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Why Fish Stomps Flax as a Source of Omega-3


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I want to thank everyone for sending in their questions and voting on the next topic. The good news is that there’s a lot of interest in all of the topics I’m researching right now. The bad news is that there was no clear winner.

I’ve decided to go ahead with the series on fish and fish oil, but I may write about thyroid and diabetes simultaneously. I’m also going to experiment with shorter (although I’m clearly breaking that rule here), but more frequent, articles. Hopefully these will be easier for me to write and for you to read.

Finally, stay tuned for the first episode of The Healthy Skeptic audio podcast, coming up next week. I’ll be interviewing Stephan Guyenet, Ph.D, on the subject of obesity and weight regulation. Stephan is a senior fellow at the University of Washington studying the neurobiology of body fat regulation. He’s also the author of Whole Health Source, which is one of my favorite health related blogs.

Before we get into talking about the benefits of fish consumption, or how how much fish or fish oil you should eat, it’s probably a good idea to start with a basic review of the omega-3 fatty acids.

Essential Fatty Acids 101

A fatty acid is a chain of carbon, oxygen and hydrogen atoms with a carboxyl group on one end. Fatty acids are classified on the basis of how many carbon atoms are in the chain, as well as how many double bonds exist within the molecule.

Fish contain a variety of fatty acids, but the ones that are believed to confer the majority of the benefits are the long-chain omega-3 fats eicosapentanaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3 fats are found exclusively in seafood and marine algae.

As you can see from the chart below, it is also possible for the body to synthesize EPA and DHA from the short-chain omega-3 alpha-linolenic acid (ALA). ALA is found in plant foods such as flax, hemp and pumpkin seeds and walnuts.

Click thumbnail for a larger version

However, research clearly indicates that the conversion of ALA to EPA and DHA is extremely limited. Less than 5% of ALA gets converted to EPA, and less than 0.5% (one-half of one percent) of ALA is converted to DHA.

A common misconception, especially amongst vegetarians and vegans, is that our need for EPA and DHA can be met by consuming flax oil and other plant sources of ALA. But the conversion numbers above clearly indicate that this isn’t the case.

Studies have shown that ALA supplements (like flax oil) are unable to raise plasma DHA levels in vegans, despite low DHA levels at baseline. (ref) So unless they are supplementing with an algae-derived source of DHA, it is likely that most vegetarians and vegans are deficient.

This is significant because researchers now believe that the majority of the health benefits we get from dietary omega-3 fats come from the longer chain derivatives (especially DHA, as I will explain below).

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Is DHA essential?

In fact, some researchers have proposed that DHA is essential. When scientists label a nutrient as “essential”, they they’re not just saying that it’s “very important”. In the context of nutrition essential means that the nutrient cannot be synthesized in the human body, and must be derived from dietary sources.

According to today’s nutrition textbooks, there are only two essential fatty acids, omega-6 linoleic acid (LA) and omega-3 alpha-linolenic acid (ALA). It is believed that as long as these fats are present in the diet, all of the longer-chain omega-3 and omega-6 derivatives can be synthesized in the body.

As I pointed out above, while this is theoretically possible, in reality the conversion doesn’t work well. This is true even for healthy people, but it’s especially true for those with nutrient deficiencies, because the conversion of ALA to DHA depends on zinc, iron and pyridoxine.

The bioavailability of iron in plant sources is poor compared to animal sources, so iron deficiency is common in vegans and vegetarians. This is another reason why they tend to be poor converters of ALA to DHA.

Several other observations support the hypothesis that DHA is essential:

  • DHA content in the tissues of all mammals is very similar despite widely varying intakes of omega-3 fatty acids. 1
  • DHA and AA, but not other omega-3 or omega-6 fatty acids, are selectively transferred across the placenta (PDF).
  • 60% of the dry matter of the brain is lipid, and DHA and AA are the most abundant fatty acids of brain phospholipids (PDF)
  • DHA status in newborns is much lower in those receiving formula with LA and ALA, than in those receiving milk or formula with pre-formed DHA (PDF)

It is possible that the primarily carnivorous diet of our ancestors, which ensured a consistently high dietary intake of DHA and AA, precluded the need to evolve efficient conversion mechanisms.

In other words, since we were eating a lot of meat and fish with pre-formed DHA and AA, our bodies didn’t need to be experts at converting ALA and LA in plants to DHA and AA. It is far easier for the body to assimilate pre-formed DHA and AA than it is to synthesize them from precursors.

What about EPA? Isn’t it essential too?

EPA is another long-chain omega-3 fatty acid that is conventionally believed to be responsible for the benefits of fish consumption.

EPA is often referred to as “anti-inflammatory”. However, according to this report on essential fatty acids by Masterjohn, EPA’s effect seems to be more of an interference with the metabolism of omega-6 arachidonic acid (AA) than the performance of any essential role itself.

Take a look at the chart again that I linked to in the beginning of the post. The fatty acids in blue boxes are less inflammatory, and those in pink boxes are more inflammatory. The chart shows that AA is used to synthesize prostaglandins that cause inflammation (indicated by the pink box on the chart). Because it has the same number of carbon atoms, EPA competes with AA for the enzymes that metabolize it. Since the prostaglandins made by EPA are less inflammatory than those made by AA (indicated by the blue box), EPA is often referred to as “anti-inflammatory”.

But while EPA is certainly less inflammatory than AA, it doesn’t make sense that the body would require an essential fatty acid just to block the inflammatory effects of of another fatty acid.

By contrast, DHA is used to synthesize compounds that play an active role in resolving inflammation. EPA only makes these compounds in the presence of aspirin (PDF). EPA is thus likely to simply be a byproduct of compromised DHA synthesis.

What does this mean to you?

Putting all of this information together yields the following conclusions:

  1. DHA is the most important of the omega-3 fatty acids, and is primarily responsible for the benefits we get from consuming them.
  2. DHA is likely to be essential, which means that you must consume it in the diet to prevent disease and ensure optimal function.
  3. The conversion of plant sources of ALA, such as flax seed oil, to DHA is poor in healthy people and even worse in people deficient in certain nutrients. Vegans and vegetarians are especially prone to be poor converters of ALA to DHA.
  4. If you’ve been buying flax oil in the hopes that it will help, you’d be far better off putting that money towards some fish or fish oil capsules.
Dietary changes over the past century have lowered the DHA status to a state of subclinical deficiency in many people. Countless studies show that this deficiency is at least in part to blame for the rising incidence of cardiovascular disease, inflammatory disease, mental and psychiatric disorders and suboptimal neurodevelopment.

DHA is not the only reason to eat fish, which is also rich in selenium and vitamin D. However, DHA is likely to be the primary reason why populations that eat fish on a regular basis have consistently been shown to healthier than those that don’t. We’ll discuss this further in the next article.

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  1. Jones PJH, Kubow S. Lipids, Sterols, and their Metabolites. In: Shils ME, et al., eds. Modern Nutrition in Health and Disease: Tenth Edition. Baltimore, MD; Philadelphia, PA: Lippincott Williams & Wilkins (2006) pp. 92-122.
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Join the conversation

  1. Sorry if you have answered this there are tons of questions. Would Ahiflower be absorbed similar to Flax and hemp?

  2. What about articles that claim omega 3 causes prostate cancer? It’s that just from fish/krill oil or any omega 3 source?

  3. Hi,

    I eat a lot of almonds and coconut, as in flaked (unsweetened) coconut, coconut milk and coconut oil. I also eat some flax seed and flax oil, pumpkin seeds, walnuts, filberts, pecans, chia seeds, brazil nuts, pecans and cashews. Could you please tell me how much omega 6 fatty acids are in these in say 100mg or at least let me know where I can obtain this information.

    • Kudos to you Chris Kresser. Your discussion of fatty acids (FA) is the best succinctly stated information. jeff’s preceding link here to a huge American Journal of Clinical Nutrition study, although not flawed for the study of plasma FA, stops short of quantifying tissue concentrations and stores of FA which is the more important study needed and suggested by that study. A deuterium marker study would serve to trace the bioavailability and pathways of FA, but how would fish be raised in heavy water in order to obtain marked oil? At at least one point in your article you use the nebulous term “DHA status”, I don’t have any comprehension of what that might mean. DHA craving tissues and processes suck-up plasma based DHA trying to maintain what has been found to be more or less a two month reserve. One can pulverize and ferment flax seeds to one’s heart’s content, but it will not even minimally increase tissue concentrations of DHA. True vegans and vegetarians have a great need to at least ingest essential algae, if for no other reason than to keep their brains turned on. Do not ingest krill oil, they are not plants and you will be responsible for millions of sins.

      • That marked DHA test shouldn’t be to hard to do. Algae can be fed with a marker and if you like, you can feed fish with those algae as well. But is the study you refer to seriously suggesting that there may not be a relationship between plasma and tissue DHA? I would have a bit more faith in the elegant (as in: not stupid) functioning of our bodies. But I can be wrong, have not read the study.

  4. I have been an ovolacto vegitarian for a long time then I decided to start eating fish. I think vegans and meat eaters depend to much nutrition supplements and not whole food. Well it is their problem. To say that grassfed beef or milk gives you adequate levels of dha,dpa and epa is stupid. I love dairy a lot but it does not give me enough animal omega 3. Conversion rates are hard to measure but they work to a certain level.

  5. I am concerned at your suggestion that flaxseed oil supplementation is of little value because of the limited bio-availability of it omega 3.
    This is in contradiction to recently published research results clearly indicating an over 60% lower relapse rate among patients suffering from Multiple Sclerosis who were taking flaxseed oil supplementation.

    International Journal of Neuroscience,
    Volume 123, Issue 11, 2013
    Association of fish consumption and omega 3 supplementation with quality of life, disability and disease activity in an international cohort of people with multiple sclerosis.

    Please be so kind as to comment.

  6. I’m missing some references here. For example for the claim of 5% conversion to EPA and 0.05% to DHA. According to research I’ve seen, it’s 8-20% to EPA and 0,5-9% to DHA. Women convert ALA to EPA at a 2,5 greater rate than men and vegetarians and vegans.

    I also read that when fish is not consumed, the body get’s better at the conversion to a point where vegans have an even higher DHA content in the blood than fish-eaters!
    Again, there’s no source for the claim that flax oil doesn’t raise DHA plasma levels.

    Would like to see them.


    • Jill,

      There could be some variables (like precursor levels) that make the conversion of ALA to EPA and DHA better. But let’s assume your best-case scenario of 20% and 9%. Given all the uncertainty in these numbers, wouldn’t you rather consume a source of oil that doesn’t require conversion at all? Wouldn’t you rather have 100%? Whatever the number, ALA is certaintly not going to give you that.

      • I am a vegetarian for about eight years now. I had a migraine a couple weeks ago. I changed my diet and I found myself feeling better. But my mind still felt cloudy. At the same time I was reading that fish oil could give you lucid dreams. So I ground up some flax seed to experiment. I had one tablespoon a few hours before bed. Yes I had those lucid dreams and kept waking up. The next morning my mind is so aware.

        I’ve kept taking more flaxseed -today I will have at least four tablespoons. I’m so aware. I can’t even describe how good it feels.

        So to say it does nothing, well I’m telling you that it definitely does.

        I think you all might be underestimating ALA as just some raw ingredient needed to create DHA, versus ALA being an important itself and all the substances it creates along the way also important. DHA clearly important too, but to say there is no conversion, I’m not so sure.

        I will get some algae sources of DHA. By the way the fish have so much DHA because they consume algae. So DHA is not an animal product. It’s all vegan. It just so happens that they accumulate it in their bodies. Horrible to think you’re killing all those fish and depleting the oceans just for some oil that you could get from algae.

    • Jill, I’ve got the article Chris referenced regarding the “5% conversion to EPA and 0.05% to DHA”. Just copying the article and it’s abstract below along with Lyle McDonald’s commentary on the piece.


      Extremely Limited Synthesis of Long Chain Polyunsaturates in Adults: Implications for their Dietary Essentiality and use as Supplements

      Plourde M, Cunnane SC. Extremely limited synthesis of long chain polyunsaturates in adults: implications for their dietary essentiality and use as supplements. Appl Physiol Nutr Metab. 2007 Aug;32(4):619-34.


      There is considerable interest in the potential impact of several polyunsaturated fatty acids (PUFAs) in mitigating the significant morbidity and mortality caused by degenerative diseases of the cardiovascular system and brain. Despite this interest, confusion surrounds the extent of conversion in humans of the parent PUFA, linoleic acid or alpha-linolenic acid (ALA), to their respective long-chain PUFA products. As a result, there is uncertainty about the potential benefits of ALA versus eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA).

      Some of the confusion arises because although mammals have the necessary enzymes to make the long-chain PUFA from the parent PUFA, in vivo studies in humans show that asymptotically equal to 5% of ALA is converted to EPA and <0.5% of ALA is converted to DHA.

      Because the capacity of this pathway is very low in healthy, nonvegetarian humans, even large amounts of dietary ALA have a negligible effect on plasma DHA, an effect paralleled in the omega6 PUFA by a negligible effect of dietary linoleic acid on plasma arachidonic acid. Despite this inefficient conversion, there are potential roles in human health for ALA and EPA that could be independent of their metabolism to DHA through the desaturation – chain elongation pathway."

    • Jill, the study that you point to is the same one jeff, above, points to. It is deficient in that it only studs plasma concentrations and not finding out if target tissues are assimilating their DHA. See my comment above.


  7. I ate fish steamed in water, together with rice and fruit, and my skin was clear and smooth, hair also had a nice shine and even with depression I had a lot of energy. I then proceeded with cooked vegan diet (low phytic acid, perfect ferritin and iron levels) but then I wanted to eat foods easy to digest and ended up with tree fruits and plants fruits, but I had enormous hunger no matter how much I ate. I craved fats, and probably needed more protein. However, my energy levels drastically plummeted, and skin broke out. I started supplementing zinc, selenium, vit. D, vit. B12, algae supplements. I got a little energy back, but skin did not improve at all. I ate lots of ALA (walnuts, lettuce, spinach) together with tree and plant fruits, but the nuts made my acne worse, and gave me pain all over the body. The cravings were so strong, that after buying kilogram of walnuts everyday, I would come out of the shop and sit, eating everything within ten minutes.
    I simply concluded that we do need fats, especially the omega 3 fats. ALA is very important but a stable source of preformed DHA/EPA from seafood, for proper DHA EPA was needed for me, and my acne still haven’t healed, but slowly they are improving and my energy levels are returning. I am very saddened as I loved the total vegan diet, and that I did not have to harm any animals by following it. I still study time to time if there really is no other option; but to all the people who want to get proper omega3 from plants: algae supplements, spinach, lettuce, berries, mangos together with nutrient dense foods, every single day; if you cannot get all your minerals from plant foods, try to supplement zinc aswell (too much zinc is bad, it gives me acne) and I forbid you avocados, nuts, chocolate, vegetable oils, seeds. Eat seeds high in ALA 2-3 hours before or after a meal because the phytic acid will disrupt vitamin B3, mineral absorption. Keep in mind you will get fat or mild protein deficiency by doing this, and this also shows why of most vegans I see their skin is so bad, and looks so barren. Look at the Japanese people who followed a traditional seafood-rich diet, their skin is very soft and hair is very strong, beautiful. Plus they are always 1st in technology; all thanks to DHA and EPA in my opinion.

    • Nice article. I have been a vegan for ages, and sometimes wonder what the fuss about omega 3 is, really. I admit, I eat almost nothing with omega 3 in it, usually. Why aren’t I keeling over? Esp. since I eat plenty of omega 6 in seeds and oils. Which supposedly would block some omega 3 uptake anyway.
      I must say, though, that when I was regularly eating a lot of flax oil for a time, my whole body and mind just felt like it was functioning smoother, somehow. Why I stopped taking it, kind of baffles me, looking back. I am about to jump back into grinding fresh flax and eating a lot again now! Hope my vegan body is utilizing it way more than the paltry 5% that study suggests. I trust a vegan body adjusts itself to do what it must.
      Still odd that I can go years with nary of drop of omega 3 in my diet and not see any real harm….

      • So are you saying that because you haven’t felt adverse effects yet, your diet isn’t doing you any harm? Or are you saying as long as you don’t feel the adverse effects yet, you simply don’t care?

        • I just wonder how, if it is really a vital food stuff, we survive at all without it. How does a body compensate for lack of it? Surely, down the ages, I would wager that barely anyone who isn’t eating raw fatty fish a lot got anywhere near their recommended RDA of omega 3.
          And what about the fact that cooking any source of omega 3 pretty much destroys it, from what I understand. Just leave a bottle of flax oil out of the fridge a day or two and it’s pretty much kaput.
          I do care, but I don’t want to be paranoid. I am quite healthy, despite eating next to no omega 3s.

          • The human body is very resilient if nothing else. It can survive under all kinds of conditions. But eventually it breaks down. It can take decades to develop cancers or CVD, after all the cumulative damage we’ve done catches up. And if you haven’t noticed, people are keeling over know from chronic diseases that didn’t use to kill us. Throughout the ages, people had varying diets and not all people’s diets had an abundance of fish, but that’s not the only source. Red meat, when grassfed, also contains high levels of omega-3s. And despite varying diets, it’s a certainly that no one had anywhere near the levels of omega 6 and 9s that many people do today, because real food simply doesn’t have the concentrated amounts like you see in modern oils/ processed foods.
            Regardless, because you feel “quite healthy” now doesn’t mean you are on a cellular level, and doesn’t mean you will feel that way tomorrow. Nothing we do guarantees health, but I feel I owe it to myself and my loved ones to make the best food decisions possible based on the information available. If more evidence indicates eating omega-3s from fish is going to preserve my health longer than not eating it, I owe it to myself to that. Ignorance is not bliss. Ignorance is pain. Our parents had an excuse for ignorance – we do not.

            • I just whipped up some chia in a blender with water this morning, and was surprised how delicious it is! I’ll try flax next, but the chia was easier to get in my village.
              I’m hooked. Gonna gobble it daily and get back that smoothness I remember from flax oil days. I agree with you about slow decay, etc. Not arguing that. Just wonder literally how are body fills those cellular keyholes when the key is completely absent. 😉

      • j,

        It is great that you can go for so long with nary a drop of DHA, however I would more likely believe that you are not a true vegan. A proper dose of animal sourced DHA could last maybe two months or so. Human ejaculate contains DHA, ALA and saturated palmitic acid, as well. Are you sure that your a true vegan?

    • Dia, your down-to-earth, personal food/nutrient experience is the best comment that I’ve read so far. ?☺

    • Do you think eating 1kg of Walnuts is healthy? I would stop eating so many nuts. That’s probably toxic to your health.

      My gf is vegetarian and eats too many sunflower seeds. She thinks there is something in them she needs.

      Try eating ground flax – four tablespoons a day. See if the desire to eat so many nuts goes down.

  8. Wow! what a wonderful and detailed discussion giving both sides views. Thanks for allowing that. I wanted to mention that there was a German doctor named Buddig who found that flax oil and cottage cheese mixed together (and I mean the old fashioned home made cottage cheese) actually cured cancer. They could tell the difference in the blood work soon after this was taken.
    I do wonder about the paleolithic diet containing much fish, if humanoids were still using stone axes, I doubt they were doing much deep sea fishing. Only woven nets in lakes and seashores. And lake fish I have heard don’t have all the essential fish oils like the sardine and shad type fish of the sea. I think nuts and seeds would have contributed quite a bit of oils and early man being an omnivore would eat whatever he could get so all those tribes living inland did fine without much fish.
    I do wonder tho on a different topic how they avoided goiter from lack of iodine which all the central us lacks in its soils and up until 50 years ago a great many people of central Europe and the US had a big problem with until they added iodine to our salt…!! lack of it can cause retardation!
    Might relate to autism since so many diets now are low salt .

    • I agree – there are a lot of good algae oil supplements out there that are high in DHA and EPA and don’t require EFA conversion. The one I use is an microalgae oil spray from Ora Organic. It’s great for taking at work (I’m a web designer) and I don’t smell like fish afterwards lol. Would recommend if anyone’s vegan and doesn’t want to eat fish.