The Omega-3 Index: what the fish oil evidence actually says.

The Omega-3 Index is the combined amount of EPA and DHA sitting in your red blood cell membranes, reported as a percentage of total fatty acids. Red cells survive about 120 days, so the number reflects two to three months of intake rather than last night's salmon. Think of it as HbA1c for fats. Most Australians who have never deliberately eaten oily fish land somewhere between 4 and 6%. The frequently quoted target is 8%.

Whether hitting that target does anything for you is a far more interesting question than the supplement aisle suggests.

Four big trials, and they do not agree

Fish oil has the strange distinction of being one of the most studied supplements in cardiology and one of the least settled. Four large randomised trials define the field, and reading them in order is the fastest way to understand why cardiologists remain split.

• ASCEND (NEJM 2018): 15,480 adults with diabetes and no known cardiovascular disease, randomised to 1 g/day of omega-3 or olive oil placebo. Serious vascular events occurred in 8.9% of the omega-3 group and 9.2% of placebo. No benefit.
• VITAL (Manson et al. NEJM 2019): 25,871 US adults over a median 5.3 years on 1 g/day. The primary composite of major cardiovascular events was not reduced. Total myocardial infarction was, at a hazard ratio of 0.72 (95% CI 0.59–0.90), which is a secondary endpoint and should be read as hypothesis-generating rather than proof.
• REDUCE-IT (Bhatt et al. NEJM 2019): 8,179 statin-treated patients with elevated triglycerides given 4 g/day of icosapent ethyl, a purified prescription EPA. The primary endpoint fell 25% (HR 0.75, 95% CI 0.68–0.83) over a median 4.9 years. That is a large effect by any standard.
• STRENGTH (Nicholls et al. JAMA 2020): more than 13,000 similar patients given 4 g/day of a high-dose EPA plus DHA carboxylic acid. Hazard ratio 0.99. Stopped early for futility.

Two high-dose trials in comparable populations, one spectacularly positive and one flat. The field has been arguing about why ever since. The leading explanation is uncomfortable for REDUCE-IT: its placebo was mineral oil, and the mineral oil group showed a 32% rise in hs-CRP, a 10.9% rise in oxidised LDL and a 16.2% rise in interleukin-6 over 12 months. STRENGTH used corn oil and saw no such drift. If the placebo arm got worse rather than the treatment arm getting better, part of that 25% is an artefact. Regulators put the attributable portion at around 3%, which does not erase the result but does not fully dispel the doubt either.

A secondary analysis of STRENGTH published in JAMA in 2021 added a further problem for the whole premise. Among people who achieved the highest blood EPA and DHA levels on treatment, outcomes were no better than in those with the lowest. If raising the number were the mechanism, that analysis should have shown a gradient. It did not.

What the observational data show, and why that is not the same thing

The case for the 8% target rests almost entirely on cohort studies, and the strongest of them is genuinely impressive. Harris and colleagues measured the Omega-3 Index in 2,500 participants of the Framingham Offspring cohort, mean age 66, and followed them a median 7.3 years. People in the highest quintile had a 34% lower risk of death from any cause and a 39% lower risk of incident cardiovascular disease than those in the lowest. In the same statistical models, total cholesterol predicted nothing while the Omega-3 Index did.

That result gets quoted constantly, usually without the caveat that follows it. A cohort study tells you that people with more EPA and DHA in their cell membranes live longer. It cannot tell you that giving EPA and DHA to someone with a low index will extend their life, because the people eating three serves of fish a week differ from everyone else in a hundred ways that no statistical adjustment fully captures. The randomised trials were built to answer exactly that question, and in general prevention they came back null.

So the index is a good marker and an unproven target. Both things are true, and the supplement industry has spent 20 years pretending only the first one is.

Reading your result

Every biomarker in The HodieLabs Evidence-Based Clinical Library is graded against published authority standards and tagged with the strength of the evidence behind it, which matters here more than usual. The Omega-3 Index is classified as emerging and adjunctive, not mainstream. The bands come from the Harris and von Schacky methodology:

• 8–12%, desirable. The range associated with the lowest cardiovascular risk in observational cohorts.
• 6–7.9%, borderline. Typical of someone eating fish occasionally.
• 4–5.9%, low. Where a large share of Australian adults sit.
• Below 4%, very low. Consistent with essentially no dietary EPA or DHA.

A low index is a reliable statement about your diet. It is a much weaker statement about your prognosis than the marketing implies. If your ApoB is clean and your index is 5%, the index is not the thing to worry about.

The atrial fibrillation problem nobody mentions on the label

This is the finding that should change how fish oil gets recommended. Gencer and colleagues pooled seven cardiovascular outcome trials in Circulation in 2021 and found that marine omega-3 supplementation raised the risk of atrial fibrillation by 25%, with the signal concentrated at doses above 1 g per day. STRENGTH itself recorded new-onset atrial fibrillation in 2.2% of the omega-3 arm against 1.3% on corn oil. REDUCE-IT showed a similar direction.

Atrial fibrillation is not a trivial finding. It carries stroke risk and often needs lifelong anticoagulation. A 25% relative increase on a low base is a modest absolute risk, but it is a real one, and it arrives with a supplement most people buy off a shelf believing it is uniformly good for the heart. Dietary fish does not carry the same signal, which is one of the sharper arguments for food over capsules.

Multi-gram dosing without a clinical reason is difficult to justify on the current evidence. If someone has established cardiovascular disease and triglycerides that stay high on a statin, prescription icosapent ethyl is a legitimate conversation with a cardiologist. Four grams a day of supermarket fish oil taken because a podcast recommended it is a different proposition entirely.

What actually moves the number

Oily fish. Salmon, sardines, mackerel, herring, trout. The Heart Foundation recommends two to three serves a week and 250 to 500 mg of combined EPA and DHA daily, which is achievable through food for most people who like fish and awkward for those who do not.

Plant sources are weaker than their reputation. Flaxseed, chia and walnuts supply alpha-linolenic acid, which the body converts to EPA and then DHA at rates usually below 10% and often below 5%, with conversion to DHA lower still. Someone eating chia daily and no seafood will generally still show an index in the low range. Algal oil supplements are the exception and deliver DHA directly, which makes them the practical option for vegetarians.

Because the index tracks red cell turnover, changes take about three months to register. Testing four weeks after starting a supplement will underestimate the eventual effect. Annual measurement is enough for most people, with a repeat at three months if an intervention is being trialled.

The Australian context

Australians live beside some of the best seafood in the world and mostly eat prawns, barramundi and flake, none of which are high in EPA and DHA. Fish oil remains one of the highest-selling complementary medicines in the country. So a fair number of people are supplementing at doses with a known arrhythmia signal and no demonstrated outcome benefit in primary prevention, while their actual dietary intake stays low.

The Omega-3 Index is not on a standard GP request form here and attracts no Medicare rebate. That is defensible given the evidence tier. It also means that anyone taking fish oil daily has no idea whether it is doing anything to their blood at all, which is a strange place for a marker that costs very little to measure.

How we use it at HodieLabs

We measure the Omega-3 Index in the Health Blueprint at our Melbourne Preventative Health & Longevity Clinic, and we read it as a nutritional marker rather than a risk score. It carries most weight when it sits low alongside a raised hs-CRP or elevated triglycerides, because that combination points at a dietary pattern worth changing regardless of what the omega-3 trials showed. It carries least weight in isolation.

Doctors here are also explicit about the evidence tier when they discuss it, which is the part that tends to go missing elsewhere. A marker labelled emerging should be presented as emerging.

If you take fish oil every morning and have never measured your index, you are running an experiment with no readout. Measure it once. If it comes back at 9%, the capsules are redundant. If it comes back at 4.5% after two years of daily dosing, something about your supplement or your absorption is not working, and that is worth knowing before you spend another decade on it.

References

1. Bhatt DL, et al. Cardiovascular risk reduction with icosapent ethyl for hypertriglyceridemia (REDUCE-IT). N Engl J Med. 2019;380:11–22.
2. Nicholls SJ, et al. Effect of high-dose omega-3 fatty acids vs corn oil on major adverse cardiovascular events in patients at high cardiovascular risk: the STRENGTH randomized clinical trial. JAMA. 2020;324(22):2268–2280.
3. Manson JE, et al. Marine n−3 fatty acids and prevention of cardiovascular disease and cancer (VITAL). N Engl J Med. 2019;380:23–32.
4. ASCEND Study Collaborative Group. Effects of n−3 fatty acid supplements in diabetes mellitus. N Engl J Med. 2018;379:1540–1550.
5. Harris WS, Tintle NL, Etherton MR, Vasan RS. Erythrocyte long-chain omega-3 fatty acid levels are inversely associated with mortality and with incident cardiovascular disease: the Framingham Heart Study. J Clin Lipidol. 2018;12(3):718–727.
6. Gencer B, et al. Effect of long-term marine omega-3 fatty acids supplementation on the risk of atrial fibrillation in randomized controlled trials of cardiovascular outcomes: a systematic review and meta-analysis. Circulation. 2021;144:1981–1990.
7. Nissen SE, et al. Association between achieved omega-3 fatty acid levels and major adverse cardiovascular outcomes in patients with high cardiovascular risk: a secondary analysis of the STRENGTH trial. JAMA Cardiol. 2021;6(8):910–917.
8. National Heart Foundation of Australia. Omega-3, omega-6 and heart health. Accessed 2026.

This article is general information, not medical advice. Speak with your GP about what testing and care is appropriate for you.