The horse’s gut is not very conducive to athleticism.The structure and function of the equine gut is complex and easily prone to digestive upset, for example at the location of the pelvic flexure. The shear size and weight of the gut, plus its persistent contents, means it must not be the easiest structure to carry around.
The efficient functioning of the hindgut plays a key role in generating energy and nutrients for metabolism and the subsequent maintenance of homeostasis. The horse has evolved to require a minimum amount of dietary fibre as a food source for the micro-organisms located in the caecum and large colon. In addition, the small intestine of the horse can digest very little starch at any one time and consequently, it struggles to deal with concentrate meals containing more than 3-4 g starch per kilogram of body weight per meal (Potter et al., 1992). Disruption of normal hindgut function by less than adequate dietary fibre or excess dietary starch often reduces the ability of the hindgut to maintain a near-neutral pH, which can lead to acidic conditions, often referred to as acidosis. Acidosis actually relates to a lowering of blood alkali reserves (this means a lack of buffer to compensate for the increased acidity) and many horses suffer from a sub-clinical form, particularly those in hard work coupled with limited access to forage. Excessive amounts of starch that are fermented in the small intestine may also have implications for performance. When volatile fatty acids (VFA) are produced as products of fermentation, they are absorbed across the mucosal barrier together with water. If absorption does not mirror production and VFA accumulate in the caecum or colon, the animal attempts to buffer the resultant pH decrease by recruiting water from the blood. This of course has consequences for osmolarity and subsequent hydration.
Unfortunately, data to try to elucidate exactly what is happening at gut level during exercise appear scarce in the horse. It is known that exercise seems to induce increases in adrenocorticotrophic hormone (ACTH) and cortisol as part of a normal response to exertion, rather than that of a stress reposnse (Harrington McKeever and Gordon, 2008) and their respective concentrations are dependent on the intensity and duration of exercise (Nagata et al., 1999). Effects of diet on response to exercise have also been studied. High hydrolysable carbohydrate diets have been shown to increase heart rate and blood lactate concentration with little or no benefit to performance. Fat supplementation has proven useful in certain situations, however, animals need time to become metabolically adapted to fat supplementation – this may be around 2-3 months for complete adaptation (Pösö et al., 2008).
As the horse employs fermentation as a major digestive mechanism at both the front and rear of the GI tract, it follows that reasonable amounts of gas are produced and require removal. It has been suggested that the escape of this gas may be impeded during exercise and result in significant discomfort to the animal (Lorenzo-Figueras and Merritt, 2008). Another issue associated with fermentation in the stomach is gastric ulcers and data exists to demonstrate an increased prevalence in horses undergoing race training (Orsini et al., 2009). Additionally, the rate of gastric emptying is potentially reduced during exertion, which may have implications for the above however, this area is lacking information. Intestinal dysmotility (abnormal gut motility) is likely to be a significant issue with regards to exertion and is an area that requires more research.
Aside from the metabolic issues of gut function often associated with performance horses, there are also physical considerations. For example, an increase in intra-abdominal pressure via contraction or tensing of the abdominal muscles could potentially have numerous consequences, such as physical dislocation of the intestines, diversion of blood flow and release of regulatory compounds (Lorenzo-Figueras and Merritt, 2008). Indeed, in humans the term ‘caecal slap’ has been assigned to the knocking of the caecum against the abdominal wall while running (Porter,
1982). Implications for this include effects on blood flow and consequently, any functions influenced by blood flow (of which digestion is one) would be affected. Traditionally, animals are restricted in their access to feed prior to exercise however, increased blood flow to the GI tract has been demonstrated in animals that have been fed rather than fasted and may be at lower risk of hypovolaemia. Maintaining sufficient water ingestion is also key to maintaining blood flow – forage versus concentrate meals encourage greater water ingestion. Additionally, concentrate meals offered 2-3hours prior to exercise can alter nutrient metabolism, such that lipid oxidation is considerably reduced, which may be detrimental to performance. Conversely, forage meals have no such effect (Pösö et al., 2008).
In summary, the effect of moderate to intensive exercise on the functioning of the gut is complex and not fully understood in the horse. However, ensuring the supply of optimal levels of nutrients and energy via quality feed, as well as promoting gut health through the potential use of yeast-based supplements, the horse may be better able to cope with the physiological challenge of exercise.