NUTRITION
CaRb CONUNdRUm Cereals, which are a major source of starch, have been part of racing diets for decades. The staple racing feed in ‘the good old days’ was oats and bran, which powered many famous racehorses home. Some would say that tradition is a poor excuse to hamper progress, especially if health and welfare are concerned. High starch rations continue to be in the spotlight as they have been implicated in many commonly occurring health issues such as gastric ulcers, tying up, laminitis, colic, and other digestive disturbances. However, starch has in the past been equally regarded as essential for horses in training to adequately replenish important muscle and liver glycogen stores and to allow a plentiful supply of glucose to the brain. Starch has also been thought to be critical for giving racehorses that competitive edge or ‘will to win.’ Practically, in my experience there are racehorses that race and win on either high, moderate, or low starch diets, so which is preferable? Let’s look at the evidence. WORDS: DR. CATHERINE DUNNETT BSC, PHD, R.NUTR PHOTO: SHUTTERSTOCK, FIONA BOYD
Oat starch is more digestible than starch from other uncooked cereals
Starch is a polymer of glucose with both straight chains (amylose) and branched structures (amylopectin). The nature of starch in different feed ingredients varies according to the proportion of each type and also on the degree of secondary folding in the structure. Horses produce the enzymes necessary to break down a variety of starch types in the small intestine, but some are more easily digested than others in their natural state. For example, oat starch is very much more digestible in the small intestine than barley or maize starch fed without processing. Cooking a cereal grain helps to unfold and gelatinize the starch making it more digestible, as does grinding to a certain extent, although other mechanical treatments such as clipping or rolling are much less effective. Starch is an efficient source of glucose, which helps to maintain the concentration in blood and also provides the building blocks to replenish muscle and liver glycogen (the storage form of glucose). Glucose is released relatively quickly from starch, as long as it is predominantly digested in the small intestine. We must also, however, appreciate that glycogen can be replenished from the fiber part of a ration. Fiber is fermented in the hindgut to produce volatile fatty acids including acetic acid, butyric acid, and propionic acid, the latter of which supports 72
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glycogen re-synthesis. The ratio of propionic acid to other volatile fatty acids is increased when more concentrate feed is fed, and this probably reflects a greater delivery of starch to the hindgut, which is not necessarily desirable. The type of forage fed may also effect the production of propionic acid.
Horses have a slow rate of glycogen re-synthesis
When compared to human athletes, Thoroughbred horses have an unusually high concentration of muscle glycogen
but have a contrastingly slow rate of re-synthesis. Early studies showed that horses fed a typical high starch racing diet (35-40% starch) required three days to return muscle glycogen concentration to baseline levels following hard work. However, more recent work suggests that the composition of the diet has little effect on the rate of glycogen re-synthesis and that providing an adequate period of time between the last sessions of hard work and racing is important.