What it means
The omega-3 index is the percentage of total fatty acids in red-blood-cell membranes that are EPA and DHA, the two long-chain omega-3 fats from fish. Because red cells live about 120 days, the index reflects three to four months of intake rather than last night's dinner. It was proposed by Harris and von Schacky in 2004 as a cardiovascular risk marker, with less than 4% defined as high risk and more than 8% as low risk.
Typical values are 4–5% in North America and Northern Europe, 8–10% in Japan.
Why it matters for longevity
EPA and DHA are built into cell membranes, where they affect inflammation signalling, heart rhythm and triglyceride handling. In the Framingham Offspring cohort, an index above 6.8% was associated with about 34% lower all-cause mortality than below 4.2% — a difference the authors compared to the effect of smoking. It is also one of the cheapest markers to correct.
What the evidence says
Observational data are consistent: higher index, lower cardiovascular and all-cause mortality (Harris 2021 pooled analysis of 17 cohorts, 42,000 people). The randomised trials are mixed because they tested doses, not index targets: REDUCE-IT (4 g/day purified EPA) cut cardiovascular events by 25% in high-risk patients; STRENGTH (4 g/day EPA + DHA) found nothing; VITAL (1 g/day) found no benefit overall.
The difference between trials is partly explained by baseline index and achieved index — participants starting low and reaching 8% benefited; those already adequate did not. Fish-oil doses needed to go from 4% to 8% are around 1–2 g of EPA + DHA per day for three to four months.
How to measure or use it
- A finger-prick dried-blood-spot test or a venous sample, reported as a percentage; retest after four months.
- Two to three servings of oily fish a week moves most people to 6–7%; reaching 8% usually needs a supplement of 1–2 g EPA + DHA daily.
- Read it alongside triglycerides and hs-CRP; omega-3s lower both at the doses that raise the index.
Related
- Biomarkers of ageing — the panel it belongs to
- Supplements that have evidence — fish oil, graded
- Inflammation and immunity — the inflammation link
Frequently asked questions
What is a good omega-3 index?
8% or above is the low-risk range; below 4% is high risk. Most Western adults are 4–5%.
How much fish oil to reach 8%?
Typically 1–2 g of combined EPA + DHA per day for three to four months, depending on starting point and body size; retest to confirm.
Does fish oil raise atrial fibrillation risk?
High-dose trials (4 g/day) showed a small increase in atrial fibrillation. At 1–2 g/day the signal is minimal; discuss it with your physician if you have a rhythm history.
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