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Does Your Horse Really Need A Low-Starch Diet?By Kentucky Equine Research · May 4, 2011

Horses derive dietary energy from a multitude of sources. A combination of fiber, starch, and fat is generally the best way to fuel performance, whether it is in the breeding shed, on the trail, or in the show ring. Over the last several years, the market has become flooded with low-starch feeds, and horse owners are responding by purchasing more and more.

Yet, low-starch feeds are not appropriate for every horse. Certain groups of horses require starch for optimal performance—growing youngsters and many equine athletes, for example. What lies behind the current low-starch craze?

What is starch?

Starch is a long, complex chain of sugar molecules. Within the plant, it represents the energy storehouse and is found in particularly high levels in grains. Starches are considered nonstructural carbohydrates because they are found within the cell wall. Fiber, on the other hand, is considered a structural carbohydrate because it lends support to the plant.

During digestion starch is broken down by enzymes into minute sugar molecules so that it can be readily absorbed. At this point, starch and sugar in the diet are handled similarly. Sugar in the diet can come from the cell contents in grass, molasses from feeds, and certain treats. These sugar molecules cross into the bloodstream and end up as glucose, which gets distributed to cells for use as energy throughout the body. The hormone that helps clear glucose from the blood and into the cells is insulin. This distribution method of getting glucose to the cells for energy, glycogen-building, or adipose storage is part of the energy-generation system of the working horse. The harder the horse is working, the more important the system is.

How are low-starch feeds beneficial?

Avoiding high amounts of starch may be beneficial in certain horses. After digestion of a starch meal, large amounts of glucose hit the bloodstream and trigger the release of insulin, which starts moving the glucose out of the blood and into cells. A rise in glucose is seen in a horse's blood for the first two hours after a meal and then a gradual drop occurs over the next four hours. Looking at the profile of insulin after ingestion of starch, it parallels that of glucose with a similar pattern of rising and falling. If there is a potential problem with the way a horse handles starch, it may be due to glucose and insulin fluctuations 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, a feed that deposits a lot of glucose into the bloodstream at one time has the potential to send significant amounts to the brain. High levels of glucose in the brain are associated with increased dopamine production and can cause heightened awareness or excitability. Certain sports depend on the heightened awareness and energy, like horse racing, but other disciplines or activities do not.

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. The 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 associated with obesity, and losing weight is one of the strategies for improving insulin sensitivity, as is decreasing starch and sugar in the diet. 

Other horses that have difficulty handling starch in their diets are those with genetic muscle disorders like polysaccharide storage myopathy (PSSM) or recurrent exertional rhabdomyolysis (RER). Horse diagnosed with PSSM are efficient at storing blood glucose in muscle cells and end up with abnormally high accumulations of one type of glycogen, which, oddly enough, cannot be easily broken down and used for energy. Keeping feeds that increase blood glucose out of the diet is the recommended strategy for managing horses diagnosed with PSSM. RER also responds to a decrease in the amount of starch and sugar in the diet but for different reasons, and horses with RER are more tolerant of limited amounts.  

High-starch diets may also pose a problem for horses when there is a shift in how starch is digested. There appears to be a limited amount of the enzyme (amylase) that digests starch in the small intestine, and this amount varies among individuals. By itself, that is not cause for concern 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 product of fermentation of starch and sugar is lactic acid, which can have a significant 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 fiber-digesting microbial population, thus upsetting the ability of the horse to get calories from the fiber. 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 5 pounds (2.2 kilograms) of grain per feeding. It appears, however, that the amount of starch that escapes digestion in the small intestine can vary enough to make certain individuals much more susceptible to hindgut acidosis. Signs of hindgut acidosis are subtle but can be cause for poor performance, sour attitude, or intermittent mild colic.

Tolerance to high amounts of starch can be quite variable among horses because of how well the body digests the starch and how it handles the cycles of glucose and insulin once the starch is digested. Finding the level of starch and sugar that an individual can tolerate may require considerable trial and error. Switching to a low-starch feed has made a difference in the attitude and performance of many horses. Trying to avoid starch completely may not be appropriate for every horse, and high-level performance horses often cannot get enough calories from a fiber-only diet to be able to maintain their weight or have sufficient energy for athletic endeavors.

How low is low-starch?

At this time, feed manufacturers can describe a feed as low-starch without having to define it exactly. Because of indiscriminate use of the terminology, legislation may be forthcoming that clearly designates what feeds can be classified as low-starch.

A low-starch feed does not contain a high concentration of cereal grain. If there is little grain in the feed, then something else must provide calories. This is where fiber and fat come into play. This relationship can be useful when trying to decipher a feed tag: the lower the starch, the higher the fiber and fat.

Another number that is often mentioned as an indicator of starch and sugar is nonstructural carbohydrates (NSC).  Finding the NSC value of the feed may require a call to the feed manufacturer. Approximate indicators of starch levels may be as follows: high, NSC >35%; relatively low, NSC =35-20%; and low, NSC <20%. When looking at the ingredients, a low-starch feed will contain a low percentage of cereal grain, and when present may be near the end of the major ingredients in the list (but still before the minerals and vitamins). Fiber ingredients that may be at the top of the list could be soyhulls, beet pulp, and alfalfa meal. Rice bran is a high-fat, high-fiber ingredient found in many low-starch feeds.

What horses benefit from low-starch, high-fiber diets?

Horses involved in various disciplines and of any breed may benefit from a low-starch diet, depending on the individual. Horses that are easily excitable, fractious to handle, obese, insulin resistant, or diagnosed with muscle disorders or hindgut acidosis are all obvious candidates.  Trying to get the horse back to a fiber-based diet is also reason enough to switch.

Low-starch feeds are not appropriate for all horses. Some horses cannot hold their weight when fed these feeds, and they need the extra punch provided by starches. Other horses might maintain their weight adequately but lack any energy when asked to perform. Further, the raw materials used in making commercial high-fiber, high-fat feeds are more expensive than common grains, so switching a horse to such a feed may be more costly.

If a low-starch feed is the right nutritional solution for an individual horse, then the health advantages outweigh any of the cost issues. However, low-starch feeds are not necessary for all horses. If the number of feed choices proves mind-boggling and you're unsure if low-starch is the right choice for your horse, seek guidance from an equine nutritionist.