As a culture, we have a peculiar relationship with carbohydrates: counting them, cutting them, craving them. The horse’s body relies on this nutrient group for the majority of its energy and calories. Carbohydrates are an essential fuel source for all horses and not to be disregarded, even amongst the du jour “low-carb” trend in human nutrition. However, the confusion arises due to the fact that not all carbohydrates are created equal. The convoluted carbohydrate mystery becomes which are the “right” ones and how to feed them? While some are the perfect fuel for equines, others can have a deleterious effect on health when not fed in moderation or when fed to horses with metabolic issues and other health conditions.
The horse is a natural grazing, hindgut fermenting herbivore with the unique ability to utilize a wide range of carbohydrates from forage, such as pasture and hay, as its main energy source by breaking them down both enzymatically in the foregut (stomach and small intestine) and through fermentation in the hindgut (cecum and colon). The modern way of feeding suggests diets based solely on pasture or hay “require” supplemental concentrated feed to support calorie requirements needed for performance or to ensure vitamin and mineral requirements are met. This way of feeding continues even with the numerous studies that correlate high grain feeding practices with digestive and metabolic disorders such as colic, laminitis, gastric ulcers, hindgut acidosis, developmental orthopedic disease, insulin resistance and myopathy conditions. In response to the many grain-associated diseases seen today, the majority of feed companies offer “low starch” or “low NSC” commercial feeds options. These options can be helpful in feeding programs for some horses, but others may thrive on a simple diet of quality forage. Hard-working horses and specific life stages may need additional calorie sources from concentrated feed sources to maintain body condition or energy above what is provided from forage alone, but others may not.
As a grazing herbivore, the horse was designed to ingest a steady supply of plant material. Upwards of 75 percent of all plant matter is made up of carbohydrates, mostly due to the most important carbohydrate for the horse: fiber. For the horse, carbohydrates are life because fiber is essential. Sugars, starches, fructan and fiber are different forms of carbohydrates that are important in equine nutrition. All of the different forms of carbohydrates provide energy to the horse and all are found to some extent in commonly fed forages, cereal grains, beet pulp, legumes and so on. But what makes them different? A lot has to do with where and how they are digested. Sugars and starches are broken down enzymatically in the small intestine to provide glucose for cellular energy (calories). Fructan is digested in the hindgut to predominantly produce lactic acid. Fibers are fermented by microflora in the cecum and colon to yield volatile fatty acids (VFAs), which provide the horse with energy.
The most confusing aspect to understanding carbohydrates may be the many ways to classify them. Some do not fall neatly into one category, and classification is consistently evolving with new research and discoveries. Based on plant physiology, they are commonly divided into structural and non-structural carbohydrates. Structural or fibrous carbohydrates form the cell wall portion of the plant that gives the cell wall shape and strength. Structural carbohydrates are often referred to as the plant fiber and include cellulose, hemicellulose and pectin (not an orthodox structural part but still considered a fiber). Lignin is also a structural portion of the plant that provides rigidity but no nutritional value to the horse as it is completely indigestible. Cellulose and hemicellulose are slowly fermentable carbohydrates that provide a significant amount of digestible energy to the horse via the VFAs — propionate, butyrate and acetate — and are essential to maintain a healthy microbial environment in the hindgut.
Non-structural carbohydrates, or NSCs, are simply the portion of the plant that do not provide structural support. Simple sugars, disaccharides, starches and fructan fall into this category. Elevated levels of NSCs have well-deserved notoriety in the equine diet as they are linked to several health issues. Foregut digestion of sugars and starches results in glucose, which can be absorbed directly into the bloodstream or stored as glycogen in the muscles and liver. Insulin responds to an elevation in glucose from food being broken down and controls glucose leaving and entering the cells. Meals containing a high volume of grain will require higher concentrations of insulin so the dietary sugar can be digested properly. In some horses, over time, the cells can become less sensitive to the effects of insulin; they become insulin resistant. The cell receptors for insulin do not respond appropriately and more insulin is needed to control blood glucose. Insulin resistance is a health concern as it is a component of equine metabolic syndrome (EMS) and a risk factor in the development of laminitis. High sugar and starch content is typically seen in feeds that contain cereal grains and, although variable, much lower levels are seen in pasture and hays. Fructan is different than the other NSCs as it is found in varying levels in cool season pasture grasses, not grains. It is resistant to foregut digestion, passing into the cecum to be fermented, along with starch that has overwhelmed the small intestine, and can lower the pH of the hindgut enough to potentially cause a variety of small and large intestinal complications including colic and laminitis.
Sugars are small molecules made up of single units called monosaccharides, double units called disaccharides and longer chains of monosaccharides linked together include oligosaccharides and polysaccharides known as complex carbohydrates. Monosaccharides or “simple sugars” are the basic carbohydrate building blocks for all sugars and include glucose, fructose, xylose, mannose, arabinose and galactose. Monosaccharides are the only form of carbohydrates that can be absorbed by the equine intestine, and glucose is the only carbohydrate that can cross the blood-brain barrier. More complex polysaccharide carbohydrates, such as starch, must be broken down into simple sugars before they can be used by the horse.
Disaccharides are made up of two simple sugars bonded together. Sucrose (glucose and fructose) is commonly found in grasses and legumes. Maltose (glucose and glucose) is produced as an intermediate in the hydrolytic digestion of starch. Lactose (glucose and galactose) is found in milk and is important for nursing foals. The enzymes maltase, sucrase and lactase are produced in the small intestine and digest their respective sugar couplings down so they can be completely absorbed in the foregut of the healthy horse
Complex carbohydrates are simple sugars chemically linked together to form long chains of monosaccharides. Oligosaccharides are considered a short-chain type of sugar that contain 3-10 sugar units. Polysaccharides have more than 10 units and include starch, pectin, as well as other fermentable fibers, such as hemicellulose and cellulose. Fructans can be considered either oligo- or polysaccharides depending on their polymerization.
Glucose, fructose, sucrose, starch and fructans are sugars commonly found in many forages, however the main carbohydrates in forage that are digestible in the hindgut by the horse include cellulose and hemicellulose. The ratio and balance of these carbohydrates in pasture grass and hay is what makes them an ideal food source for horses.
Starches are long, branched chains of glucose molecules that are broken down to sugar through enzymatic hydrolysis in the small intestine and readily absorbed to provide energy. However, this type of non-structural carbohydrate can also be rapidly fermented in the hindgut. Starch is a major carbohydrate fraction in commonly fed cereal grains such as corn, oats, barley and wheat. It is also the main storage carbohydrate for warm season grasses like coastal Bermudagrass and legumes like alfalfa, although typically at much lower levels compared to grains.
Starch has long been a contentious word when feeding horses and with good reason as high levels of dietary starch are correlated with digestive disturbances. The digestive tract of the horse is very efficient at digesting starch with upwards of 95 percent total digestibility within the gastrointestinal tract, regardless of the source. Starch is subject to enzymatic hydrolysis in the small intestine, but it can also be rapidly fermented in the cecum. Starch is easily broken down to glucose in the small intestine and readily absorbed into the bloodstream for energy. However, if the volume of dietary starch overwhelms the capacity of the small intestine, which happens often if there is too much starch in a single meal, it will not be completely digested and will spill over into the hindgut for fermentation. The by-product of starch fermentation is lactic acid. An elevation of this acid in the hindgut will lower the pH and kill off fiber-digesting bacteria in the gut. Endotoxins will be released and absorbed in the bloodstream. This is a contributing factor to carbohydrate overload, hindgut acidosis and possible colic and laminitis. The key to starch in the equine diet is to provide an appropriate volume so complete digestion can occur in the small intestine to prevent excess undigested starch from reaching the cecum and colon.
Fructans are water-soluble sugar chains of fructose molecules and are the main storage carbohydrate for cool season grasses such as timothy, orchard, fescue and brome. They mainly accumulate in pasture grasses. In cereal grains, the starch level remains relatively stable; this is not the case with fructan levels in grass, which can vary widely and can contribute anywhere between 5 to 50 percent of the dry matter (DM) in some grasses. Higher levels of fructans are usually seen during periods of rapid grass growth, such as springtime, or after increased precipitation in the summer or fall. Overgrazed pastures and stressed grasses tend to also have higher fructan levels. Concentrations vary daily, typically peaking in late afternoon and evening and having the lowest levels in the early morning.
Fructan content in grass is tolerated by most horses at moderate levels. However, as fructan levels rise, especially after times of rapid growth or stress, elevated intake of these pasture sugars can result in health issues, particularly in susceptible horses. Fructan is resistant to enzymatic hydrolysis; the horse does not have the necessary digestive enzymes in the foregut needed to break apart the bonds that hold the fructose molecules together. Because of this, they move relatively undigested into the hindgut of the horse and are then subjected to microbial fermentation. Similar to starch overload, the bacteria and protozoa in the cecum ferment fructans into lactic acid. This decreases the pH of the hindgut, making it more acidic. The acidic environment kills the beneficial bacteria releasing endotoxins that enter the blood and creates inflammation. Current research shows excessive consumption of fructans from pasture grasses can cause cecal acidosis, colic, laminitis or, more specifically, pasture associated laminitis. Horses also may vary in personal sensitivity to fructan consumption where one horse in a pasture may have issues; others grazing the same grass may not. Ponies, heavier breeds, overweight horses, insulin resistant or metabolic horses and horses with previous laminitic issues are at a higher risk for health issues associated with fructan content in pastures.
Often overlooked even as a carbohydrate, fiber is ironically the most important in the equine diet. For humans, fiber does not provide much nutritional benefit except to promote intestinal motility. Fiber is everything for the horse. Like humans, horses do not have the enzymes capable of digesting fibers in the stomach or small intestine, but they are uniquely capable of using fiber from forage sources to fulfill a huge part of their energy requirements due to the ability to ferment fiber via specialized microflora in the hindgut. In fact, the fermentation of fiber has the capability to supply the horse with 30-70 percent of its total digestible energy requirements. Hindgut fermentation in the cecum and colon produces the VFAs — acetate, butyrate and propionate — that enter the bloodstream and are metabolized within the cells for energy (calories). This is the way horses are able to extract energy from the fibrous hay and pasture they consume. The pH of the hindgut should be maintained above a pH of 6 for optimal populations of fiber-digesting microbes.
Although forage consists of varying levels of sugars, starches and fructans, it is predominately made up of fiber. Fiber can vary widely in quality and digestibility. This is a good thing as horses are individuals, and not all of them need the highest quality, most digestible pasture or hay. For instance, the easy keeper would do poorly and likely suffer health consequences if its main feed source was a very digestible, rich hay. The potential calorie yield from fiber depends on how easily it can be broken down by the gut microbes. The main fibrous constituents of plants include cellulose, hemicellulose, pectin and lignin. Cellulose, hemicellulose and pectin can all be fermented and digested to varying degrees by the microbes in the hindgut; lignin is completely resistant to microbial breakdown. Fibers are typically divided into soluble and insoluble types, which dictates their digestibility. Cellulose and hemicellulose are both considered insoluble fibers; they can’t dissolve in water. These are located in the cell walls to give structure to the plant. Hemicellulose is more completely broken down by microbial fermentation; cellulose to a lesser extent. Insoluble fibers tend to be less nutritious than their soluble counterparts. There are many soluble fibers; the main ones in equine nutrition are pectin, glucan and mucilage. Pectin is a soluble fiber, dissolvable in water and forms a highly digestible gel in the hindgut. It is easily broken down to produce VFAs, and its energy contribution can be considerable. Beet pulp, for example, contains high pectin levels and very little starch and can yield nearly as many calories as whole oats. Soluble fiber is also a very good prebiotic and provides support for the health of the fiber-digesting hindgut microbes.
Besides being an abundant source of energy, a constant supply of fibrous forage greatly influences gut motility and maintains the integrity of the delicate microbial population in the hindgut that is the fermentative powerhouse and is essential to the well-being of the horse. Debunking any myths that it is “just fill,” providing horses with adequate and appropriate fibrous feed sources helps to reduce the risk of common equine disorders such as colic, gastric ulcers, hindgut acidosis, as well as some stereotypic behaviors, such as cribbing and wood chewing. Forage-based diets of pasture and hay provide a high fiber content that favors slow fermentation in the hindgut providing a critical energy source and encouraging healthy digestion.
|Horses are uniquely capable of using fiber from forage sources to fulfill a huge part of their energy requirements due to the ability to ferment fiber via specialized microflora in the hindgut. In fact, the fermentation of fiber has the capability to supply the horse with 30-70 percent of its total digestible energy requirements.||
(Plant Fiber) Helpful because it provides a significant amount of digestible energy and essential to maintain intestinal motility and a healthy microbial environment in the hindgut.
|Fructan content in grass is tolerated by most horses at moderate levels. However, as fructan's levels rise, especially after times of rapid grass growth or stress, elevated intake of these pasture sugars can result in health issues like colic and pasture associated laminitis, particularly in susceptible horses.||
High levels of starch and sugar can have health repercussions as their breakdown results in excessive amounts of glucose. Insulin controls glucose moving into and exiting cells. A higher amount of insulin is needed to control glucose levels when digesting foods high in starch and sugar. Over time, the cells can become less sensitive to the effects of insulin; they become insulin resistant. Fructans must be fermented in the hindgut and at high levels can acidify the hindgut potentially leading to colic and pasture associated laminitis.
|Amounts from cereal grains should be limited for all horses.|
|Because starch digestion results in more energy and calories available than the digestion of fiber, grains are often used to provide horses in work with extra fuel. For high performance horses relying on increased levels of starch, it may be beneficial to split meals into multiple servings daily (less than 4 lbs/meal). Horses at maintenance do best on a lower starch content in their diet. Horses with the following conditions are good candidates for a low starch or forage-only diet: obesity, insulin resistance, metabolic syndrome, chronic colic, laminitis, developmental orthopedic diseases, polysaccharide storage myopathy and recurrent exertional rhabdomyolysis (tying-up).|
Are horses “low-carb” diet candidates? Yes and no. Carbohydrates are a horse’s best source for energy and here’s why: fiber is a carbohydrate. It is not just gastrointestinal fill, although this is exceedingly important too, but the horse ingests huge amounts of fiber daily, creating its own energy from the breakdown and fermentation of fiber in the hindgut. This is a unique fact that does not crossover into human nutrition. Once considered stalky filler that keeps stalled horses busy to prevent behavioral issues, fiber is now understood to be the most important energy source for the horse. If fed approximately 2 percent of their total body weight daily in good quality forage, energy needs will be met for most horses at maintenance and is the best foundation for all equine diets. Certain carbohydrates like starches and sugars should be limited for all horses. Fiber should not be limited in the equine diet. Use concentrates as a supplement for body condition or energy needs, not as a cornerstone of a feeding program. If there is concern about the sugar, starch or fructan content in your forage, consider a hay or pasture analysis, which can provide the nutrient content of your forages. Most horses will excel on a naturally low starch and high fiber diet because that is what pasture grasses and hay provide. The different types and levels of carbohydrates that constitute forages will always be the best guide for what horses will thrive on. Feeding horses the way they were designed to eat is the single most influential, proactive measure that we can do for equine health.
Feeding a horse the natural diet he is designed to eat is the single most influential, proactive measure we can take to achieve optimal health. The carbohydrate is the best source of energy because of fiber.
by Emily Smith, MS,