Understanding the roles of carbohydrates, fats, and proteins in the equine diet
If you’ve ever used a fitness or meal-tracking app on your phone, you’re probably familiar with the term “macros.” Short for macronutrients, they’re the three main suppliers of nutrients in human and horse diets: carbohydrates, fats, and protein. They’re necessary for all facets of life, from energy provision to cell membrane structure support. In this article we’ll describe the function of macronutrients in a horse’s diet and the sources from which they’re derived.
Carbs are the main calorie source for helping meet a horse or pony’s daily energy requirements. Every equid needs a certain amount of energy per day for maintaining body condition and essential biological functions (otherwise known as maintenance requirements). Pregnancy, lactation, growth, training, and performance increase energy requirements above maintenance.
Carbs in the diet can be classified into two groups by plant structure or by how they’re digested:
- Structural carbs include the fibrous portion of plants (aka fiber).
- Nonstructural carbs (NSCs) include the polysaccharides cellulose and hemicellulose.
Because horses are continuous grazers, it makes sense that structural carbs provided by pasture and hay constitute the greatest percentage of the total diet and meet 50-100% of the horse’s total energy needs. Grains with fibrous hulls, such as whole oats, and byproduct feeds, such as beet pulp, also contribute to the diet’s structural carbohydrates.
How do horses utilize structural carbs? Equids are not capable of digesting fiber without the help of billions and billions of bacteria residing in the hindgut (the intestinal tract behind the small intestine). Microbial fermentation breaks fiber down into usable forms of energy known as volatile fatty acids.
The three main volatile fatty acids (VFAs) microbial fermentation produces are propionate, acetate, and butyrate, which travel through the large intestine’s walls and into the bloodstream. The body can use acetate, produced in the highest concentration of the three, for energy or convert it into long chain fatty acids to store for later use. Propionate can end up in the liver for conversion to glucose (sugars) in a process known as gluconeogenesis. Butyrate is the preferred energy source of the cells lining the intestinal tract.
Fiber could almost be its own macronutrient. Besides contributing to daily energy requirements, fiber supports a healthy microbial population in the hindgut and helps prevent colic, gastric ulcers, hindgut acidosis, and unwanted behaviors such as cribbing or stall-walking. However, researchers have yet to quantify a fiber requirement for horses.
Nonstructural carbohydrates include all the mono-, di-, and oligosaccharides and the polysaccharide starch. Digestive enzymes typically break NSCs down in the foregut (everything ahead of the large intestine) to simple sugars, or monosaccharides, that get absorbed through the small intestine. The body can immediately use simple sugars that enter the bloodstream for energy, store them as glycogen in the muscle and liver, or use them for fat synthesis. Nonstructural carbs not digested in the small intestine (due to large concentrate meal size, for example) get fermented in the hindgut. Certain NSCs (e.g., fructans found in fresh pasture) have been known to resist enzymatic hydrolysis (breakdown) in the small intestine, passing through to ferment in the hindgut instead. When the hindgut bacteria ferment these NSCs, they produce lactate rather than VFAs, potentially disrupting the hindgut environment and increasing colic or laminitis risks.
Cereal grains are by far the largest NSC sources, although pasture and hay do contain various percentages of sugars. Common cereal grains include oats, barley, and corn. Byproduct feeds, such as wheat middlings, corn gluten, and rice bran, also contribute to the diet’s starch content.
Common Carbohydrate Sources in Equine Rations
|Structural Carbohydrates||Nonstructural Carbohydrates|
|Baled legume or grass hay||Corn|
|Forage cubes or pellets||Barley|
|Oat hulls||Wheat bran|
Fats serve a variety of functions in horses, including transporting fat-soluble vitamins; delivering essential fatty acids, which are not made by the body; providing cells with structural integrity; and serving as precursors to hormones and other signaling molecules. Fats and oils fall into a class of molecules called lipids.
Structurally, all fats contain:
- A single glycerol molecule—a chain of three carbon atoms, each with a hydroxyl group (oxygen and hydrogen) bound to it; and
- Fatty acids—long hydrocarbon (containing hydrogen and carbon) chains.
Of course, fat’s most recognized function is providing energy for cells, particularly heart and muscle cells, with glucose and glycogen being the other major sources of cellular energy.
“In most species of animals, muscle cells are either designed to rely mostly on glucose/glycogen for energy or … on fat for energy,” says Beth Valentine, DVM, PhD, Dipl. ACVP, professor of anatomic pathology at Oregon State University, in Corvallis. “But in the horse, studies found that even muscle cells designed to use glucose/glycogen for energy also have fatty acid transporters on their surface.”
This is understandable when you consider that horses do not have specific fat requirements but, rather, fatty acid requirements.
Dietary fats and fatty acids vary in length and in the types of bonds linking carbon molecules internally. Single bonds link carbon atoms in saturated fats. Conversely, one or more double bonds link carbon atoms in unsaturated fats. Once ingested, enzymes called lipases in the horse’s stomach begin to break down the fat bonds, with most digestion taking place in the small intestine. After absorption, fatty acids get transported to the muscle, liver, adipose (fat) tissue, or elsewhere as needed for storage or use. Fats that don’t get broken down and absorbed in the small intestine travel to the hindgut, where they get excreted in the feces.
“The horse evolved to ferment the fiber in forage to fatty acids, courtesy of large colon (the second part of the hindgut) bacteria,” says Valentine. These fatty acids get absorbed into the bloodstream, and some are converted to sugars used to fuel cells like those in the brain, which rely almost entirely on sugars for energy. The horse’s body uses other fatty acids for immediate energy and to ensure adequate fat stores for future energy needs when food is scarce.
Grains, seeds, and nuts (such as peanuts) that horses might encounter while browsing can supply additional fats, she says.
Researchers have identified drastic differences in fat source digestibility in horses. Fats from forages appear to be 55% digestible, compared to 100% for oils. Common fat sources include grain-based and oil-based fats.
“Many horses on pasture or hay will obtain sufficient fatty acids from the forage,” says Valentine. “However, there are many horses that appear to be metabolically different and whose muscle cells seem to need additional fat to allow for normal muscle health, function, and performance.”
In these scenarios you might need to supply additional fat in your horse’s diet.
Common Fat Sources
|Grain-Based Fat Sources||Oil-Based Fat Sources|
|Flaxseed or linseed||Corn Oil|
|Stabilized rice bran||Canola Oil|
|Whole soybeans||Soybean meal|
|Sunflower seeds||Rice bran oil|
|Copra meal||Sunflower oil|
|Canola meal||Coconut oil|
|Wheat germ||Peanut oil|
Most equine body tissues contain protein, with muscle containing the most. “Proteins help make up body tissues, including everything from muscle to hair; function as enzymes and hormones in the metabolic machinery of the horse; and play an important role in the immune system as antibodies,” says Sara Mastellar, PhD, assistant professor at Ohio State University ATI, in Wooster.
Proteins consist of various-sized chains of about 22 different amino acids linked together by peptide bonds. Horses can synthesize most of the amino acids they need (considered nonessential amino acids) but, again, they must derive some amino acids from the diet—essential amino acids. Scientists have identified 10 essential amino acids for the horse, including arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
In addition, the horse can convert amino acids, which comprise proteins, into glucose for energy. However, Mastellar says, it takes much more energy for the body to produce glucose from amino acids. The end product is nitrogen, which must be excreted in urine. Therefore, carbohydrates and fats remain preferable energy sources over protein.
When horses consume protein, pepsin in the stomach begins the breakdown process by attacking the amino acid peptide bonds. Further down, in the small intestine, protease enzymes from the pancreas continue breaking proteins down into single amino acids, which then travel through the gut wall and into the pool of free amino acids in the blood. Little information exists supporting the large intestine’s ability to absorb amino acids, and most absorption occurs before protein and amino acids reach the large intestine.
“There is at least some protein in many common feedstuffs for horses,” says Mastellar. A feed’s protein quality depends on digestibility and essential amino acid profile. Crude protein, while listed on all feed tags, is simply a measure of the nitrogen concentration and does not indicate the presence of essential amino acids. A high-quality protein source for horses must provide lysine and methionine.
“Feed and supplement companies may also include in their formulations single amino acids that are not part of a protein, such as lysine, threonine, methionine, and leucine,” adds Mastellar.
Forages that tend to be higher in protein include legumes such as alfalfa and clovers. Forage protein gets digested in the hindgut and, although it might contribute to the horse’s overall protein requirements, it’s not a major source of amino acids. Other relatively high-protein feedstuffs include whey, which is milk protein, and oilseed meals, such as soybean and canola, says Mastellar.
Common Protein Sources
|Grain Protein Sources||Forage Protein Sources|
|Whole soybeans||Fresh pasture|
|Soybean meal||Legume hay (e.g. alfalfa, clover)|
|Brewer’s dried grains|
|Distiller’s dried grains|
|Corn gluten meal|
The three major macronutrients—carbohydrates, fats, and protein—make up the bulk of the equine diet and are the main nutrient sources necessary for all aspects of life, including reproduction, exercise, growth, and maintenance. Work with an equine nutritionist to ensure the macronutrients in your horse’s diet meet his daily nutritional needs.