Omega-3 index
The omega-3 index is a blood test that measures how much of the fat in your red-blood-cell membranes is EPA and DHA, the omega-3 fats from oily fish. It is the most useful single test of omega-3 status, and people with higher readings live longer in observational studies. Its popular 8% target comes from heart research, though, not from proof that reaching it extends life.
If you don't eat oily fish, one test can show whether you are low (under 4%) and whether a supplement has lifted you towards 8%. Treat 8% as a heart-health goal, and reconsider your dose above 12%, especially before surgery or if you take blood thinners.
What it measures
EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are the two long-chain omega-3 fats in oily fish. The omega-3 index is their combined share of the fatty acids in red-blood-cell membranes, expressed as a percentage and measured from one blood sample. For omega-3 itself, including doses and products, see Omega-3.
| Omega-3 index | Status |
|---|---|
| Under 4% | Low; associated with higher heart risk and higher death rates |
| 4–8% | Intermediate |
| 8–12% | The usual target range |
| Over 12% | High; reconsider the dose, especially before surgery or with blood thinners |
Higher readings go with longer lives, in observational studies
In a long-running US heart study that measured the index, people in the top fifth (above 6.8%) had about a third lower death rates than those in the bottom fifth (below 4.2%).[1] Across 17 long-running cohorts of about 42,000 adults, those in the top fifth for blood omega-3 had about 15–18% lower death rates than those in the bottom fifth.[2]
Three things limit what this means:
- These are observational links. People who eat fish tend to be healthier in many other ways, and randomized trials of omega-3 supplements show essentially no effect on deaths.
- The 8% target is a heart-health goal, not a longevity threshold. It comes mainly from cardiovascular research; the death-rate studies compare top and bottom fifths rather than testing cut-offs.
- The test has a commercial backer. The omega-3 index is promoted heavily by a testing company whose founder co-authors much of this research.
How much omega-3 it takes to move it
From a five-month dosing trial in young adults who ate little fish, the researchers estimated that about 1.2 g a day of EPA+DHA would lift a 75 kg adult from a typical starting index of about 4.3% to 8%, with heavier people needing more; 1.8 g a day gave an average of about 9.5%.[3] For athletes, a sports-nutrition position stand estimates that about 1.4 g a day of extra EPA+DHA lifts a typical index to 8%.[4]
People who eat oily fish two to three times a week often reach the target without a supplement. Farmed salmon alone may not be enough: fillets bought in US shops in 2023 held less than half the EPA+DHA of older food tables, so two 150 g portions a week supply only about 0.4 g a day.[5]
Who tends to test low
Athletes and people who skip fish often test at 4–5%, at the bottom of the intermediate band, even when they think their diet is healthy. In 30 collegiate athletes the average index was 4.6%.[6] In 404 college football players it was 4.4%, and not one reached 8%.[7]
What it doesn't tell you
A high index is not a reason to push the dose. In heart trials, atrial fibrillation, an irregular heart rhythm, became more common above about 1.5 g a day of EPA+DHA in people already at high heart risk, and one pooling of heart trials saw a slight rise even at 1 g a day or less. The advice to reconsider the dose above 12%, especially before surgery or with blood thinners, is a precaution rather than a tested rule. People with higher blood omega-3 levels had less atrial fibrillation in a large UK cohort, but that is observational and does not show that supplementing protects you.[8]
The best-known omega-3 aging-clock trial, in adults over 70 taking 1 g a day, did not report an omega-3 index at all.
What to do
- If you eat oily fish twice a week or more, you probably don't need the test, unless your only oily fish is farmed salmon.
- If you don't, test once, start about 1–1.5 g a day of EPA+DHA if you are low, and retest after about five months, roughly the time red-cell membranes take to settle at a new level. If you have atrial fibrillation or are at high risk of it, agree the dose with your doctor first; even this band is not completely free of rhythm risk.
- Aim for the 8–12% range, and don't chase higher. Above 12%, reconsider the dose, especially before planned surgery or if you take blood thinners.
Further reading
- Harris WS et al. 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.[9]
- Harris WS et al. Blood n-3 fatty acid levels and total and cause-specific mortality from 17 prospective studies (FORCE consortium). Nature Communications 2021.[10]
- Jäger R et al. International Society of Sports Nutrition Position Stand: Long-Chain Omega-3 Polyunsaturated Fatty Acids. J Int Soc Sports Nutr 2025.[11]
- Heileson JL et al. A Cross-Sectional Analysis of Whole Blood Long-Chain ω-3 Polyunsaturated Fatty Acids and Its Relationship with Dietary Intake, Body Composition, and Measures of Strength and Power in Collegiate Athletes. J Am Nutr Assoc 2023.[12]