Omega-3 index

What it measures

The omega-3 index expresses EPA plus DHA as a percentage of total fatty acids in red blood cell membranes. Because membranes turn over slowly, the result reflects intake over months, which is what makes it more useful than a plasma measurement that responds to a single meal.

It is one of the few nutrition markers that answers a question people otherwise guess at: whether the fish oil is doing anything measurable.

The target range and where it comes from

Harris and von Schacky proposed a desirable index above 8 percent and an undesirable index below 4 percent, based on the relationship between red cell EPA and DHA content and cardiovascular death across populations. Typical American values sit near 4 to 5 percent, while Japanese populations commonly exceed 8.

Those cutoffs are derived from observational relationships, not from a randomized trial that assigned people to an index. That distinction matters, because supplement trials targeting doses rather than blood levels have produced mixed cardiovascular results.

How to use it

It is most useful as a before and after test. Measure, change intake, measure again in three to four months. Absorption varies enough between people that dose alone is a poor predictor of blood level.

It is not a test to repeat frequently, and it does not need to be perfect. Moving from 4 percent to the middle of the range is where any benefit plausibly sits.

References

1. Harris WS, von Schacky C. The Omega-3 Index: a new risk factor for death from coronary heart disease? Prev Med. 2004.
2. Harris WS, et al. Blood omega-3 fatty acids and death from COVID-19: a pilot study. Prostaglandins Leukot Essent Fatty Acids. 2021.