Anything you read about equine nutrition will tell you about the evils of starch. But how many horse owners really understand WHY it is so important to avoid starch in their horse’s diet?
First of all- what is starch? Starch is the energy store of plants. Also called non-structural carbohydrates (NSCs), starches are found inside of plant cells. Starch consists of two types of molecules, amylose and amylopectin.
The digestion of starch in horses begins in the small intestine with degradation by the enzyme alpha-amylase breaking the molecules into smaller units. Amyloglucosidase from the intestinal brush border then hydrolyzes these units, releasing glucose. This glucose can then be transported from the lumen of the small intestine, through the intestinal epithelial cells and into the bloodstream so that they can be used. This is what happens in an ideal situation with a manageable amount of starch. However, when too much grain is fed, or when pasture grasses are too high in NSCs, the limiting factor becomes the amylase. When there is not enough amylase present, the starch will not be broken down in time for this process to happen in the small intestine. Instead, starch will enter the cecum. This is the first part of the large intestine of the horse, and is an area heavily inhabited with microorganisms. These microorganisms aid the horse in digestion by producing digestive enzymes so that they can break down plant material. When a large amount of starch reaches the hindgut, microbes will ferment it. There are several disadvantages and even potential dangers to fermentation of starch in the hindgut of your horse.
First of all, fermenting the starch is a much less efficient usage than breaking it down earlier. Not many nutrients will be gained from it at the point of the hindgut. When starch is broken down prior to the cecum, glucose is able to be absorbed through the small intestine, and this is how the horse is able to gain maximum nutrition.
Additionally, a by-product of microbial fermentation is lactic acid. This lowers the pH of the hindgut, causing something called “hindgut acidosis”. This acid production can lead to several problems in the horse. It can cause a general discomfort that may lead to attitude and behavioral problems. It also may mean gas buildup that could lead to gas colic. In addition to this, lactic acid from the starch-fermenting bacteria may cause the “good” bacteria to die off and release toxins into the horse’s bloodstream. This can lead to system-wide inflammation and laminitis.
Now that you understand why people say to avoid starch in your horse’s diet, you may be concerned about the starch that IS present. You can’t avoid starch altogether, and some starch is needed to meet the nutritional requirements of your horse. The best thing that you can do to make sure that your horse safely digests their starch is to ensure that they have adequate digestive enzymes to break it down in their small intestine. This will not only help prevent digestive upsets, but will also help maximize their digestive efficiency. Horses have a limited amount of amylase, and it has also been found that their amylase enzymes are not as efficient as some bacterial and fungal amylase enzymes. Supplemental amylase is very beneficial in the diet of any horse, and will also help your peace of mind.
Here is a very interesting study about equine alpha-amylase and starch digestion: http://www.livestocklibrary.com.au/bitstream/handle/1234/19993/191.pdf?sequence=1