The equine gut simply hasn’t evolved to digest fat.

Horses can't digest fat, full stop. The natural equine diet of lean grasses, herbs, leaves, barks etc., are naturally very low in fat content, hence why horses have evolved over the course of evolution to cope with low food fats, specifically the EFAs (Essential Fatty Acids), aka the omegas 3 & 6 - in grass forage, so sloshing a load of extra liquid oil into the feedbowl does not a happy gut system make - seriously.

A quick heads-up - this page is a bit of a 🤓 Science Alert ...

For starters, unlike us and dogs, horses don’t have a gall bladder, the primary function of which is to store, then release enough 'emulsifier', aka bile - a bit like our washing up liquid poured into a sink over a pile of greasy pans - to degrade fat to fatty acids which can then be absorbed through the small intestine so the liver can make body fat. The horse's bile comes directly from the liver via a small trickle - to match their natural 'trickle' feeding practice - just enough to emulsify the small amount of EFAs from their forage.

This is one of the many reasons why the feed industry frustrates the heck out of me, because we see so often the phrase ‘Lightly coated with xxx oil’ on so many bags. If you look at the wild horse, their constant roughage feeding provides enough EFAs, and the gut microbiome also produces a %. It’s different for our domesticated horse because we’ve messed with their lifestyle and biology, which is why we have to add EFAs into the diet.

It frustrates me all the more that you hear some vets advise to feed oil, or worse, corn oil (gahhh!!!) to a horse for 'energy'. I had a client contact me just the other day to let me know her vet had suggested this - horrors! I can only assume that because vets don’t cover nutrition in their training, they don’t know about either a horse's inability to digest fat or the fact that their energy only comes from fibre digestion in the hindgut.

So what’s this oil/fat thing all about?

Stand by your guns cos here's the science 🤓. One of the bile’s many jobs is to help with the digestion of the fats in the diet. So now let’s quickly remind ourselves that fats aren’t water soluble – if you add oil to anything, it swims to the top. If you mix your own vinaigrette you'll know what I mean - vinegar plus oil; shake it, and watch the oil head for the top of the jar.

Now to horses, and pretty much most of what horses eat is water soluble, as are their stomach fluids too. And here’s the problem – it’s all to do with those digestive enzymes again because they're all water-soluble too. The enzyme that degrades natural fat in forage - lipase - is water soluble, not fat soluble. Which I know sounds wrong but there it is. Thing is, if lypase was fat soluble it would dive right in to the fat and get on with it, but it isn’t, so … the fat has to be made water soluble first by the bile trickle from the liver.

Note I'm saying 'fat' here and not 'oil', to distinguish between the EFAs/fatty acids meant to be in the natural diet of a horse from forage/roughage, and us adding liquid pouring oil - the latter here is what this page is all about.

So how does the horses's gut convert the fat to water soluble? Back to bile, which has molecules that are both water and fat soluble – they’re called emulsifers and they act just like a detergent, like soap. And whether horse, human or dog, the whole feed is shaken about in the gut system via intestinal peristalsis, so fat molecules that usually cling together are now shaken into smaller particles. The longer it’s shaken up, the smaller the fat becomes, turning into what's known as fat mycelles – tiny fat balls with emulsifier mixed in. So, now we have tiny fat mycelles with a water soluble surface so the lipase can start digesting the fat inside the bubble.

Back to the liver, and without a gall bladder to release bile as per humans or dogs, the equine liver constantly produces a bile trickle, continually secreting it to the small intestine to match the way a horse eats, because, remember, the horse is a continual trickle forager with their forage having very low EFA fat levels, so what little fat there is can instantly be emulsified by the continual trickle of bile from the liver.

So now we get to the issue of adding extra liquid oil to the feedbowl, and this delicate dietary low-fat process is about to change dramatically, simply because the small intestine isn’t in any way, shape or form, evolved to deal with a high oil load because, remember - all digestive enzymes are water-soluble and the liver doesn't produce much bile!

So, since the extra volume of fat/oil cannot be fully - or immediately - emulsified, it starts to spread over the rest of the gut contents, i.e. the starches and proteins. So, amylase, the starch enzyme digester, can’t reach the starch due to the fat layer; pepsin, the protein enzyme digester, can't get to the protein because of the fat layer, and so on. Back to our sink full of glasses/dishes/greasy pans in the sink - without washing-up liquid everything gets covered with a greasy layer.

In theory, yes we’ve added something (possibly) beneficial – calories/energy as far as the vets think - to the feedbowl, but in practice? No nutrient benefits can be absorbed by the horse because the digestive enzymes can't get to what they're meant to digest, which carries the knock-on risk of large undigested starch and protein particles reaching the hindgut, which presents a major colic/lami risk ☹

There's another reason we shouldn’t add liquid oil to our horse, and that's because 50% of it will swim straight through the small intestine directly into the hindgut. Remember, the hindgut microbes' only job is to degrade the cellulose fibre from the forage/roughage which provides the horse's energy, so if starch/protein/oil gets into the hindgut, it's toxic to those hindgut fibre-fermenting microbes. So, if these vital microbes get killed off, hey ho, there's dysbiosis/SIBO and leaky gut, alongside a major colic/lami risk, plus ... no energy is created either.

Pulling this together

Adding extra oil in liquid form to the feedbowl creates a huge metabolic problem. Despite thinking it adds energy, ultimately it results in loss of energy, and risks a major lami/colic episode too.

That said, feeding fat in a meal or seed form such as linseed, is much safer because it's not a 'floating' oil, and will be easily digested and utilised as building materials for many molecules in the body such as cell membrane or hormones. However, it doesn't hurt to still be aware of the fat content %.