be found in the cell wall of plants as fibre (e.g. hemi-cellulose and cellulose), or as a form of stored energy called starch. The horse itself can also store energy as a polysaccharide known as glycogen. 

Carbohydrates can be divided into two groups, simple and complex. These two groups are distinguished by a simple difference in structure which dictates whether or not they can be broken down by enzymes in the horse’s small intestine. 

Simple carbohydrates

Simple carbohydrates, or Non-Structural Carbohydrates (NSC), include sugar and starch. 

The monosaccharide units in NSCs are bound by ß-linkages (fig. 2a). These can be broken down by enzymes, such as amylase, in the small intestine of the horse to produce glucose. However amylase is only produced in small quantities by the horse and therefore can only hydrolyse [break down by chemical reaction with water] a limited amount of starch at a time. 

Glucose is then absorbed into the bloodstream and transported to various tissues in the body, where it can either be used to provide energy immediately, or stored as glycogen or fat. Glycogen is stored in the muscles or liver, where it can be utilised as fuel at a later time as required.

If large quantities of starch are fed, excess undigested starch passes into the hindgut where it is digested by acid producing bacteria, including amylolytic [starch-digesting] bacteria. Consequently, numbers of amylolytic bacteria increase and the hindgut becomes more acidic, which can result in a number of problems for the horse. 

Complex carbohydrates

Complex carbohydrates, or structural carbohydrates, occur in the cell wall portion of the plant and are referred to as fibre. In contrast to simple carbohydrates, the sugar units are bound by ß-linkages (fig. 2b) which cannot be broken down by the enzymes in the horse’s small intestine. 

Therefore, fibre passes to the large intestine where it is fermented by the cellulolytic (fibre-digesting) bacteria which can break the bond. This microbial fermentation produces volatile fatty acids, mainly acetate, propionate and butyrate, which can be used as energy sources. 

Not all fibres passing into the hindgut are fermented equally (fig. 3). Of the fibre reaching the hindgut, pectin is believed to have the highest digestibility, followed by hemi-cellulose and then cellulose. 

Lignin is not at all digestible and, although technically not a carbohydrate, it is closely related and an important promoter of gut motility. Lignin rich forage, such as straw, can be useful in limited quantities to prolong fibre consumption in good-doers on restricted forage intake. 

Misconceptions associated with the use of carbohydrates

Low NSC diets are recommended for horses in many situations, such as those prone to gastric ulcers, diarrhoea, colic, laminitis, Insulin Dysregulation (ID), PPID (Cushing’s), tying-up, fizzy behaviour, Developmental Orthopaedic Disease (DOD) and stereotypical behaviour. However, it is a misconception by some that this means their diet should be ‘low-carbohydrate.’ Fibre is a carbohydrate and should form the basis of all horses’ diets, especially those with any of the issues above. 

Sugar and starch is present, although the levels vary, in grass and hay/haylage, as well as in the raw materials used in hard feeds. Glucose is the primary energy source used by tissues in the body, especially within the central nervous system, including the brain. Blood glucose levels of the horse are maintained within tight parameters. 

Therefore, NSCs should not, and cannot, be eliminated from the diet completely. If the diet provides insufficient amounts, other nutrients may be converted to glucose inefficiently to meet essential demand. 

What does this mean for your customers’ horses?

A constant supply of fibre (complex carbohydrate) is essential to maintain a healthy digestive system, with optimal gut motility and balanced hindgut microflora. Most horses should be offered forage ad-lib and it is beneficial for as much of their calorie needs as possible to be met by their forage. 

Grass early in its growth cycle (i.e. spring), tends to be lower in fibre (structural carbohydrates) and higher in sugar/starch (NSCs). Haylage cut at this time will also reflect these nutritional values. This type of forage is more appropriate for competition horses, poor-doers, and pregnant or lactating mares. 

Conversely, as grass matures through the growing season, fibre (including cellulose and lignin) content rises and NSCs reduce. This makes the grass, and hay cut at this time, better suited to those needing a high fibre and low sugar and starch diet, such as over-excitable horses, good-doers or those with PPID (Cushing’s) or laminitis. 

For a horse with low-energy requirements, ad-lib high-fibre forage, balanced using a top specification low-calorie feed balancer or multi-supplement, may be all that they require providing a salt lick is available 24/7. 

A horse with higher calorie requirements may not be able to meet their needs with good-quality forage alone. A top specification conditioning feed balancer will enable the horse to utilise maximum nutrients from his forage whilst the addition of an appropriate blend, straights or compound feed can provide more concentrated energy sources.

Feeding a diet that is low in sugar and starch is beneficial to most horses. Therefore, before reaching for cereal-grains or cereal containing compounds, products containing highly digestible fibre sources (‘super-fibres’) should be used to supply additional calories. Examples of ‘super-fibres’ include sugar beet pulp and soya hulls. The addition of oil can also be useful to increase the Digestible Energy (DE) of the feed, without the need for high starch ingredients. 

When higher starch diets are necessary, it is important to feed cereal-grains which are digested well in the horse’s small intestine. This means that provided excessive quantities are not fed, overflow of starch into the hindgut is less likely. 

Pre-caecal digestibility as it is termed, is not only influenced by the cereal grain used but also whether they are fed raw, or have been processed or cooked. 

Raw oats have the highest pre-caecal digestion rate (95%); all other cereal grains such as wheat, barley or maize need cooking before they are fed to a horse. 

Summary

• Carbohydrates are the main source of energy for a horse.
• Complex, or structural, carbohydrates (fibre) should form the bulk of a horse’s diet whenever possible.
• The digestion of simple, or non-structural, carbohydrates (sugar and starch) in the horse’s small intestine is limited. Therefore intake should be controlled.
• The appropriate level of sugar and starch will vary according to individual requirements.