Understanding Hydrolysable Carbohydrates (HC)

Key Message

Hydrolysable carbohydrates (HC) are the sugars and starches that are digested into glucose and cause insulin to rise. For horses with Equine Metabolic Syndrome (EMS) or insulin dysregulation, controlling HC intake is one of the most important steps in preventing laminitis.

At a Glance

✓ Hydrolysable carbohydrates (HC) are the sugars and starches that raise blood glucose and insulin.
✓ HC is calculated by adding ESC + starch.
✓ Most horses can safely digest HC, but horses with EMS or insulin dysregulation often produce an abnormally high insulin response.
✓ High insulin, not grass itself, is responsible for most modern cases of laminitis.
✓ Grass, hay and bagged feeds can all contain HC.
✓ You cannot tell how much HC a forage contains by looking at it.
✓ Hay testing is the best way to know what your horse is eating.
✓ For horses with EMS or insulin dysregulation, ECIR recommends forage containing 10% HC (ESC + starch) or less.
✓ Understanding HC makes choosing feeds much simpler.

 

The Full Lesson

Why do we need to understand carbohydrates?

Carbohydrates are the horse's main source of energy.

The important thing to understand is that not all carbohydrates behave the same way.
Some are broken down into glucose and absorbed into the bloodstream. These are the carbohydrates that cause insulin to rise.

Others are fermented by the bacteria living in the horse's hindgut and have very different effects.

Understanding the difference helps explain why modern laminitis management focuses on some carbohydrates more than others.

What are hydrolysable carbohydrates?

Hydrolysable carbohydrates, usually shortened to HC, are the carbohydrates that are digested in the small intestine into glucose.

As glucose enters the bloodstream, insulin is released.

For most horses this is a completely normal process.

However, horses with EMS or insulin dysregulation often produce much higher insulin levels than normal after eating the same amount of carbohydrate.

Because high insulin is now recognised as the major driver of most laminitis cases, managing HC intake has become one of the foundations of prevention.

The ECIR Group defines HC as:

ESC + Starch

This simple equation is used throughout the ECIR educational material because these are the carbohydrates most closely linked to the insulin response.

What is ESC?

ESC stands for Ethanol Soluble Carbohydrates.

Don't let the name worry you.

In simple terms, ESC measures the simple sugars in forage that are rapidly digested and absorbed.

These sugars quickly become glucose in the bloodstream and stimulate insulin release.
For horses with insulin dysregulation, these sugars are particularly important because they can produce a much larger insulin response than they would in a healthy horse.

What is starch?

Starch is another carbohydrate that is digested into glucose.

Like ESC, starch contributes to the horse's insulin response.

Many grains are naturally high in starch, but starch is also found in pasture, hay and some commercial feeds.

When we calculate HC, both ESC and starch matter because both raise glucose and insulin.

Why don't we just use NSC?

Many horse owners have heard the term NSC (Non Structural Carbohydrates).

For many years, NSC was widely used when discussing laminitis.

The difficulty is that NSC also includes fructans.

Fructans behave very differently from sugars and starch.

Because fructans do not directly raise blood glucose or insulin, the ECIR Group now recommends focusing on HC (ESC + starch) when managing horses with EMS or insulin dysregulation.

This approach gives a much clearer picture of the carbohydrates most relevant to insulin control.

 

What about fructans?

Fructans have probably caused more confusion than any other carbohydrate.

For many years they were thought to be the main reason horses developed pasture-associated laminitis.

We now know the picture is more complex.

Fructans are not digested into glucose in the small intestine.

Instead, they pass to the hindgut where they are fermented by bacteria.

Very large experimental doses of purified fructan have been used to create severe hindgut disturbance and inflammatory laminitis in research horses.

However, this is a different process from the hyperinsulinaemia-associated laminitis that now accounts for most naturally occurring cases.

That doesn't mean fructans are unimportant.

It simply means that, when managing horses with EMS or insulin dysregulation, the main dietary focus is on controlling HC because these are the carbohydrates that directly influence insulin.

 

Where do hydrolysable carbohydrates come from?

HC are found in many parts of the horse's diet, including:
• Fresh pasture
• Hay
• Grain
• Sweet feeds
• Some commercial feeds
• Treats such as apples and carrots
This is why preventing laminitis isn't simply about removing grass.
The horse's entire diet needs to be considered.

 

Why should I test my hay?

One of the most common questions owners ask is whether they can tell if hay is suitable by looking at it.

Unfortunately, the answer is no.

A soft green hay may be perfectly suitable.

A dry, stemmy hay may be high in HC.

The appearance, smell and texture of hay tell us very little about its carbohydrate content.

The only reliable way to know is through laboratory analysis.

A hay test allows you to see the ESC and starch values so the diet can be based on facts rather than guesswork.

 

Should I soak hay?

If your hay hasn't been tested, soaking is often recommended as a temporary way of reducing soluble sugars.

The ECIR Emergency Diet recommends soaking untested hay while waiting for the analysis.

However, soaking does not remove the same amount of sugar from every hay.
Some hays lose relatively little.

Others lose much more.

For that reason, soaking should be viewed as a temporary management tool rather than a substitute for hay testing.

 

What should I aim for?

For horses with EMS or insulin dysregulation, the ECIR Group recommends forage with:
HC (ESC + starch) of 10% or less

This recommendation forms the basis of the ECIR Emergency Diet and many long-term feeding programs.

Once the hay has been tested, the rest of the diet can be balanced to meet the horse's individual nutritional needs.

The most important thing to remember - Hydrolysable carbohydrates are not "bad."

Healthy horses eat HC every day without developing laminitis.

The problem occurs when a horse cannot regulate its insulin response.

Understanding HC helps you make better feeding decisions, but it is only one part of the bigger picture.

Successful laminitis prevention also depends on maintaining a healthy body condition, investigating EMS and PPID where appropriate, providing regular hoof care and introducing exercise when the horse is sound.

 

Continue Learning

If you'd like to explore these topics in more detail, the following lessons build on the information in this guide.
• Why Horses Get Laminitis
• Preventing Laminitis
• The Emergency Diet
• Understanding Equine Metabolic Syndrome
• Understanding PPID
• Reading a Hay Analysis
• Managing Pasture for the Metabolic Horse

 

Further Reading

This lesson has been prepared using the following primary resources. If you would like to explore this topic in more detail, these are excellent places to continue your learning.

ECIR Group
• Diet – Nutritional Needs for EMS and PPID/Cushing's Horses
• Frequently Asked Questions (Hydrolysable Carbohydrates, ESC, NSC, WSC and Fructans)
• Equine Metabolic Syndrome

NO Laminitis! Conference Proceedings (2025)
• Dr Katy Gustafson – Nutrition Considerations for Equine Hyperinsulinemia
• Dr Katy Gustafson – Factors Affecting Forage Carbohydrates
Dr Eleanor Kellon
• Horse Sense Blog articles relating to hydrolysable carbohydrates, hay testing, insulin dysregulation and feeding horses with EMS and PPID.