Between the Atkins diet and the South Beach craze, the role of carbohydrates has taken center stage in the human diet. And, as with other health trends, it is now filtering through to our domesticated animal kingdom. Unlike humans, though, horses are built to thrive on a high-carbohydrate diet, so long as those carbs are predominantly insoluble carbs.
In horses, carbohydrates have gained a dubious reputation as the number-one contributor to a variety of disorders, including colic and laminitis as well as undesirable behavioral changes such as hyperactivity or nervousness. Carbs have also been fingered as being an important factor in Developmental Orthopedic Diseases (DOD), tying up, and Cushing’s Disease. Since the equine diet consists mostly of carbohydrates, though, carbs, per se, are not all bad. But it is possible to get too much of a good thing.
In order to figure out exactly what role carbohydrates should play when it comes to feeding horses, the relationship between carbohydrates and disease needs further study. In the meantime, it helps to start with information about the different types of carbohydrates, soluble and insoluble, and the basic equine digestion process.
Understanding the Digestion Process
According to Jim Ward, DVM, graduate from Texas A&M and equine nutrition consultant for Nutrena, nature designed horses to be continuous grazers. In the wild, they can forage up to 16 hours a day as they seek to satisfy their nutritional requirements.
Their stomachs, which comprise only 7 percent of the digestive tract, are designed to process small but steady amounts of feed. The stomach secretes hydrochloric acid (HCI) and pepsin to break down matter rapidly, taking just 30 to 90 minutes to complete, moving it on so as to accommodate the steady stream of material entering the system.
“...it is carbohydrates . . .that provide most of fuel for equines.”
Describing the next stop in the process, Dr. Ward explains, “The small intestine measures 50 to 70 feet long. This is where the majority of the nutrients, including protein, some carbohydrates, and fat, are digested, and where many essential vitamins and minerals are absorbed. From here, most of the matter and liquids continue on to the hind-gut, known as the cecum and colon, which digests fiber (insoluble carbohydrates) and the remaining soluble carbohydrates (starches and sugars), utilizing specialized bacteria and protozoa in a fluid environment. In the large intestine or hind-gut, minerals are absorbed and many vitamins are synthesized using microbial action. Finally, the undigested material is eliminated through the rectum and anus as feces or manure.”
There are six main categories of nutrients which are necessary to a horse’s survival: water, carbohydrates, protein, fats, vitamins, and minerals. Aside from water, Dr. Ward points out that almost 90 percent of nutrient intake is used to provide a horse’s energy requirement. Although the densest energy forms are found in fats, it is carbohydrates, soluble and non-soluble, that provide most of fuel for equines.
Soluble carbohydrates, such as starches and sugars, provide immediate energy. The most common sources are corn, oats, barley, and molasses. Starch breaks down into simple sugars, referred to as glucose, which are absorbed by the small intestine and assimilated into the blood stream. The hormone insulin continues the process by transferring glucose to the liver and muscles, which store glucose to provide energy on demand. This energy reserve, termed glycogen, is the resource that provides the immediate power bursts essential to physical activity. Horses involved in intense anaerobic activities, such as racing, must have adequate soluble carbohydrates in their diet to maintain adequate muscle glycogen levels.
Unlike soluble carbohydrates, insoluble carbohydrates are derived from the fiber source, cellulose. The nutrients found in forages such as hay, grass, beet pulp, and soybean hulls are the main suppliers of insoluble carbohydrates. These are digested by microbes in the cecum and large intestine. Taking more than 24 hours to process, insoluble carbohydrates are transformed into volatile fatty acids, which are eventually absorbed and held by the liver in reserve as an additional energy source.
The Relationship Between the Two
Dr. Ward contends that trouble often starts as a consequence of feeding excess amounts of soluble carbohydrates, which are present in grain. The starches, when broken down in the stomach and small intestine, are thought to release a surplus of glucose and insulin into the blood stream. Excessive starch and sugar intake, as with a high starch/sugar diet in humans, can lead to metabolic upsets in both the young developing horse as well as in the older horse and has been associated with hyper- excitability and behavioral problems. Alternatively, when not adequately broken down, residual starches enter the cecum and are left to ferment.
This is where many colic and associated gastro-intestinal disorders originate. “Starch overload will cause a number of conditions, from gas formation, lactic acidosis, loss of bacterial flora, endotoxin production, and pain—all in varying degrees, depending on the amount that has entered the cecum and the amount of time that has elapsed,” states Dr. Ward. “As this sequence of events progresses, intestinal motility (peristalsis) is reduced, and abdominal distension from gas production increases. Since horses are unable to belch, having a one-way valve between the stomach and esophagus, episodes of violent rolling in response to pain may occur, possibly leading to displacement and entrapment of segments of intestine. Circulation of the bowel will subsequently become compromised causing necrosis (death) of the affected area and leading to rupture and spillage of contents into the abdominal cavity. Shock, circulatory collapse, and death follow shortly.”
Insoluble carbohydrates (fiber), on the other hand, are fundamental to an efficient digestive system, and therefore should not be reduced in an effort to curb starches and sugars in the diet. A general guideline is to feed at least one to two pounds of insoluble carbohydrates for every 100 pounds of body weight per day. For example, a 1,000 pound adult horse would need an estimated 10 – 20 pounds of hay, and may be fed even more depending on its workload and condition.
A Balanced Diet
Dietary research indicates that by increasing dietary fiber (insoluble carbohydrates) and fat while reducing soluble carbohydrate intake will benefit overall health in horses that are engaged in light to medium performance or pleasure activities, while at the same time reducing the risks linked with starch overload. Fats are especially effective in helping to control the rate at which glucose is released into the system. In the end, Dr. Ward recommends only feeding the amount of concentrates necessary to maintain sufficient body condition and balanced nutrient levels.