Authors |
MacLean
CH, Newberry SJ, Mojica WA, Khanna P, Issa AM, Suttorp MJ, Lim YW, Traina
SB, Hilton L, Garland R, Morton SC.
|
Abstract |
CONTEXT:
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OBJECTIVE: To synthesize published and unpublished evidence to determine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.
DATA SOURCES: 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Trials, and CAB Health; unpublished literature sought through letters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.
STUDY SELECTION: 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.
DATA EXTRACTION: 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.
DATA SYNTHESIS: 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.
|