A long time ago in a galaxy far, far away…
Our horses have evolved as herd dwellers, designed to trickle feed on high fibre low starch diets. Domestication and requirements imposed on horses by modern life has forced diet adaptation. Our hectic lives have impacted our horses and it has become, in many circles, a requirement that our horses consume and perform on concentrate based rations with limited access to forage. Chosen carefully and fed in appropriate quantities, such feeds can provide the performance that humans require. But at what cost to the horse? The physiology of the equine tract is poorly adapted to fit with this regime and can lead to ill health and behavioural problems.
Internal fusion reactions
To consider the effects of starches and sugars, we must first look at how our equine jedi metabolise and meet their energy requirements. Namely by carbohydrates, which are categorised by their site of digestion. This is by enzymatic hydrolysis in the small intestine and by fermentation in the hind gut. Non-starch polysaccharides found in grasses and conserved forages provide the bulk of energy requirements through hind gut fermentation. But here I am concentrating on looking at supplementary feeding of starches and sugars. These are known as non-structural carbohydrates (NSC).
Sugars are the building blocks of dietary carbohydrates. In simple form they are glucose, fructose, galactose and mannose. These monosaccharides are the most basic energy storage molecules and more complex sugars are formed by joining two or more simple sugars together. Monosaccharides are water soluble and are the form in which carbohydrates are carried around the body in the bloodstream.
Starch is commonly found in cereals such as oats, corn, wheat and barley and is formed by long chains of linked glucose molecules. This is a bit like the highly charged particles fuelling star wars spaceships!
Fig 1. Example of monossacharide glucose molecule 
In recent years our knowledge of factors influencing NSC content in horse feeds has advanced considerably. We can affect the starch content of cereal grains via a number of factors such as crop variety, stage of growth and time of harvest, environmental factors such as weather conditions and artificial factors such as the use of fertiliser.
The Disturbance In The Force
Our equines are ill equipped to digest large quantities of starch and sugar. This is due to the fact that the equine gastrointestinal tract is perfectly equipped to extract the majority of energy requirements through fermentation of dietary fibre in the hind gut. There is limited capacity for digestion in the small intestine. The problem with feeding starches therefore becomes apparent as starch and sugar requires digestion mainly in the small intestine. Research has found that the reported mean daily concentrated feed in Australian racehorses was 7kg across two meals  which is typical of a feeding pattern for performance horses. However, in 2006 research had shown that the capacity for starch and sugar digestion in a horse is estimated at 1kg per meal for a 500kg horse . Considering the 2006 research, there is a very large amount of starch and sugar which goes undigested in the equine. Once undigested starch passes into the hind gut where fermentation occurs, the acidity of the large colon can increase and there can be a disruption of the microbial population. This gives the potential for digestive upset and disease. The speed with which the feed passes through the small intestine influences digestibility. This is affected by meal size, feeding frequency and chewing. In the words of the great Han Solo “Without precise calculations we could fly right through a star or bounce too close to a supernova, and that’d end your trip real quick, wouldn’t it?”
It’s Not Fair
All cereals are not created equal. In 2012, a study by Brokner et al.  showed that the starch content of maize and barley is higher than oats. Oats have a greater digestibility in the small intestine than maize or barley. Due to the lower digestibility of maize and barley, more of it passes into the hind gut to be fermented. However, heat treatment of maize and barley including micronising, steam flaking and extruding increases their digestibility in the small intestine.
Rate of passage is also affected by particle size and horses that do not chew well due to age or infirmity will allow large particles to pass into the digestive system.
Keeping Balance In The Force
So how do we fuel our equines without causing an inbalance in the force? Obviously we wish to give them enough energy to perform the work we ask of them whether it be hacking, or competing in a discipline. Muscle contractions require energy which is derived from digestion and chemical interaction. Research has shown that fats yield energy in horses and yet generate 3% less heat than glucose. Heat reduction may be beneficial for performance horses who are competing under heat-stress conditions .
We can divide carbohydrates into three categories. The first is the hydrolysable group CHO-H which is starches and sugars broken down in the small intestine, the second is the fermentable group CHO-FR which is things such as young grass digested in the hindgut by fermentation and the third group is the slowly fermentable group CHO-FS which is feed such as mature grasses and late cut hay which is also fermented in the hind gut at a slower rate. Each of these types of carbohydrate has a different effect on the horse and can be seen in the table below:
It Is Pointless To Resist
Insulin resistance is defined as a decreased sensitivity or responsiveness to insulin-mediated glucose disposal or inhbition of hepatic glucose production . In other words, the cells in our horses fail to respond normally to the hormone insulin. This is a serious disturbance in the force of the jedi resulting in raised blood sugar levels. Insulin resistance has been linked to laminitis, equine polysaccharide storage myopathy (EPSSM), recurrent exertional rhabdomyolysis (RER), developmental orthopaedic disease (DOD), colic and hindgut acidosis. Whilst this can affect any horse, there does seem to be an insulin resistant phenotype which exists in horses. This genetic defect means these horses are more prone to developing disorders if fed diets high in starches and sugars .
Laminitis and Insulin Resistance
There are three theories that currently exist as regards laminitis and insulin resistance.
- Insulin resistance diminshes the uptake of glucose by lamellar cells. This is detrimental as the hoof tissue has a high requirement for glucose.
- Insulin resistance disturbs vascular function. This causes the laminae to be more prone to injury when exposed to other factors and so promotes the development of laminitis.
- There is evidence to suggest that obesity related insulin resistance is due to an inflammatory state which originates in adipose tissue (fat). Fat produces inflammatory hormones which disrupt insulin signalling . This then worsens the insulin resistance and the horse is stuck in a vicious circle.
Before domestication a horse with the insulin resistant phenotype may have been desireable. This is because such horses can survive harsh conditions. However, under the present day domestic conditions, the feeding of starch and sugars to this type should be very limited to prevent laminitis. I have heard it said many times that ponies are more prone to laminitis than horses. This is not so. Any horse or pony is prone to laminitis. It is just that insulin sensitivity is low in ponies which is possibly why they have a higher incidence of laminitis.
D.O.D and Insulin Resistance
Developmental Orthopaedic disease in young horses has also been linked to insulin problems. Again this comes down to the impatience of humans. The feeding of concentrate meals with high grain content provokes growth hormone secretion and is mediated by Insulin Growth Factor (IGF-1). High IGF-1 concentrations may affect the maturation of cells containing cartilage (chondrocytes). This increases the risk of developmental orthopaedic disease. In 2007 a study was released whereby 24 foals had been studied over a 2 year period . This study found that horses fed high glycaemic meals had raised levels of IGF-1 in their plasma during the spring compensatory period. Taking this into account it may be that we wish to limit grains in youngsters. This means we raise balanced horses with healthy joints, well prepared for the day they are ready for backing.
The hyperdrive is leaking
Hindgut Acidosis & Sugars and Starches
Starch and fructan overload can disturb hindgut fermentation and bacterial flora. Carbohydrate overload causes major changes in the hindgut. This includes reproduction of streptococcal species, a lowering of the PH and an increase of intestinal permeability. An increase in intestinal permeability means that there can be entry of endotoxins, bacterial components and histamines into the circulation. In laminitis this causes systemic and lamellar inflammatory responses . Hindgut acidosis which occurs in this way may be a primary factor in triggering metabolic related diseases in horses with genetic predisposition to insulin resistance.
This is a genetic condition which causes horses to be efficient at storing blood glucose. This description in itself indicates a necessity that such horses have limited to no access to sugar and starches.
Effects of Exercise
Exercise is essential to regulate metabolic function. It is possible that exercise training interacts with diet adaptation to reverse diet-induced changes in glucose transfer when our horses are at rest.Higher concentrations of dietary sugars may cause lowered glycogen usage in muscles. This may be beneficial in disciplines such as endurance riding .
The Dark Side Clouds Everything
We now come to possibly one of the hottest topics being debated at the moment. Can feed be ‘heating’ and making owners reach for the nearest calming supplement?
Concentrate meals rich in NSC’s are broken down by converting starch to glucose. About 2 hours after such a feed, the circulating glucose in the blood may rise. This is referred to as the glycaemic response. Feeding our horses such carbohydrates on a ‘meal’ basis creates peaks and troughs in sugar levels. Horses fed on fibre have lower and more even responses in blood glucose levels. It is the peaks and troughs in blood sugar levels due to starch digestion which is suggested to affect behaviour.
A rise in blood glucose levels leads to an increase in the release of serotonin (a brain hormone linked with pleasure). Horses experiencing a glycaemic peak will also experience heightened insulin production. Due to this, tyrosine (an amino acid) is diverted from its normal pathway to the brain. This allows an increase in the chanelling of tryptophan to the brain. In turn, the brain increases serotonin production . The increase in serotonin levels causes increased reactivity which can cause our horses to behave seemingly irrationally. This is usually the point at which owners will complain of their horses becoming difficult to handle and instead of the calm and composed jedi, we suddenly see more of an R2D2 with it’s circuits blown! Levels of tryptophan have been found to differ between horses fed a high fibre and fat diet and those fed a high starch diet . The savvy owner may want to consider this the next time they open a sack of feed with high levels of starch.
But what about molasses? Everything these days seems to contain molasses. These evil sugars are definitely from the dark side! They may be easily digestible in the small intestine but this is what makes them so dangerous. Consuming molasses may result in excitability simply because they are so easily digested. And when molasses are provided with cereals it’s like sending the millenium falcon into hyperspace! The blood glucose response is completely exaggerated.
Already Know You That Which You Need
Many studies show that diets high in fibre and fat reduce reactivity levels. This results in horses with improved performance, lower cortisol concentrations, lower heart rates and fewer startle reactions. In essence our jedis are balanced and focused. And in consideration of what we feed them let us consider not only grains and molasses but also the treats which we so dearly cling to when currying favour with our horses. Carrots and other root vegetables are sugar in disguise. They are an absolute no go for our horses. Instead choose the humble apple, and even this in moderation. In the words of the great Master Yoda ” Mind what you have learned. Save you it can.”
 Richards N., Hinch G.N., and Rowe J.B (2006) The effect of current grain feeding practices on hindgut starch fermentation and acidosis in the Australian racing Thoroughbred. Australian Veterinary Journal 84(11) pp.402-407
 Shafrir E. and Ziv E. (1998) Cellular mechanism of nutritionally induced insulin resistance. The desert rodent Psammommyms obesus and other animals in which insulin resistance leads to detrimental outcome. Journal of Basic Clin. Physiol. Pharm. 9:347-385
 Belknap J.K., Giguere, S, Pettigrew A., Cochran A.M, Van Eps A.W. and Pollitt C.C (2007) Lamellar pro-inflammatory cytokine expression patterns in laminitis at the developmental stage and at the onset of lameness: innate vs. adaptive immune response. Equine Vet. Journal Vol.39 1:42-47
 Jose-Cunilleras E, Hinchcliff KW, Sams RA, Devor S.T and Linderman J.K (2002)Glycemic index of a meal fed before exercise alters substrate use and glucose flux in exercising horses. J Appl Physiol 92:117-128.
 Hale, C., Hemmings, A.J., Bee, S.E. (2011). The effects of a high starch, cereal-based diet compared to a low starch, fibre-based diet on reactivity in horses. In: Lindner, A. (Ed.). Applied Equine Nutrition and Training. Wageningen: Wageningen Academic Publishers, 227-231
 Wilson, A.D., Badnell-Waters, A.J., Bice, R., Kelland, A., Harris, P.A., Nicol, C.J.(2007). The effects of diet on blood glucose, insulin, gastrin and the serum tryptophan: Large neutral amino acid ratio in foals. The Veterinary Journal. 174(1), 139-146