This piece, authored by IFFO's Dr Brett Glencross, was published in the January 2024 edition of International Aquafeed magazine.

I recently attended a meeting with a couple of the world’s best lipid nutrition experts and I put to them the simple question: Is there more to fish oil than energy and omega-3’s? The answer I got back was a little surprising in that “it depends on which omega-3’s are you talking about; short-chain, long-chain, or the very-long-chain ones”. It was the opening of a Pandora’s Box of sorts, and so the conversation went on to clarify things.

Short-chain (SC) omega-3’s are those fatty acids we often find in vegetable oils like linseed, camelina and canola oils. The typical fatty acid here we are talking about is 18:3n-3, known as alpha-linolenic acid (ALA as it is often abbreviated to). This fatty acid has 18-carbons, 3-unsaturated bonds, with the first of them 3-carbons from the terminal (or omega) end of the carbon chain. It was argued that this fatty acid (ALA) was in reality, the true “essential” omega-3 fatty acid, as virtually no animal can make this one, only plants can do that, so for animals it has to be obtained via the diet. But its levels are low to non-existent in fish oils, so it clearly wasn’t the one we were focussing on in our original question.

Long-chain (LC) omega-3’s though are those fatty acids that have 20- or 22- carbons and contain two or more of those unsaturated bonds. There are about half a dozen that are found naturally in fish oils, but the fatty acids here we are (usually) talking about include 20:5n-3, known as eicosapentaenoic acid (EPA as it is often abbreviated to), or 22:6n-3, known as docosahexaenoic acid (usually abbreviated to DHA). These are the fatty acids that are most often talked about when we mention “essential” fatty acids, as in most animals they have significant levels of biological activity affecting a whole range of physiological mechanisms (usually for the better). But there are some animals that can make these long-chain omega-3 fatty acids from ALA, but for most animals it is clearly more beneficial if they are obtained via the diet. And as for our original question. Yes, for these fatty acids their levels in fish oils are relatively high and they are widely recognised as one of the best sources available for these nutrients. So much so that fish oils are now traded on a combination of their energy value and EPA+DHA value. So, that kind of reaffirmed the original question nicely.

But, what about these very-long-chain (VLC) omega-3 fatty acids? Well, these are those fatty acids that typically have 24- or more- carbons and contain two or more of those unsaturated bonds. There are lots that are found naturally, but the level of these fatty acids is generally less than 0.2% of most fish oils. Among the more common ones are 24:6n-3, 26:6n-3 and 28:8n-3. Why haven’t we heard more about these? One comment I got was that most labs turn their gas chromatograph (the machine that measures fatty acids) off once they get the DHA data, and these VLC things come way after that. So, for they most part they are just ignored. However, among the few studies that have followed things further they find that these VLC are often constrained to certain tissues like brain, eye, or testes where they are parts of important structural lipids (fats) that are assumed to have some important role. Most studies on these things have had such a small amount to play with that the studies are often constrained to cell culture trials and the like. In those studies, the VLC show some interesting properties in affecting things like wound healing. But notably these VLC omega-3 although not abundant, fish oil remains the best source of them. So, maybe in the future fish oil will continue to be valued on their energy+omega-3 content, but just what constitutes that omega-3 might evolve.