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A Quick Guide To MacroNutrients

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:

  1. Structural carbs include the fibrous portion of plants (aka fiber).
  2. 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 colicgastric 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.


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.

Published by Thehorse.com