The difference between ALA and EPA/DHA, and why it's not just chemistry

New here? Our guide to omega-3 in horses explains ALA, EPA and DHA from first principles →

Omega-3 is not a single substance. It is a name given to a group of related fatty acids, and the ones your horse eats are not the ones its tissues use.

When people talk about omega-3, they tend to talk about it as though it were one thing — one nutrient, one effect, one question to answer: is my horse getting it or not?

In reality omega-3 is a collective term for a group of related fatty acids that share a similar chemical structure. Three of them matter for horse owners: ALA, EPA and DHA. They are different molecules, they come from different places, and they do different jobs, which is why the question "is my horse getting omega-3?" is less useful than it sounds.

What a horse actually eats

Almost all of the omega-3 in a horse's natural diet is ALA — alpha-linolenic acid. It is the dominant fatty acid in vegetative forage, concentrated in the leaf, which means a horse grazing good spring pasture is taking in a considerable amount of it every day without anybody supplementing anything.

The plant-based sources owners reach for are the same molecule in more concentrated form. A scoop of micronised linseed supplies ALA, linseed oil supplies ALA, and chia supplies ALA — as does the grass in the field and, in much reduced quantity, the hay in the barn.

What none of them supply is EPA or DHA.

Why a horse is in a different position from you

This is where the horse differs from the animals most omega-3 advice is written about, and it is the part worth understanding properly.

A human eating oily fish takes in EPA and DHA ready-made, and a dog fed a diet containing fish oil does the same. Their tissues receive a supply of long-chain omega-3 that never had to be manufactured internally at all, so whatever their conversion machinery manages, it is topping up something already arriving in the diet.

A horse is a strict herbivore. Nothing in a horse's natural diet contains meaningful quantities of preformed EPA or DHA — not grass, not hay, not haylage, and not any of the plant oils or seeds routinely added to a feed. Every molecule of EPA and DHA in a horse's cell membranes has either been built inside the horse from ALA or supplied deliberately from outside the natural diet.

That makes the conversion step load-bearing in a way it simply isn't for a fish-eating species, because there is no second route.

Short-chain and long-chain — what the difference means

Fatty acids come in different lengths, and that length is not just a structural detail: it determines what a fatty acid can do in the body and how much work the body has to do before it can use it.

ALA is a short-chain omega-3, an 18-carbon molecule and an essential nutrient, meaning the horse cannot manufacture it and must obtain it from what it eats. That makes it genuinely important, and it is not the point of contention.

But short-chain means starting material. The horse cannot use ALA directly in the tissues where omega-3 is expected to have an effect, and has first to extend and modify it into longer forms: EPA at 20 carbons and DHA at 22.

EPA and DHA are structurally more complex, and that complexity is what makes them functionally active. These are the forms that get incorporated into cell membranes — in joint fluid, in the lining of the airways, in muscle, in skin beneath the surface coat — and once they are there they influence how those cells behave, including how they respond to inflammation. They are not building blocks waiting to be converted; they are what the pathway is trying to produce.

The raw material and the finished product

A straightforward way to think about it is that ALA is the raw ingredient and EPA and DHA are what gets made from it.

When the diet provides ALA, the horse has to manufacture EPA and DHA itself through a series of chemical steps that progressively extend and modify the short-chain molecule. When EPA and DHA are provided directly — from algae, for example, where these fatty acids originate in the first place — that manufacturing step is bypassed and the body receives the finished product rather than the raw material.

This is the fundamental difference between plant sources like linseed and direct sources of EPA and DHA. It is not that one is good and one is bad, but that they enter the pathway at different points: one arrives as raw material requiring further processing, the other arrives ready to use.

Why this matters for supplementation

Two horses can both be described as receiving omega-3 supplementation while sitting in quite different situations. One is receiving ALA, a building block that depends entirely on conversion. The other is receiving EPA and DHA, the molecules the body would otherwise have had to produce, supplied ready-made.

From a labelling perspective both are being supplemented with omega-3. From a biological perspective, one horse's tissues depend on a manufacturing process that may or may not run reliably, and the other's do not.

There is a practical consequence to this that catches owners out. Because EPA and DHA work by being built into cell membranes, and because membranes renew gradually — an equine red blood cell lives roughly 140 to 150 days — nothing here happens quickly. Whichever route the fatty acids arrive by, the tissue picture changes over months rather than weeks, which is why supplementation trials in horses run for 60 or 90 days and longer rather than a fortnight.

Whether the conversion route actually delivers in a horse, and what happened when researchers went looking, is the subject of the next article.