title: Effects of omega-3 fatty acids on cancer risk: a systematic review.
abstract: BACKGROUND
  Omega-3 fatty acids are purported to reduce the risk of cancer. Studies have reported mixed results.
  
  
  OBJECTIVE
  To synthesize published and unpublished evidence to determine estimates of the effect of omega-3 fatty acids on cancer risk in prospective cohort studies.
  
  
  METHODS
  Articles published from 1966 to October 2005 identified through MEDLINE, PREMEDLINE, EMBASE, Cochrane Central Register of Controlled Trials, and CAB Health; unpublished literature sought through letters to experts in the neutraceutical industry.
  
  
  METHODS
  A total of 38 articles with a description of effects of consumption of omega-3 fatty acids on tumor incidence, prospective cohort study design, human study population; and description of effect of omega-3 among groups with different levels of exposure in the cohort were included. Two reviewers independently reviewed articles using structured abstraction forms; disagreements were resolved by consensus.
  
  
  METHODS
  Two reviewers independently abstracted detailed data about the incidence of cancer, the type of cancer, the number and characteristics of the patients, details on the exposure to omega-3 fatty acids, and the elapsed time between the intervention and outcome measurements. Data about the methodological quality of the study were also abstracted.
  
  
  RESULTS
  Across 20 cohorts from 7 countries for 11 different types of cancer and using up to 6 different ways to categorize omega-3 fatty acid consumption, 65 estimates of the association between omega-3 fatty acid consumption were reported. Among these, only 8 were statistically significant. The high degree of heterogeneity across these studies precluded pooling of data. For breast cancer 1 significant estimate was for increased risk (incidence risk ratio [IRR], 1.47; 95% confidence interval [CI], 1.10-1.98) and 3 were for decreased risk (RR, 0.68-0.72); 7 other estimates did not show a significant association. For colorectal cancer, there was 1 estimate of decreased risk (RR, 0.49; 95% CI, 0.27-0.89) and 17 estimates without association. For lung cancer one of the significant associations was for increased cancer risk (IRR, 3.0; 95% CI, 1.2-7.3), the other was for decreased risk (RR, 0.32; 95% CI, 0.13-0.76), and 4 other estimates were not significant. For prostate cancer, there was 1 estimate of decreased risk (RR, 0.43; 95% CI, 0.22-0.83) and 1 of increased risk (RR, 1.98; 95% CI, 1.34-2.93) for advanced prostate cancer; 15 other estimates did not show a significant association. The study that assessed skin cancer found an increased risk (RR, 1.13; 95% CI, 1.01-1.27). No significant associations between omega-3 fatty acid consumption and cancer incidence were found for aerodigestive cancer, bladder cancer, lymphoma, ovarian cancer, pancreatic cancer, or stomach cancer.
  
  
  CONCLUSIONS
  A large body of literature spanning numerous cohorts from many countries and with different demographic characteristics does not provide evidence to suggest a significant association between omega-3 fatty acids and cancer incidence. Dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer.
authors.affiliation: Southern California Evidence-Based Practice Center, RAND Health, Santa Monica 90407-2138, USA. maclean@rand.org
authors.forename: Catherine H
authors.initials: CH
authors.lastname: MacLean
authors.forename: Sydne J
authors.initials: SJ
authors.lastname: Newberry
authors.forename: Walter A
authors.initials: WA
authors.lastname: Mojica
authors.forename: Puja
authors.initials: P
authors.lastname: Khanna
authors.forename: Amalia M
authors.initials: AM
authors.lastname: Issa
authors.forename: Marika J
authors.initials: MJ
authors.lastname: Suttorp
authors.forename: Yee-Wee
authors.initials: YW
authors.lastname: Lim
authors.forename: Shana B
authors.initials: SB
authors.lastname: Traina
authors.forename: Lara
authors.initials: L
authors.lastname: Hilton
authors.forename: Rena
authors.initials: R
authors.lastname: Garland
authors.forename: Sally C
authors.initials: SC
authors.lastname: Morton
mesh_terms: D019587:Dietary Supplements; D015525:Fatty Acids, Omega-3; D006801:Humans; D015994:Incidence; D009369:Neoplasms / Q000453:epidemiology*; D012306:Risk
chemical_list: D015525:Fatty Acids, Omega-3