Written by Dr. Tom Shurlock for and on behalf of GWF Nutrition Limited.
The Impact of Starch & Sugar on Equine Diets
Copyright: GWF Nutrition Limited - Not for Reproduction.
Not so long ago, feeding starch was seen to be perfectly acceptable, especially in the form of oats. Over recent years the “expert” opinion is that starch and sugar is not to be fed and fibre is the way to go.
As with all biological systems, the truth is somewhere in between. What makes it all the more confusing is the relationship between starch, sugars and fibre and the technical terms for them which all seem similar.
All these products are carbohydrates. Within this classification are a group of carbohydrates called sugars. Their structure tends to be cyclic (atoms arranged into a circle) and there are about 18 sugars of importance to nutrition ranging from glucose to deoxyribose (as in DNA), as disaccharides (lactose, sucrose), oligosaccharides (fructans) and polysaccharides (Starch, glycogen).
Because they are fairly complex molecules in their layout they can vary. This does not make them different chemicals but can give them different properties. If we look at the glucose molecule (glucose being, arguably, the “first” of the sugars) we can see that at position 1 the other atoms can swop position:
H and OH at position 1 on Alpha and Beta glucose
Chemically, these are identical molecules and both will be broken down by the metabolic process of glycolysis.
The difference is the way they join to form oligosaccharides and polysaccharides (short and long chains of sugars). Sugars join by condensation. A water molecule (H2O) is released from the two OH’s at position 1 & 4, forming a link via the remaining O. Depending on the configuration at position 1 we get either an alpha, or a beta link:
Alpha Link Beta Link
It is this difference that characterises starch and sugar on one side and fibre on the other. Simply because the beta link is the wrong shape for the enzymes produced by any animal to break them. They are indigestible. However, some bacteria, yeasts and protozoa can break these links by a process called fermentation and their fermentation products are absorbed and used as “slow release energy”.
The horse evolved to use fibre as its main source of energy. Its hindgut is a large fermentation chamber where those microbes that can attack the beta link produce a range of short chain acids – the VFA that are absorbed and metabolised. The environment of the hindgut is such that it can be disrupted if high levels of alpha linked sugars and starch reach it. There are bacteria living higher up in the small intestine that live off starches and sugars in competition with their host, the horse.
If we provide too much starch and sugar in the diet the horse’s own enzymes and absorptive processes cannot cope and the sugars flood into the hindgut along with those bacteria that feed off them. Their end products are more acidic than the fibre fermenters, which results in the disruption of the environment and this can lead to changes in the populations, death of the fibre fermenters and a rise in toxic end products called 'endotoxins'.
When starch is fermented by various bacteria one of the main end products is lactic acid. In the small intestine this is used by other bacteria that convert it to other products. For whatever reason this does not happen in the hindgut, so starch reaching this area without being absorbed will produce lactic acid and this acidification will add to the process of environment change.
Starch and sugars, in themselves, are not bad. Glucose, the base unit of starch, is an essential energy source. The breakdown of glucose generates ATP for muscle contraction and is particularly important for the fast twitch, power, muscle fibres. It is one of the few nutrients that cross the blood-brain barrier and, at the gut level, is involved in the active transport of nutrients. As such, glucose is regarded as an essential nutrient.
So why does an essential nutrient become a danger to the horse? As we explained earlier it is because it can be fermented and, when it is fermented in the wrong place, it disrupts the normal environment of the hind gut. This is mainly down to how we feed horses.
The horse is a grazer; continuously eating small amounts of grass, which is chewed and swallowed. It enters the stomach where the stomach acids start to process the starches and sugars for when they enter the small intestine. Here the slightly acidic environment is suitable for the horse’s enzymes to break down the starch into shorter chain sugars. These in turn are broken down by other enzymes into sugars and disaccharides, which are small enough units to be absorbed across the gut lining. As the stomach is small for such a large animal and acid secretion is continuous (in humans, acid secretion only occurs over the period of a meal), feed is not held there long and passes into the small intestine. Passage through the small intestine is fairly rapid, enzyme secretion is relatively low and so uptake of starch/sugar is low. For a wild horse living off tundra grass and scrub this is not a problem as the sugar levels in his environment are extremely low. The problem really only occurs for the domestic animal.
Feeding a horse a cereal-rich diet in itself is not a problem, it is the amount we feed that is important. As the horse’s system can only cope with a certain level of starch (sugar is not such a problem as it may not need to be enzymatically broken down) and the ability of enzyme output to convert it to sugar, there may be too much for this to happen. Additionally, there is a time factor. The passage of feed through the small intestine is rapid and there simply isn’t enough time for all of it to be absorbed. We can affect this though, by choosing suitable starch sources or modifying the starch molecule.
Starch is an energy storage system. It varies in its structure between cereals, oilseeds and roots & tubers and also within these groups. For example, looking at a starch granule under the microscope, there is a difference between cereal starches.
Individual Starch Granules (x4000) of Oats (top left), Barley (top right) and Maize (above).
Of particular note is the uneven and pitted surface of the oat starch granule; this allows amylase (the enzyme that breaks down starch) to attach to and penetrate the granule, whilst the maize granule presents an almost impermeable barrier. It simply means that the oat starch is broken down more easily and so will be absorbed. Oat starch has a digestibility value of about 90% compared to around 30% for maize or barley. We can improve these values by cooking as this opens up the granules with steam, allowing greater access by enzymes.
However, at the end of the day, we can only improve digestibility so far, so there will always be a trickle of starch into the hind gut. As with everything a little is not a problem, it’s when large amounts enter the hind gut.
Apart from the inability of the horse to process and digest large quantities of starch and sugar, there can be more serious problems associated with a high sugar / high starch diet. These include Cushing’s Disease, Exertional Rhabdomyolysis, Insulin Resistance and Polysaccharide Storage Myopathy. The common factor in all of these is a malfunction in the biochemistry of the processing of glucose to provide energy and it mainly relates to the initial decomposition of glucose (or the provision of glucose stored in the body as glycogen). Energy metabolism occurs in two phases, the first being exclusively for glucose and other sugars (fat, protein and “slow release nutrients” can all enter the system at a later point) so switching to these nutrients can spare glucose energy as far as possible.
Two other factors need to be taken into account. Overfeeding of starch and sugars can lead to nutritional laminitis. The lactic acid generated in the hind gut opens up the gut wall and allows the absorption of endotoxins that contribute to inflammation in the hooves.
The other factor: Cereals, the main starch source in equine feeding, can add to the acidity of the stomach. Not only do they not absorb the stomach acids they can be fermented, fuelling acid generating bacteria. These have been shown to be able to penetrate the mucosal stomach layer and contributing to ulcers.
Feeding starch and sugar to a horse is unavoidable. As sugars are the end product of photosynthesis it is inevitable that it will be present in a horse’s diet. There will be times (when on spring grass, for example) that the starch/sugar load will be high, or health is compromised and it would be sensible to replace some of this material with a low starch product. It has been recommended that an active horse in good condition should eat no more than 1 gram of starch per kg body weight per meal. As a result it can easily be seen that low starch products need to be included to ensure daily intake remains manageable.
Products like No Starch Fibregest can be fed alongside hard feeds, or replacing high sugar products to ensure intake can be moderate and therefore not impact on the normal gut function of a herbivore.