Omega-3: what dose of EPA and DHA, for what effect
In brief
Marine omega-3 has a demonstrated effect, and it is more precise than what is usually said. The largest assessment available, 86 randomised trials and nearly 163,000 participants, establishes with a high level of certainty that it lowers triglycerides by around 15 %, in proportion to the dose. That is where it is most useful, and where quantity genuinely counts. On cardiovascular events, by contrast, the benefit observed is slight and does not grow with the dose: worth knowing, so you can choose a product according to your goal rather than a general promise.
Omega-3 is among the best-studied nutrients on the market, with tens of thousands of participants across the trials. That abundance is an opportunity: it makes it possible to say precisely what it does, at what dose, and for whom, instead of settling for a vague promise.
That is the purpose of this article: to identify where its effect is most solid, where it is less so, and what that changes when choosing a product. It is for information and does not replace your doctor’s assessment.
EPA, DHA, ALA: three molecules, two origins
Marine omega-3
EPA and DHA are described as long-chain and come from oily fish and algae. They are the ones most studied and the ones found in fish oil capsules. DHA is particularly present in the retina and the brain.
Plant omega-3
Alpha-linolenic acid, or ALA, comes from plants: walnuts, flaxseed, chia, rapeseed. The body converts it into EPA and then DHA, but at a low rate. That is why ALA and marine omega-3 are not interchangeable, and why they were assessed separately.
What 86 randomised trials show
The scale of the synthesis
The Cochrane review on the subject brought together 86 randomised trials and 162,796 participants, with durations from 12 to 88 months and doses from 0.5 g to more than 5 g per day, nineteen trials having used at least 3 g daily[1]. By its authors’ own account, it is the most complete assessment carried out to date of the effects of omega-3 on cardiovascular health.
The main results
Increasing marine omega-3 had little or no effect on all-cause mortality, nor on cardiovascular events, nor on stroke, nor on arrhythmias[1]. For overall mortality and cardiovascular events, the certainty of that conclusion is described as high, which is rare and means new trials are unlikely to overturn the result.
What moves slightly
There is a signal on the coronary side: marine omega-3 might slightly reduce coronary mortality and coronary events, with a low level of certainty[1]. That is not nothing, but the size of this benefit deserves to be put into plain language, which the next section does.
The figure that puts it all in scale
How many people for one benefit
The authors supply a figure marketing never quotes: the number of people who would need to be supplemented to avoid one event. It is 167 people to avoid one coronary event, and 334 people to avoid one coronary death[1].
How to read it
It means that if 167 people take omega-3 for years, one alone will avoid a coronary event they would otherwise have had; the other 166 will not gain that benefit. This is not an argument for throwing away your capsules, it is a matter of scale. Public health decisions are made with figures like these, and individual decisions would gain from drawing on them.
The most counter-intuitive result
One last finding is worth knowing: in the pre-planned analyses, the effects differed neither by trial duration nor by dose of marine omega-3[1]. In other words, the widespread idea that “you just have to take the right dose” is not supported by this analysis for cardiovascular events. Increasing the dose does not turn a neutral result into a positive one.
| Outcome | Effect | Certainty |
|---|---|---|
| All-cause mortality | Little or no effect | High |
| Cardiovascular events | Little or no effect | High |
| Coronary events | Slight reduction, 167 people for one event avoided | Low |
| Stroke | No effect | Moderate |
| Triglycerides | Fall of around 15 %, proportional to the dose | High |
The only solid, dose-dependent effect
Triglycerides
This is the clearest result in the whole review, and it carries high certainty: increasing marine omega-3 lowers triglycerides by around 15 %, in a dose-dependent way[1]. Here the dose genuinely counts, unlike for clinical events.
Why that matters
It maps out a reasoned use: marine omega-3 has a measurable, reproducible biological effect on one specific blood measure. Lowering a number is not, however, the same as avoiding a cardiac event, and that is exactly what the contrast between these two results within the same analysis shows.
And plant omega-3
For ALA, the review concludes there is little or no difference in overall and coronary mortality. Two slight signals appear: a possible small reduction in cardiovascular events and a probable reduction in the risk of arrhythmia[1]. The numbers involved are far smaller, which calls for caution.
Fish, capsules or algae
Eating fish
The review notes that there is little data on the effects of eating fish itself, since most trials tested capsules[1]. That does not mean fish is of no interest: it means it has been little studied in this trial format, and that it also provides protein, iodine and selenium that a capsule does not.
The Swiss question
Switzerland is a landlocked country and fish consumption here remains modest. That is often put forward as a reason to supplement. It is debatable: given the results above, the question is less about making up a presumed shortfall than about knowing precisely what benefit you expect.
Krill, esters, triglycerides: does the form change anything?
This is the most common selling point on the shelf, and a randomised double-blind trial tested it directly. Sixty-six adults received the same dose of EPA and DHA for four weeks, around 1.3 g per day, in three different forms: fish oil as ethyl esters, fish oil as triglycerides, or krill oil. The blood levels reached at four weeks did not differ significantly between the three, and the omega-3 index measured in red blood cells was comparable[2].
The practical conclusion is simple and saves money: at an equal dose of EPA and DHA, the form matters little. The relevant purchase criterion is therefore the actual amount of EPA and DHA per capsule, shown on the back of the box, and not the promise of a superior form. One point for context: this trial was conducted by authors affiliated with an ingredient manufacturer, which does not invalidate it but is worth knowing.
Algal oil
It supplies EPA and DHA directly without going through fish, which makes it the relevant source for vegetarians and vegans. In terms of effects, it provides the same molecules and is therefore read through the same lens.
What to do, in practice
Distinguish three situations
If you are in good health, the order of priority remains your diet as a whole, without expecting a major protective effect from a capsule. If you have raised triglycerides, that is the one area where the effect is clear and dose-dependent, and it is discussed with a doctor. If you have known cardiovascular disease or a treatment under way, the decision is theirs too.
This file is above all an invitation to choose with full knowledge. Marine omega-3 does something precise and well established: it lowers triglycerides, with an effect proportional to the dose and a high level of certainty. That is a legitimate goal, and the one worth investing in. On overall cardiovascular prevention, by contrast, expectations gain from being adjusted, which spares disappointment and misdirected purchases.
To carry on with how to read the evidence for a nutrient, two articles complete this one: our file on coenzyme Q10 and the one devoted to vitamin E.
Frequently asked questions
Does omega-3 protect the heart?
Its best-established effect concerns triglycerides, with a fall of around 15 % in proportion to the dose and a high level of certainty. On cardiovascular events themselves, the Cochrane review, which brings together 86 randomised trials and nearly 163,000 participants, sees a slight benefit for coronary heart disease and no clear effect on overall mortality. The value is therefore real but targeted: a precise tool for triglycerides, not cardiovascular insurance.
What dose of EPA and DHA should you take?
The answer depends on the goal, and one finding is surprising: in the Cochrane review’s pre-planned analyses, the effects on cardiovascular events differed neither by dose nor by duration. Increasing the dose therefore does not turn a neutral result into a positive one. For triglycerides, by contrast, the effect really is proportional to the dose. That is the only area where the question of quantity genuinely arises, and it is discussed with a doctor.
Does omega-3 lower triglycerides?
Yes, it is the most solid result in the entire review: a fall of around 15 %, proportional to the dose, with a high level of certainty. It is a real and reproducible biological effect. One shortcut must be avoided though: lowering a blood measure is not the same as avoiding a cardiac event, as the contrast with the other results of the same analysis shows.
Is it better to eat fish or take capsules?
The review notes that there is little data on the effects of eating fish itself, since most trials tested capsules. That does not disqualify fish: it also provides protein, iodine and selenium that a capsule does not. In practice, including oily fish in your diet remains a reasonable approach, without expecting a spectacular protective effect from it.
Do flaxseeds replace fish oil?
Not exactly. Plants provide alpha-linolenic acid, which the body converts into EPA and then DHA at a low rate. The review does in fact analyse the two separately: for ALA, it concludes there is little or no difference in mortality, with two slight signals, a possible small reduction in cardiovascular events and a probable reduction in the risk of arrhythmia, in far smaller numbers.
How good are algal oils for vegetarians?
They supply EPA and DHA directly without going through fish, which makes them the relevant source for vegetarians and vegans, in whom the conversion of ALA remains limited. In terms of expected effects, they provide the same molecules as fish oil and are therefore read through the same lens: a clear effect on triglycerides, a limited effect on cardiovascular events.
Are there any precautions to take?
Yes. At high doses, omega-3 can affect blood clotting: mention it if you take an anticoagulant or an antiplatelet medicine, and before surgery. As with any supplement, mention it in consultation just as you would a medicine. And to be clear: a food supplement does not treat, prevent or cure cardiovascular disease.
Why do we hear so much good about omega-3?
Because observational studies, which compare populations, show favourable associations, and because the biological mechanism is plausible. But once you move to the randomised trial, where who receives what is drawn by lot, the expected benefit shrinks sharply. This is a classic gap in nutrition, and it is precisely why systematic reviews of randomised trials are authoritative on this kind of question.
Sources and references (verified on PubMed)
2 sources- Abdelhamid A.S., Brown T.J., Brainard J.S. et al. (2020). Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease.
- Yurko-Mauro K., Kralovec J., Bailey-Hall E., Smeberg V., Stark J.G., Salem N. (2015). Similar eicosapentaenoic acid and docosahexaenoic acid plasma levels achieved with fish oil or krill oil in a randomized double-blind four-week bioavailability study.