Gastrointestinal Disease, Colic, and Carbohydrate Nutrition

How is feeding carbohydrates related to gastrointestinal (GI) disease? Ray Geor, BVSc, MVSc, PhD, Dipl. ACVIM , professor, Paul Mellon Distinguished Chair, and director of research at Virginia Tech's Middleburg Agricultural Research and Extension Center in Middleburg, Va. began his discussion of this topic at the 2007 American Association of Equine Practitioners Convention held Dec. 1-5 in Orlando, Fla., by noting the conflict between GI physiology and the way horse owners tend to fed modern horses, especially those with high athletic demands. The propensity to feed high-grain and high-concentrate diets instead of relying on high-fiber diets has increased the incidence of colic. Geor said this is a historic problem, quoting from an 1857 source: "An excess of corn may induce violent indigestion and gripes."

One study from 1997 indicated that on 31 farms, the risk of colic increased 4 ½ times when horses were fed moderate to large amounts of grain (5-10 pounds).

An increased risk of colic also is related to a change in diet; particularly in the first week after a diet change, there is a chance of simple colonic obstruction or distention, but risk diminishes 15-28 days following dietary changes.

Geor stressed that other contributing factors should be considered, such as a horse's level of physical activity, breed, age, season, and the area's weather, but, in general, there is an increased risk of colic with high-grain concentrate diets and with recent dietary changes. So, the question is, "Why?"

Geor said there is a disturbance of the hindgut (which includes the large intestine and rectum) ecosystem related to delivery of undigested starch and other rapidly fermentable carbohydrates to the hindgut. A limited capacity for starch digestion in the small intestine contributes to overflow of starch to the hindgut. There is also a lag or transition time in the speed of microbial adaptation to dietary changes. He pointed out that maintaining stability of the hindgut microbial community is of paramount importance to gastrointestinal (GI) health.

There are three types of carbohydrates:

Hydrolyzable Starches and sugars are digested in the small intestine to generate glucose, but these ferment rapidly in the stomach or the large intestine.

Rapidly fermentable Fructans and oligosaccharides (saccharides of a small number of component sugars) that rely on Lactobacillus, Streptococcus, and Clostridium spp, with lactate production as a byproduct.

Slowly fermentable Cellulose that relies on Fibrobacter and Eubacterium spp that consume lactate.

If a large load of rapidly fermentable carbohydrates enters the hindgut, overgrowth of lactate-producing bacteria leads to increased lactate and gas production, acidity of the hindgut, a die-off of Gram-negative bacteria, and the release of endotoxin and other substances. The result is disruption of the mucosal layer of the intestinal lining, absorption of endotoxin, more gas distention, and altered gut motility with the potential for an intestinal volvulus (twist). This sequence of events results in an acute and severe colic.

A more chronic and less severe intestinal disruption develops from a decrease in fiber-fermenting, acid-utilizing microbes. This leads to chronic acidosis, digestive inefficiency, weight loss, altered fecal consistency, and the development of stereotypic behaviors such as cribbing.

Hindgut acidosis develops subsequent to sudden introduction of grain feeding or an abrupt increase in amount of grain fed. Acidosis also results from large grain concentrate meals, lush spring pasture grazing, or from forage that is high in nonfiber carbohydrates, such as what occurs with rich legume hay. Studies indicate there is a stepwise decrease in cecal pH relative to increasing amounts of grain and the size of a starch meal. Larger grain meals increase risk of starch bypass of the small intestine with rapid fermentation in the large intestine. The effects of this depend on the horse's adaptation to the types of starch and size of meal.

Cecal acidity is associated with feeding grain starch of low digestibility, such as is seen with corn. The higher digestibility of oats mitigates this effect, whereas barley and corn starch are more resistant to digestion and lead to acidosis. Oats are about 84% digested before reaching the cecum as opposed to corn at 45% or less digestion. Heat treatment or reduction in particle size (micronization or extrusion) is needed for improved pre-cecal digestibility and retention of normal cecal pH.

Geor also mentioned how diet affects equine gastric ulcer syndrome (EGUS), which is reported to have an incidence of 60% in pleasure and show horses, and 85-90% in racehorses. The "trickle" feeding pattern of pastured horses (small intake of feed over a long period of time) might have a protective effect that is coupled with the high-fiber diet of pasture grass. Saliva production doubles when eating hay and is continuous when grazing; saliva has a powerful buffering effect on stomach acid. Alfalfa has an additional buffering effect on stomach acid, whereas recent findings do not support any benefits from corn or rice oil in preventing development of EGUS. Overall, it is important to remember that a horse has a small stomach and large hindgut and a limited capacity for starch digestion in the small intestine. In general, the best strategies rely on feeding smaller and more frequent meals with less reliance on cereal or sweet feeds.

Other supplements were discussed, such as "stabilizing" products for the hindgut ecosystem. A live yeast preparation or prebiotic (a food substance intended to promote the growth of certain bacteria in the intestines; 10 grams per day of Saccharomyces) mitigates the decrease in pH that occurs with high-starch meals, but of greater importance is the feeding of high dietary fiber, i.e., forage.

As for probiotics (dietary supplements containing potentially beneficial bacteria or yeasts), Geor pointed out that data is lacking due to the absence of well-designed scientific studies. In addition, there are issues regarding product quality, and some studies show adverse effects in foals receiving probiotics containing Lactobacillus pentosus. Another new supplement (made by Kentucky Equine Research) that is advertised as a hindgut buffer is purported to mitigate moderate decreases in cecal pH following a grain meal or when grazing lush pasture.

Geor summarized his recommendations to reduce colic risk:

  • Minimize the flow of rapidly fermentable carbohydrates to the cecum and large colon;
  • Limit size of grain, sweet feed, or pelleted feed per meal to no more than 4 ½ pounds of starch for a 1,100-pound horse;
  • Increase the number, not the size, of meals to at least three feedings per day and provide free-choice hay when possible;
  • Feed starch sources that are high in pre-cecal digestibility, such as oats. If using corn or barley, these need heat treatment, such as extrusion, popping, or micronization;
  • Slowly introduce changes in cereal or sweet feed to increments of less than 1 pound per day over seven to 10 days;
  • Use alternative concentrated energy sources such as oils or rice bran or highly digestible super fibers, i.e., beet pulp or soya hulls;
  • Encourage long-stem forage intake to at least 1-1 ½% of body weight, i.e., 12-17 pounds per day for a 1,000-pound horse
  • When switching hay batches, blend over seven to 10 days.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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