title: Absolute versus relative measures of plasma fatty acids and health outcomes: example of phospholipid omega-3 and omega-6 fatty acids and all-cause mortality in women.
abstract: OBJECTIVE
  In a well-characterised community-based prospective study, we aimed to systematically assess the differences in associations of plasma omega-3 and omega-6 fatty acid (FA) status with all-cause mortality when plasma FA status is expressed in absolute concentrations versus relative levels.
  
  
  METHODS
  In a community sample of 564 women aged 25-75 years in Queensland, Australia, baseline plasma phospholipid FA levels were measured using gas chromatography. Specific FAs analysed were eicosapentaenoic acid, docosapentaenoic acid, docosahexaenoic acid, total long-chain omega-3 FAs, linoleic acid, arachidonic acid, and total omega-6 FAs. Levels of each FA were expressed in absolute amounts (µg/mL) and relative levels (% of total FAs) and divided into thirds. Deaths were monitored for 17 years and hazard ratios and 95% confidence intervals calculated to assess risk of death according to absolute versus relative plasma FA levels.
  
  
  RESULTS
  In total 81 (14%) women died during follow-up. Agreement between absolute and relative measures of plasma FAs was higher in omega-3 than omega-6 FAs. The results of multivariate analyses for risk of all-cause mortality were generally similar with risk tending to inverse associations with plasma phospholipid omega-3 FAs and no association with omega-6 FAs. Sensitivity analyses examining effects of age and presence of serious medical conditions on risk of mortality did not alter findings.
  
  
  CONCLUSIONS
  The directions and magnitude of associations with mortality of absolute versus relative FA levels were comparable. However, plasma FA expressed as absolute concentrations may be preferred for ease of comparison and since relative units can be deduced from absolute units.
authors.affiliation: Cancer and Population Studies Group, QIMR Berghofer Medical Research Institute, 300 Herston Road, Herston, Brisbane, QLD, 4006, Australia. Kyoko.Miura@qimrberghofer.edu.au.
authors.forename: Kyoko
authors.identifier: http://orcid.org/0000-0002-2805-7522
authors.initials: K
authors.lastname: Miura
authors.affiliation: Cancer and Population Studies Group, QIMR Berghofer Medical Research Institute, 300 Herston Road, Herston, Brisbane, QLD, 4006, Australia.
authors.forename: Maria Celia B
authors.initials: MCB
authors.lastname: Hughes
authors.affiliation: Queensland Health, Pathology Queensland, Herston, QLD, Australia.
authors.forename: Jacobus P J
authors.initials: JPJ
authors.lastname: Ungerer
authors.affiliation: Cancer and Population Studies Group, QIMR Berghofer Medical Research Institute, 300 Herston Road, Herston, Brisbane, QLD, 4006, Australia.
authors.forename: David D
authors.initials: DD
authors.lastname: Smith
authors.affiliation: Cancer and Population Studies Group, QIMR Berghofer Medical Research Institute, 300 Herston Road, Herston, Brisbane, QLD, 4006, Australia.|CRUK Manchester Institute, University of Manchester, Wilmslow Road, Manchester, UK.
authors.forename: Adèle C
authors.initials: AC
authors.lastname: Green
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D015415:Biomarkers / Q000097:blood; D002423:Cause of Death; D015331:Cohort Studies; D015525:Fatty Acids, Omega-3 / Q000032:analysis / Q000097:blood*; D043371:Fatty Acids, Omega-6 / Q000032:analysis / Q000097:blood*; D005260:Female; D005500:Follow-Up Studies; D006304:Health Status; D006801:Humans; D008875:Middle Aged; D009026:Mortality; D009752:Nutritional Status; D010743:Phospholipids / Q000097:blood* / Q000737:chemistry; D016016:Proportional Hazards Models; D011446:Prospective Studies; D011793:Queensland / Q000453:epidemiology; D012042:Registries
chemical_list: D015415:Biomarkers; D015525:Fatty Acids, Omega-3; D043371:Fatty Acids, Omega-6; D010743:Phospholipids
keywords: All-cause mortality; Biomarkers; Omega-3 fatty acids; Omega-6 fatty acids; Prospective studies