Polysaccharide carbohydrate comprised of linked glucose molecules which is produced by plants to store energy, mainly in their seeds and grain. Grain is therefore high in starch content whilst forage feeds are very low starch. Starch is an excellent energy source for many horses when fed in moderation, but there are two significant issues around feeding high starch feeds like grain which are of concern.
- Digestive upset & colic. Starch is only present in small amounts in the natural, mainly forage diet of horses and is digested by enzymes (amylase) in the small intestine. If starch intake increases to the extent that it overwhelms the capacity of the small intestinal enzymes to digest it, then some will pass undigested into the hind gut. Once there it will undergo digestion by microbial fermentation. One of the by-products of that is lactic acid. If present in sufficient quantity this acid can damage the lining of the hind gut and leak into the circulation causing metabolic acidosis. Gut toxins may also permeate the bowel wall in so called ‘’leaky gut syndrome’’ with consequences such as colic, circulatory disturbances, systemic shock, and laminitis. Disturbance to the normal microbiota caused by the excess acid and overgrowof the bacteria which feed off starch can also result in gas formation and altered gut motility, again potential causes of colic. These negative consequences depend largely on how much starch reaches the hind gut and how accessible it is to fermenting microbes.
- Insulin Resistance. When starch is digested as it should be, in the small intestine, the product is glucose which is absorbed into the bloodstream provoking the insulin response. As blood glucose rises, the pancreas releases more insulin prompting the uptake of that glucose by tissues like muscle and liver. The blood glucose level falls in an hour or two, followed by a fall in insulin level as the high blood glucose stimulus is reduced. That’s the normal “insulin response’’. However, when high starch diets are fed over long periods as is common for hard working horses on high grain diets, the system can become overloaded. Constant driving of the insulin response by high blood glucose from digested starch eventually blunts the response and the horse is said to have become ‘’Insulin Resistant’’. Consequences include a variety of metabolic disturbances including increased risk of developmental orthopaedic diseases like OCD in young horses, and Laminitis.
Grain processing techniques such as micronization and extrusion can enhance small intestinal starch digestion and render it safer so far as reducing overspill into the hind gut. Processing will not however reduce the impact of starchy feedstuffs on blood sugar and the potential for insulin resistance.