Understanding Equine Metabolic Syndrome

Understanding Equine Metabolic Syndrome (EMS)

Key Message

Equine Metabolic Syndrome, or EMS, is a metabolic tendency in which the horse develops abnormally high insulin levels, usually because the body does not respond to insulin normally.

The most serious consequence of EMS is laminitis.

EMS is strongly influenced by genetics, diet and exercise, but not every horse with EMS is overweight. Some horses remain relatively lean while still producing dangerously high insulin.

The good news is that EMS can often be managed very successfully through appropriate diet, weight management where required, and regular exercise when the horse is comfortable to move.

At a Glance

• High insulin is the hallmark of EMS.
• EMS and PPID are different conditions, although a horse can have both.
• The biggest danger associated with EMS is laminitis.
• Horses with EMS often gain weight easily or develop a cresty neck and unusual fat deposits, but not all horses with EMS are fat.
• Some breeds and individuals appear to have a stronger genetic tendency towards EMS.
• Horses with EMS are less able to tolerate diets high in sugars and starch.
• The most important dietary carbohydrates to control are hydrolysable carbohydrates, HC, calculated as ESC + starch.
• Appropriate forage, regular exercise and maintaining a healthy body condition form the foundation of management.
• EMS is diagnosed through appropriate insulin testing, not simply by looking at the horse.
• Horses should generally not be fasted before routine EMS testing under the ECIR protocol.
• PPID should also be investigated where appropriate, particularly in older horses.
• Some severely affected horses continue to have very high insulin despite excellent management. Medication may sometimes be considered in these refractory cases.

The Full Lesson

What is Equine Metabolic Syndrome?

EMS as a metabolic type characterised by high blood insulin, or hyperinsulinaemia, usually associated with insulin resistance.

Insulin is a normal hormone. Its job includes helping glucose move from the bloodstream into cells where it can be used for energy.

In an insulin resistant horse, the cells do not respond normally to insulin. The body compensates by producing more insulin to achieve the same result.

Blood glucose may therefore remain quite normal while insulin becomes abnormally high.

It is this high insulin that presents the greatest danger to the horse.

Why is high insulin so important?

Research has shown that very high insulin concentrations can directly cause laminitis.

The hoof changes produced experimentally by high insulin resemble those seen in naturally occurring pasture associated laminitis.

Hyperinsulinaemia associated laminitis accounts for around 80 to 90 per cent of laminitis cases.

The goal is to control insulin and prevent laminitis.

What about PPID?

EMS and Pituitary Pars Intermedia Dysfunction, or PPID, are separate conditions.

Historically there was considerable confusion between the two because both can be associated with abnormal fat deposits, high insulin and laminitis. EMS was even referred to in the past as "Peripheral Cushing's". We now know they are different conditions.

Broadly:

• EMS is managed primarily with diet and exercise.
• PPID is a disorder of the pituitary gland and generally requires medication such as pergolide or sometimes cabergoline.

A horse can also have both EMS and PPID, in which case both need to be addressed.

What does an EMS horse look like?

Many affected horses are what owners describe as "easy keepers". They may gain weight easily and require far fewer calories than other horses.

Common signs may include:

• A cresty neck
• Fat around the tail head
• Fat behind the shoulders
• A fatty sheath or udder
• Puffiness or fat deposits above the eyes
• A history of pasture associated laminitis
• High insulin despite normal blood glucose

A Horse (above) showing a typical cresty neck.

However, a horse does not have to be fat to have EMS. Dr Eleanor Kellon's 2021 NO Laminitis! presentation emphasises that many horses with EMS are normal weight, and that laminitis may actually be a more important clinical sign than obesity.

The 2025 conference proceedings make the same practical point. Horses that are not generally obese may still have focal fat deposits and hyperinsulinaemia severe enough to put them at significant laminitis risk.

So appearance alone cannot diagnose or rule out EMS.

What causes EMS?

There appears to be a strong genetic component.

The ECIR material describes carbohydrate intolerance as part of the individual horse's metabolism, with some horses and breeds naturally more susceptible than others.

Diet and lifestyle then influence how strongly that susceptibility is expressed.

A horse that is genetically predisposed may cope relatively well when lean, active and eating an appropriate forage based diet, but develop worsening hyperinsulinaemia when exposed to more calories, higher HC pasture or feeds, and reduced exercise.

The relationship is therefore better thought of as a combination of:

• Genetic susceptibility
• Diet
• Exercise
• Body condition
• Age
• PPID where present

rather than one single cause.

Does obesity cause EMS?

Obesity and EMS are commonly seen together, but being fat does not automatically cause EMS.

Dr Kellon's 2021 review points out that attempts to induce obesity experimentally in normal horses have not consistently created insulin resistance, and many naturally occurring EMS horses are not obese.

Excess weight still matters.

An overweight EMS horse is consuming more energy than it needs, and reducing excess body fat is often an important part of management. However, obesity should be considered a warning sign and management problem, not a diagnostic test for EMS.

That distinction matters because otherwise lean EMS horses may never be tested until they develop laminitis.

Why are some breeds more susceptible?

Long before EMS was formally recognised, horse owners noticed that certain ponies and hardy breeds were more likely to develop cresty necks and pasture associated laminitis.

Above - Two Pony breeds, both obese, an extremely high risk for Laminitis!

The 2006 Virginia Tech pony study discussed throughout the ECIR conference material demonstrated an association between genetics, insulin resistance, diet and subsequent laminitis risk.

These horses evolved in environments where food was often scarce and the ability to use available energy efficiently was an advantage.

Under modern management, with improved pasture, readily available hay and relatively little movement, that same metabolic efficiency can become much harder to manage.

This does not mean every pony or easy keeping breed has EMS but we should be more alert to the possibility in these cases.

What does diet have to do with EMS?

Diet is central to EMS management because insulin rises in response to the carbohydrates that ultimately provide glucose.

The dietary carbohydrates of greatest concern are hydrolysable carbohydrates, or HC.
HC is calculated as:

ESC + starch

For an EMS horse, forage containing 10% HC or less is recommended for most individuals.

A note about the words "insulin resistance" and "insulin dysregulation"

Much of the veterinary literature now uses insulin dysregulation as a broad term for abnormal insulin responses.

Dr Eleanor Kellon has argued that insulin resistance remains the more accurate description for most EMS horses because the available evidence does not convincingly demonstrate that the excessive insulin response occurs independently of insulin resistance.

Can EMS be cured?

The ECIR material describes EMS as a metabolic type rather than a disease with a simple cure.

For many horses, this means the tendency towards excessive insulin responses will always need to be considered.

With appropriate management, many horses can return to normal insulin levels, achieve a healthy body condition, exercise normally and live comfortable, active lives.

The management may need to remain in place long term.

Think of it less as a treatment course that eventually finishes and more as understanding what this particular horse's metabolism can tolerate.

What if diet and exercise aren't enough?

Every horse should first have the fundamentals addressed carefully:
• Confirm the diagnosis
• Control HC intake
• Control PPID if present
• Correct the overall diet
• Introduce exercise when safe

Some horses remain severely hyperinsulinaemic despite appropriate management.

For these refractory cases, ECIR discusses medications including metformin and, more recently, SGLT2 inhibitor drugs such as canagliflozin or ertugliflozin.

These medications are not substitutes for diet and management.

SGLT2 inhibitors in particular require careful veterinary supervision, dietary adjustment and monitoring because important side effects can occur.

For most owners reading an introductory EMS lesson, the important message is simply that additional options now exist for genuinely difficult cases.

Continue Learning

If you'd like to explore these topics in more detail, the following Lessons from Missy build on the information in this guide.
• Why Horses Get Laminitis
• Understanding Hydrolysable Carbohydrates
• Diagnosing the Cause
• Understanding Insulin Testing
• Understanding PPID
• Preventing Laminitis
• Managing Pasture for the Metabolic Horse
• Exercise and the Metabolic Horse