CARBOHYDRATES AND THEIR PRECISE DEFINITIONS
Ing. Lucie Humeni
Carbohydrates form a dominant part of the horse’s ration, yet misconceptions continue to appear. The most common one is the confusion of the term “sugars” with the entire group of these nutrients. In reality, this is a much broader category. Carbohydrates include both rapidly available energy sources and structural components of forage with different metabolic pathways and different importance for gastrointestinal health.
Carbohydrates and precise definitions of terms
From a biochemical perspective, carbohydrates are divided according to molecular complexity into:
- Monosaccharides (sugars) – glucose, fructose, galactose, pentoses
- Disaccharides (two sugar molecules) – sucrose, maltose, lactose
- Polysaccharides (multiple sugar units) – starch, glycogen, cellulose
- Fiber – a less precisely defined fraction consisting of cellulose, hemicellulose, and pectin
It is essential to emphasize that sugar is not synonymous with carbohydrates. Correctly, it refers only to monosaccharides.
Digestion of individual carbohydrate fractions in the horse
The horse is a typical hindgut fermenter, meaning it relies on microbial fermentation in the large intestine, especially the cecum. This fundamentally affects how different carbohydrates are utilized.
Digestion in the stomach and small intestine
Monosaccharides and disaccharides are almost completely absorbed in the small intestine. Starch is enzymatically broken down here into maltose and subsequently into glucose, which enters the bloodstream. However, the capacity of this process is limited. With higher starch intake, part of it escapes digestion and passes further along the digestive tract.
The large intestine as a fermentation chamber
Fiber is fermented by microorganisms in the cecum and colon. The products of fermentation are volatile fatty acids, gases, and heat. Volatile fatty acids represent a stable and long-term energy source that covers a significant portion of the body’s energy requirements.
The situation changes significantly when larger amounts of starch or rapidly fermentable sugars reach the cecum. This can occur with an unbalanced ration or, for example, a single large meal of cereal grains. In this case, rapid fermentation of starch and sugars occurs, leading to lactic acid production and a sharp drop in intestinal pH.
This process leads to dysbiosis and may trigger a range of problems such as colic, diarrhea, and systemic inflammatory responses (e.g., damage to sensitive hoof lamellae structures, resulting in laminitis).
Functional importance of starch and sugar
Rapidly digestible carbohydrates have their role, especially when immediate energy is required for replenishing muscle glycogen and supporting high-intensity performance. In sport horses, a higher starch proportion has been shown to improve glucose utilization during exercise, particularly in anaerobic work.
Fiber as the foundation of physiological nutrition
From an evolutionary perspective, the horse is adapted to a high intake of fiber. Microorganisms in the cecum allow efficient utilization of structural carbohydrates that would otherwise be indigestible. Adequate fiber intake in the ration supports a stable microbiome, ensures continuous energy production, and reduces fluctuations in blood glucose levels. Modern nutritional trends therefore favor a higher proportion of fiber at the expense of starch.
Energy value of carbohydrates
Differences in energy content:
- Sugars and starch: approx. 16–18 kJ/g
- Fiber: approx. 6–9 kJ/g
While sugars and starch provide rapidly available energy, fiber is a slower but metabolically more stable source. Fats represent the most energy-dense form, which explains their use when reducing starch content in the diet.
Carbohydrate content in feeds
Typical sources of starch and sugars are cereal grains:
- oats
- barley
- corn
These ingredients have a high concentration of non-structural carbohydrates and significantly increase the glycemic response. However, attention must also be paid to feeds such as alfalfa and grass hays, where carbohydrate content depends on growth stage, environmental conditions, and plant species.
Feeds with a higher fiber content include:
- beet pulp
- grass hay products
- alfalfa
- wheat bran
Wheat bran is somewhat controversial. It contains less starch than whole grain; however, its content depends on the degree of milling. The more finely the grain is milled, the lower the starch content.
Conclusion
Distinguishing between individual carbohydrate types is essential for properly formulating a horse’s ration. Sugars and starch are important for performance, but their excess poses a real health risk, especially in horses without significant workload. Fiber is the foundation of the equine diet and supports long-term health.
Sources
- Geor, R.J., 2010. Digestive strategy and flexibility in horses with reference to dietary carbohydrates.
- Harris, P., Geor, R., 2009. Primer on dietary carbohydrates and glycemic index in equine nutrition.
- Strauch, S. et al., 2017. Evaluation of an in vitro system to simulate equine foregut digestion.
- Julliand, V., Grimm, P., 2017. The impact of diet on the hindgut microbiome.
- Cichorska, B. et al., 2014. Nutrient digestibility in horse nutrition – A review.
- Zeyner, A., 2008. Energy providing nutrient sources.
- Harlow, B.E. et al., 2016. Effect of dietary starch on equine fecal microbiota.
- Jansson, A. et al., 2002. Effects of dietary starch and sugar on exercise metabolism.
- Pratt-Phillips, S.E., Lawrence, L.M., 2013. Nutrition of the performance horse.
- Cipriano-Salazar, M. et al., 2019. Dietary components and digestive disturbances in horses.
- Ermers, C. et al., 2023. Fiber requirements in horses and consequences of deficiency.
