WHY WOULD LOW STARCH BE GOOD? Avoiding high amounts of starch may be
benefi cial in some horses for a couple of reasons. After digestion of a starch meal, large amounts of glucose hit the blood- stream and trigger the release of insulin, which starts clearing the glucose out of the blood and ferrying it into cells. If a horse’s blood profi le is looked at for several hours following the ingestion of a starch meal, a rise in glucose is seen for the fi rst two hours and then glucose gradually drops over the next four hours. Looking at the profi le of insulin after ingestion of starch, it parallels that of glucose with a similar pattern of rising and falling. If there is a po- tential problem with the way an individual handles starch, it can possibly be because of the glucose and insulin fl uctuations in the bloodstream. Rising glucose levels can have an effect
on the brain of some horses and make them more excitable. Because glucose is the only fuel that passes the blood-brain barrier, something that can deposit a lot of glucose into the bloodstream at one time can have the potential to send signifi cant amounts to the brain. High levels of glucose in the brain are associated with increased do- pamine production and can cause height-
ened awareness or excitability. Certain sports depend on the heightened aware- ness and energy, like horse racing, but are not necessary for the pleasure horse. The other part of the glucose and insulin
cycle that can go awry in some horses is the ability of insulin to do its job and get glucose delivered into the cell. Insulin resistance occurs when insulin loses its effectiveness and the levels of glucose and insulin remain higher in the bloodstream for longer than normal. High insulin levels can cause a host of problems, the worst of which is making a horse more susceptible to laminitis. Insulin resistance is often as- sociated with obesity, and losing weight is one of the strategies for improving insulin sensitivity, as well as decreasing starch and sugar in the diet. Other individuals that have difficul-
ties with handling starch in the diet are horses with genetic muscle disorders like polysaccharide storage myopathy (PSSM) or recurrent exertional rhabdomyolysis (RER). PSSM horses are quite efficient at tucking away blood glucose into the muscle cell and end up with abnormally high amounts of glycogen, which, oddly enough, cannot be easily broken down and used for energy. Keeping foods that cause increases in blood glucose out of the diet is the recommended strategy for horses diag-
nosed with PSSM. RER also responds to a decrease in the amount of starch and sugar in the diet but for different reasons, and they are more tolerant to limited amounts. Another instance in which high starch
can pose a problem for a horse is when there is a shift in how the starch is digested. There appears to be a limited amount of the enzyme that digests starch in the small intestine (amylase), and the amount var- ies among individuals. By itself, that does not pose a problem for starch digestion because if there is any starch left in the digesta when entering the cecum there are microbes that will ferment it. The problem lies in that the end prod-
uct of fermentation of starch and sugar is lactate, which can have a signifi cant effect on the pH of the cecum. If high levels of starch are fed, the subsequent drop in pH in the cecum may affect the balance of the fi ber-digesting microbial population, thus upsetting the ability of the horse to get calories from the fi ber. This is referred to as “hindgut acidosis.” It is this effect of large grain meals on hindgut pH that is the basis for the recommendation of never feeding more than fi ve pounds of grain per feeding. However, it appears that the amount of starch that escapes digestion in the small intestine can be variable enough
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