Creation of a database to assess effects of omega-3, omega-6 and total polyunsaturated fats on health: methodology for a set of systematic reviews

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Objective: To create a database of long-term RCTs comparing higher with lower omega-3, omega-6 or total PUFA, regardless of reported outcomes, and develop methods to assess effects of increasing omega-6, ALA, LCn3 and total PUFA on health outcomes.

Design: Systematic review search, methodology and meta-analyses.

Data Sources: Medline, Embase, WHO International Clinical Trials Registry Platform,, and trials in relevant systematic reviews.

Eligibility Criteria: Randomised controlled trials (RCTs) of ≥24 weeks duration assessing effects of increasing alpha-linolenic acid (ALA), long-chain omega-3 (LCn3), omega-6 or total polyunsaturated fats (PUFA), regardless of outcomes reported.

Data Synthesis: Methods included random-effects meta-analyses, sensitivity analyses. Funnel plots were examined and subgrouping assessed effects of intervention type, replacement, baseline diabetes risk and use of diabetic medications, trial duration and dose. Quality of evidence was assessed using GRADE.

Results: Electronic searches generated 37,810 hits, de-duplicated to 19,772 titles and abstracts. We assessed 2155 full text papers, conference abstracts and trials registry entries independently in duplicate. Included studies were grouped into 363 RCTs comparing higher with lower omega-3, omega-6 and/or total PUFA intake of at least 6 months duration – the Database. Of these 363 included RCTs, 216 RCTs were included in at least one of our reviews of health outcomes, data extracted and risk of bias assessed in duplicate (Dataset 1, Supplementary File 1). Ninety five RCTs were included in the database but not included in our current reviews (Dataset 2, Supplementary File 2). Of these 311 completed trials, 27 altered ALA intake, 221 altered LCn3 intake, and 16 trials altered omega-3 intake without specifying whether ALA or LCn3. Forty one trials altered omega-6 fats and 59 total PUFA. The remaining 52 trials (Dataset 3, Supplementary File 3) are ongoing though 13 (25%) appear to be outstanding, or constitute missing data.

Conclusions: This extensive database of trials is available to allow assessment of further health outcomes.
Original languageEnglish
Article numbere029554
JournalBMJ Open
Issue number5
Publication statusPublished - 24 May 2019


  • Systematic review methodology
  • database of trials
  • Fatty Acids, Omega-3
  • Fatty Acids, Omega-6
  • Fatty Acids, Unsaturated
  • Meta-Analysis as Topic

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