If you google "hyperlipidemia in horses" you’ll see terms like ‘life-threatening’ and ‘potentially fatal’ ... No question - it's a serious condition. And yet it’s not something we rarely talk about in our EMS world. That said, it’s something worth being aware of as it can catch us out – often when we're trying to do the right thing.

So what is hyperlipidemia?

In simple terms, hyperlipidemia is a metabolic emergency response, occurring if/when we reduce  feed/calorie intake too quickly to slim our EMS pony down. Thing is - this lowers energy levels, so the metabolism panics, triggering a reaction to draw on its muscle protein and fats reserves to be used instead for energy.

Here’s the sequence:

• Feed intake drops
• Energy availability falls
• The body switches to emergency fuel mode
• Fat reserves are mobilised into the bloodstream to be used for energy.

But here’s where the problem lies. The horse isn’t designed to rely on fats or proteins as a primary energy source in the way other species can. Which means… instead of being used efficiently, those circulating fats begin to accumulate in the blood.

And when that process runs unchecked, it can place significant strain on the liver and wider metabolic system.  Worse case? Liver and renal failure, which yes, can be fatal.

When does it tend to happen?

Most commonly, we see hyperlipidemia in situations where:

• Feed has been reduced too far, too quickly
• Weight loss is pushed aggressively
• The horse is already metabolically compromised (EMS/IR/PPID)
• Stress is also part of the picture - which naturally happens if the horse doesn't have adlib access to forage.

It's not random - it's the body reacting to pressure. Hence why we need to be particularly mindful in native types and ponies, where this response is known to be more easily triggered.

The key shift in understanding

Not so long ago, the focus was often on restriction at all costs - less grass, less feed, less energy. Today? What we now understand is this - over-restriction can be just as destabilising as overfeeding - because the body doesn’t recognise “dieting” - it recognises scarcity - and it responds accordingly.

Stress plays a role here too – elevated stress hormones can further disrupt metabolic signalling, inducing further elevated insulin to an already sensitive system.

Starving a horse slim is not the solution

This is where hyperlipidemia becomes particularly relevant for our EMS horses. No question, the best prevention for IR/EMS horses is simply by not allowing our horses to become obese. But while weight management is important, pushing intake too low can backfire metabolically – it’s crucial not to starve the weight off your horse.

To quote Dr Kellon:

“The primary treatment for horses with IR and/or Cushing's is a low carb and mineral-balanced diet. Any pasture turnout should be eliminated until all signs of IR are totally absent, and reintroduced only with extreme caution. Hay with low carbohydrate content is recommended, ensuring total feed intake never drops below 1.5% of body weight.

Attempting to achieve weight loss by reducing feed intake below 1.5% can worsen IR and precipitate hyperlipidemia. Hyperlipidemia is potentially life threatening, especially in ponies, so DON'T STARVE the weight off your horse or pony.”

In short? Attempting to achieve weight loss by reducing feed intake below 1.5% of body weight can worsen insulin resistance and precipitate hyperlipidemia.

So how do we prevent it?

Simple. Proper diet and weight control. And above all, avoiding the temptation of rapid weight loss methods such as underfeeding, and/or using indigestible feeds - such as straw - as a primary feed.

At their core, horses are hindgut fibre fermenters,  and crucially, this is where their energy comes from – the fermentation of fibre. Not from fats, not from proteins, and certainly not from starvation.

Disrupt this vital, species-appropriate foundation, and the whole system starts to wobble:

• A low sugar, low carb, high fibre diet is the cornerstone in regulating blood sugar levels, avoiding the excess calories that contribute to obesity and prevent fat accumulation.
• Adlib quality meadow hay to satisfy hunger is essential:
• Without food, and being a trickle feeder, a horse will panic, which releases cortisol (the unfriendly stress hormone), which induces further elevated insulin.
• Equally, allowing the stomach to empty risks gastric ulcers developing. A wild horse can simply wander off and find more grass, but the domesticated horse is entirely dependent on how – and when – we feed them.
• Fibre intake must stay consistent as the gut microbiome needs to remain stable.

A quick word on straw

I've so often seen porky EMS-type ponies kept in all day in a stable, with a small haynet of just straw to last them all day, their owners clearly thinking this is a way of reducing calorie intake. And while it can have a place alongside forage in some situations, it should never be a replacement for it.

Equally, both Dr Kellon and Dr Fritz warn against relying on indigestible feeds like straw. Here’s why:

1. Calories between hay and straw - there’s not much difference. If you’re already feeding a mature hay or one chosen for low sugar and starch levels, the difference is only a 7% calorie reduction.
2. Straw’s sugar levels can be as high as 6.2%, and starch up to 4.3%.
3. With significant differences in the fibre ratios, straw is more difficult for the hindgut to ferment, which may result in bloated belly/diarrhea/faecal water, especially in older horses.
4. Offers less nutritional prebiotic support for the hindgut biome, meaning poor production of essential byproducts.
5. Protein? Deficient – around half that of hay.
6. Mineral levels - lower phosphorus and magnesium than hay.
7. A high risk of toxic nitrate levels.

So while it may “fill the stomach”, it starves the horse of energy, and doesn’t support the horse’s metabolic needs. And if overall energy intake drops too low? We’re back to that same metabolic stress response – triggering the need to find other stored energy resources. Cue hyperlipidemia.

As Dr Kellon says, we’re better off double netting our hay, or investing in a slow-feeder system.

The liver’s role in all of this

The liver is central to metabolic regulation - it decides what gets used, what gets cleared, and what gets stored. And in our EMS horses, the liver is often already under increased demand – processing sugars, managing energy flow, and supporting wider metabolic balance.

So when large amounts of circulating fat enter the system? It’s the liver that has to deal with it. And like any system under pressure, it has its limits. 

The takeaway

If there’s one message to take from all of this, it’s this - metabolic health is about balance, not extremes - not underfeeding, and not chasing quick fixes in either direction.

Instead:

• Maintain consistent fibre intake
• Avoid sudden or aggressive restriction
• Support the whole metabolic system
• Work with the horse’s biology, not against it

Why? Because the horses most at risk aren’t always the ones that look the worst on the outside… they’re often the ones whose internal systems are already under strain.

Final thoughts

Hyperlipidemia is a very real reminder of how sensitive the metabolic system can be. And perhaps more importantly, it reinforces a message we come back to time and time again:

• It’s not just about weight
• It’s not just about grass
• It’s about how the whole system is coping

When we understand this, we make better decisions - and our horses feel the difference.

Links to:

• EMS/IR page - Learn more about managing the metabolic horse
• Laminitis page - How this links to laminitis risk
• Feeding our Horses Healthy page - Why fibre is foundational