Carbohydrates in the Horse: Understanding Sugars, Starch and Fibre
Carbohydrates are often misunderstood in equine nutrition. Some owners think to avoid them altogether, while others unknowingly feed far more than their horse needs.
The truth is that carbohydrates are an essential part of the horse’s diet. The key is understanding that not all carbohydrates are the same, and they affect the horse’s body in very different ways.
What Are Carbohydrates?
Carbohydrates are one of the horse’s primary sources of energy and can be divided into three main groups:
- Sugars
- Starches
- Fibres
All carbohydrates are made up of sugar molecules called saccharides.
Simple Carbohydrates
The simplest carbohydrates are known as monosaccharides, meaning they consist of a single sugar molecule.
Examples include:
- Glucose
- Fructose
These are rapidly absorbed and provide readily available energy.
Two sugar molecules joined together form a disaccharide.
Examples include:
- Sucrose (table sugar)
- Lactose (milk sugar)
Complex Carbohydrates
Larger carbohydrate molecules are known as polysaccharides or complex carbohydrates.
These include:
- Starch
- Cellulose
- Hemicellulose
- Pectin
- Fructans
Although these are all carbohydrates, they are digested very differently by the horse.
How Horses Digest Carbohydrates
Horses digest different carbohydrates in different parts of the digestive tract.
Sugars and Starch
Sugars and starch are primarily digested in the small intestine.
Digestive enzymes break starch into glucose, which is then absorbed into the bloodstream.
Glucose can be:
- Used immediately for energy.
- Stored as glycogen in the liver and muscles.
- Converted to body fat if consumed in excess.
Fibre
Fibre is completely different.
Horses cannot digest fibre using their own enzymes.
Instead, fibre travels to the hindgut, where trillions of beneficial microbes ferment it.
During fermentation these microbes produce volatile fatty acids (VFAs) such as acetate, propionate and butyrate.
These VFAs provide more than 70% of the horse’s daily digestible energy, making fibre—not grain—the horse’s primary fuel source.
Why Too Much Starch Can Cause Problems
The horse’s ability to digest starch is limited.
Only a certain amount can be broken down by enzymes in the small intestine during each meal.
When excessive starch is fed, undigested starch escapes into the hindgut.
Unfortunately, the hindgut microbes are designed to ferment fibre—not large amounts of starch.
Rapid fermentation of starch produces lactic acid, lowering the hindgut pH and creating a condition known as hindgut acidosis.
As acidity increases:
- Beneficial fibre-digesting microbes die.
- Fibre digestion becomes less efficient.
- Energy production decreases.
- Harmful bacteria may proliferate.
- Inflammation increases.
The consequences may include:
- Colic
- Loose manure
- Hindgut dysbiosis
- Laminitis
- Reduced appetite
- Poor performance
Carbohydrates and Metabolic Disease
High-starch and high-sugar diets can also influence blood glucose and insulin concentrations.
After a horse eats starch or simple sugars:
- Glucose enters the bloodstream.
- The pancreas releases insulin.
- Insulin allows glucose to enter cells where it is used or stored.
What Is Insulin Resistance?
In horses with insulin dysregulation or insulin resistance, body tissues respond less effectively to insulin.
As a result:
- More insulin is required to maintain normal blood glucose.
- Blood insulin concentrations remain elevated for longer.
- Chronic hyperinsulinaemia greatly increases the risk of laminitis.
Unlike diabetes in people, many insulin-resistant horses continue to maintain relatively normal blood glucose concentrations because they produce large amounts of insulin to compensate.
The Role of Leptin
Leptin is another important hormone produced by body fat.
Normally leptin signals the brain that enough energy has been stored, helping regulate appetite.
Some overweight horses develop leptin resistance, where these signals become less effective.
The horse continues to eat despite having adequate energy reserves, making weight loss increasingly difficult.
Leptin resistance commonly occurs alongside insulin dysregulation in horses with Equine Metabolic Syndrome (EMS).
Understanding Feed Analyses
If your horse has insulin dysregulation, Equine Metabolic Syndrome (EMS), PPID (Cushing’s disease) or a history of laminitis, analysing your forage is one of the most valuable management tools available.
Several carbohydrate measurements may appear on a forage analysis.
Understanding what they mean is important.
Non-Structural Carbohydrates (NSC)
NSC is the total amount of easily digestible carbohydrates and generally includes:
- Sugars
- Starch
- Fructans
While useful, NSC does not tell you which carbohydrates are present.
Two hays may have identical NSC values but affect horses very differently.
Water-Soluble Carbohydrates (WSC)
WSC measures:
- Simple sugars
- Fructans
Because fructans are included, WSC alone cannot predict how much glucose will enter the bloodstream.
Ethanol-Soluble Carbohydrates (ESC)
ESC measures primarily:
- Glucose
- Fructose
- Sucrose
These carbohydrates are rapidly digested in the small intestine and produce the greatest blood glucose and insulin responses.
For horses with insulin dysregulation, ESC and starch are often a very useful measurements.
Why NSC Alone Can Be Misleading
Imagine two hay samples.
Both contain 15% NSC.
Hay A contains:
- 10% fructans
- 5% sugars and starch
Hay B contains:
- 10% sugars and starch
- 5% fructans
Although both report the same NSC value, they are very different nutritionally.
Hay B is likely to produce a much larger insulin response and therefore presents a greater risk for horses with insulin dysregulation.
Hay A may present less of a blood glucose challenge but, if consumed in large amounts, could still disturb the hindgut microbiome because fructans are fermented there.
This is why nutritionists increasingly evaluate ESC, WSC and starch separately, rather than relying solely on NSC.
What Carbohydrate Levels Should You Aim For?
For horses with Equine Metabolic Syndrome, PPID or insulin dysregulation, many equine nutritionists recommend:
- ESC + starch below 10% (combined)
- Total dietary starch and sugar kept as low as practical while still meeting nutritional requirements
If hay analysis is unavailable and your horse is particularly sensitive, soaking hay for approximately 30–60 minutes may reduce some water-soluble carbohydrates. However, the amount removed varies considerably depending on the hay and soaking conditions, with the anount fo reduction only being as much as @3%.
The Bottom Line
Carbohydrates are an essential source of energy for horses—but not all carbohydrates behave the same way.
Fibre supports the hindgut microbiome and provides the majority of your horse’s daily energy.
Sugars and starch provide rapidly available energy but should be fed appropriately to avoid digestive and metabolic problems.
Rather than focusing on a single number such as NSC, evaluate the type of carbohydrate your horse is consuming, particularly if your horse has insulin dysregulation, Equine Metabolic Syndrome, PPID or a history of laminitis.
The goal is not to eliminate carbohydrates—it is to choose the right carbohydrates, in the right amounts, for the individual horse.
Nutrition is the science of prevention.
