What Is the Omega-3 Index?

The omega-3 index is the share of the fatty acids in your red blood cell membranes that are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), the long-chain omega-3s found mainly in oily fish. Harris and von Schacky proposed it in 2004 as a risk factor for death from coronary heart disease: an index of 8% or more was associated with the most protection, and 4% or less with the least. It’s a good measure of how much EPA and DHA you’ve been eating over the past few months, but whether raising it lowers your risk is less clear than those numbers suggest.

This article is general information, not medical advice.

What It Measures

The fatty acids in red blood cells reflect your intake over roughly the previous 120 days, as the US Office of Dietary Supplements (ODS) notes, while levels in plasma or serum move with your recent meals. Some labs report EPA and DHA in plasma or serum phospholipids instead, a different measure with different numbers: the ODS gives about 3% to 4% as the average for US adults not taking supplements. Check that a result is the red blood cell index before comparing it with the 4% and 8% marks.

Where 4% and 8% Come From

The thresholds come from studies of people’s EPA and DHA levels and their later risk, not from trials of raising them. In a 2021 pooled analysis of 17 prospective studies, Harris and colleagues found that people in the highest fifth of circulating long-chain omega-3s had a 15% to 18% lower risk of death from any cause than those in the lowest fifth, after adjusting for other risk factors, with similar links for cardiovascular disease, cancer, and other causes. Alpha-linolenic acid (ALA), the plant omega-3, showed no such link.

Observational studies like these can’t rule out that people with high levels differ in other ways, such as eating more fish and less processed food. Some of the authors also have commercial interests in fatty acid testing, which the paper declares.

What Trials Found

Randomized trials of supplements have been more mixed:

• VITAL (Manson and colleagues, 2019) gave about 26,000 US adults 1 g a day of marine omega-3s or placebo for a median of 5.3 years. It found no significant reduction in major cardiovascular events or cancer overall, though total heart attacks were lower, a secondary finding.
• A 2021 meta-analysis of 7 trials (Gencer and colleagues) found that omega-3 supplements were linked to a 25% higher risk of atrial fibrillation, more so at doses over 1 g a day.

So a low index is a reasonable prompt to eat more oily fish, but a supplement isn’t a sure way to lower risk, and higher doses aren’t free of it. Talk to your doctor before taking high-dose supplements, especially with a heart rhythm condition.

How Much EPA and DHA to Eat

There’s no Recommended Dietary Allowance for EPA or DHA. The National Academies set an Adequate Intake only for ALA, 1.6 g a day for men and 1.1 g for women, and, as the ODS notes, established no specific intake recommendations for EPA, DHA, or the other long-chain omega-3s. Cold-water fatty fish, such as salmon, mackerel, herring, and sardines, are rich in them, while leaner fish and shellfish have less.

Following It in Kaizora

Omega-3 Index is a lab result on Labs, logged with its time, laboratory, and printed range. Dietary Eicosapentaenoic Acid and Dietary Docosahexaenoic Acid come from the foods you log from the food database, or from Cronometer’s import; Apple Health’s export doesn’t record them. Since the index reflects months of eating, compare it with your EPA and DHA intake by month on Correlations, and retest after a few months of a change rather than weeks.

The Short Version

• The omega-3 index is EPA plus DHA as a percentage of red blood cell fatty acids, reflecting about four months of intake.
• In observational studies, 8% or more was linked with the lowest heart risk and 4% or less with the highest.
• Trials of fish oil supplements haven’t reliably cut cardiovascular events, and higher doses are linked to more atrial fibrillation.
• There’s no RDA for EPA or DHA; cold-water fatty fish are rich in them.

Sources

• Harris WS, von Schacky C, “The Omega-3 Index: a new risk factor for death from coronary heart disease?” Preventive Medicine 39, no. 1 (2004): 212–220. doi:10.1016/j.ypmed.2004.02.030
• Harris WS et al., “Blood n-3 fatty acid levels and total and cause-specific mortality from 17 prospective studies,” Nature Communications 12 (2021): 2329. doi:10.1038/s41467-021-22370-2
• Manson JE et al., “Marine n−3 fatty acids and prevention of cardiovascular disease and cancer,” New England Journal of Medicine 380, no. 1 (2019): 23–32. doi:10.1056/NEJMoa1811403
• Gencer B et al., “Effect of long-term marine ω-3 fatty acids supplementation on the risk of atrial fibrillation in randomized controlled trials of cardiovascular outcomes: a systematic review and meta-analysis,” Circulation 144, no. 25 (2021): 1981–1990. doi:10.1161/CIRCULATIONAHA.121.055654
• Office of Dietary Supplements, National Institutes of Health, Omega-3 fatty acids: fact sheet for health professionals.