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Omega-3 While Fasting: Does It Affect Your Index?

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At a glance

  • What the test measures / EPA plus DHA in red blood cells, expressed as a percentage of red-cell fatty acids [1]
  • Fasting before the draw / Red-cell EPA reflects intake over the past month or two, not a single morning [2]
  • Red cell EPA half-life / 28 days, steady state at 180 days [2]
  • Cholesteryl ester EPA half-life / 4.8 days [2]
  • Cut-offs from the index paper / In observational data, 8% or above was associated with the greatest protection against coronary heart disease death and 4% or below with the least [1]

Does fasting change the Omega-3 Index?

A brief fast is unlikely to meaningfully change the result. The index is built from red-cell EPA plus DHA [1], and red-cell EPA reflected intake over the past month or two in a feeding study [2].

In an 18-month controlled study, 58 men took 0, 3, 6 or 9 g of fish oil a day for 12 months. EPA in erythrocyte membranes had an incorporation half-life of 28 days and reached steady state after 180 days [2].

How fast different blood fractions move

In the same study, EPA in serum cholesteryl esters plateaued after 4 to 8 weeks with an incorporation half-life of 4.8 days. The authors describe cholesteryl ester EPA as reflecting intake over the past week or two, while erythrocyte EPA reflects the past month or two [2].

In a 12-month double-blind study of 204 adults given EPA and DHA capsules to mimic 1, 2 or 4 weekly servings of oily fish, time to maximal incorporation ranged from days in plasma phosphatidylcholine to months in mononuclear cells to more than 12 months in adipose tissue [3].

What actually moves your number

Sustained intake over weeks to months is what the kinetic data describe. In the 204-person capsule trial, a dose response was seen for EPA and DHA in all pools except red-cell EPA, which did not reach statistical significance (P = 0.057) [3]. In the feeding study, each extra gram per day of fish oil raised erythrocyte EPA by 2.1 mass % [2].

The prescription omega-3-acid ethyl esters (Lovaza-type) label is for lowering triglycerides in adults with severe hypertriglyceridemia [4]. Over-the-counter omega-3 fish oil softgels are dietary supplements [5], so read the EPA and DHA amounts on the label rather than the total fish oil figure.

Frequently asked questions

Do I need to fast before an Omega-3 Index blood test?
A brief fast is unlikely to change the result. In an 18-month controlled study, red-cell EPA reflected intake over the past month or two, with an incorporation half-life of 28 days and steady state at 180 days [2].
I ate salmon the night before my blood draw. Is my result falsely high?
In the feeding study, red-cell EPA reflected intake over the past month or two, while cholesteryl ester EPA reflected intake over the past week or two [2].
Should I take my fish oil the morning of the test?
Red-cell EPA has an incorporation half-life of 28 days [2], and the index is built from red-cell EPA plus DHA [1]. The studies cited here did not test a morning dose.
How long after changing my dose should I retest?
Erythrocyte EPA reached steady state only after 180 days in the 18-month controlled study, so a retest a few weeks after a dose change will not show the full effect [2].

References

  1. Harris WS, Von Schacky C. The Omega-3 Index: a new risk factor for death from coronary heart disease?. Prev Med. 2004;39(1):212-20. The Omega-3 Index: a new risk factor for death from coronary heart disease?
  2. Katan MB, Deslypere JP, van Birgelen AP, Penders M, Zegwaard M. Kinetics of the incorporation of dietary fatty acids into serum cholesteryl esters, erythrocyte membranes, and adipose tissue: an 18-month controlled study. J Lipid Res. 1997;38(10):2012-22. Kinetics of the incorporation of dietary fatty acids into serum cholesteryl esters, erythrocyte membranes, and adipose tissue: an 18-month controlled study
  3. Browning LM, Walker CG, Mander AP, West AL, Madden J, Gambell JM, et al. Incorporation of eicosapentaenoic and docosahexaenoic acids into lipid pools when given as supplements providing doses equivalent to typical intakes of oily fish. Am J Clin Nutr. 2012;96(4):748-58. Incorporation of eicosapentaenoic and docosahexaenoic acids into lipid pools when given as supplements providing doses equivalent to typical intakes of oily fish
  4. U.S. National Library of Medicine, DailyMed. Omega-3-acid ethyl esters capsules (Lovaza-type generic) prescribing information. Omega-3-acid ethyl esters capsules (Lovaza-type generic) label
  5. Omega-3 Supplements: What You Need To Know | NCCIH. Omega-3 Supplements: What You Need To Know | NCCIH
Evidence overview for Omega-3 While Fasting: Does It Affect Your Index?