The results from observational epidemiological studies have been inconsistent with regards to the link between dietary omega-3 intake and endocrine-related gynaecological cancer, such as endometrial and ovarian cancers.
Analysing the association
Researchers at South Korea’s National Cancer Centre therefore conducted a meta-analysis of observational studies to investigate this association. Using PubMed, EMBASE, and Cochrane library, they selected 10 studies — six case control and four cohort — for inclusion in the final analysis.
They also performed a sub-group meta-analysis by methodological quality, types of cancer, study design, and omega-3 fatty acids.
The meta-analysis of all 10 studies revealed that when the highest and lowest dietary intakes were compared, total omega-3 fatty acid consumption was not significantly associated with the risk of both endometrial and ovarian cancers.
When it came to the meta-analysis by the type of study, the dietary intake of omega-3 fatty acids was not linked to the risk of endocrine-related gynaecological cancer in the cohort studies, but was associated with a significant risk reduction in five of the case-control studies.
This discrepancy might have been the result of certain biases — case-control studies in general are more sensitive to selection and recall biases than prospective cohort studies, as either a case or control group “might not represent the whole population because any group was a non-random sample from the population”, which could lead to selection bias.
At the same time, cancer patients typically recall their dietary intakes differently from healthy controls. For instance, they may state that they consumed less omega-3 rich foods, such as fatty fish, while healthy controls usually report a higher consumption of such foods anyway. This may result in the true effect of dietary omega-3 intake being overestimated.
The subgroup meta-analyses by study quality, type of omega-3 fatty acids (ALA, EPA DHA and DPA), and type of cancer also produced no significant association between dietary omega-3 intake and the risk of endocrine-related gynaecological cancer.
The case against cancer
Despite the lack of a significant association between dietary omega-3 intake and the risk of endocrine-related gynaecological cancer, there have been several hypotheses surrounding omega-3 fatty acids’ potential protective effects against this type of cancer.
Previous studies have found three overarching mechanisms behind the pleiotropic anti-inflammatory and immunosuppressive properties of omega-3 fatty acids such as EPA and DHA: the modulation of nuclear activation (for example, nuclear factor-κB), suppression of arachidonic acid-cyclooxygenase-derived eicosanoids, and alteration of the plasma membrane micro organisation (lipid rafts).
Pre-clinical models have also shown that omega-3 fatty acids can reduce tumour cell proliferation and migration, and promote tumour cell apoptosis by inhibiting major targets for the treatment of endometrial cancer.
The researchers also wrote: “It has been reported that omega-3 fatty acids had anti-proliferative and anti-carcinogenic effects on epithelial ovarian cancer cell lines.
“However, those anti-cancer effects of omega-3 effects were not observed in our meta-analysis of observational epidemiological studies.”
Size matters
They added that this was the first meta-analysis to have reported on the associations between dietary omega-3 intake and endocrine-related gynaecological cancer risk, and acknowledged that there should have been a larger sample size for better results.
They also said more large prospective cohort studies were warranted to confirm their findings, and that only one study had taken into consideration varying dietary factors as adjustment variables.
Furthermore, the results were inapplicable to Asians, as the published data had come from Western countries (mainly the US).
Different methods of cooking fish between Western and Eastern countries, as well as the frequent intake of raw fish in countries like Japan and Korea, may have influenced the risk of endocrine-related gynaecological cancer.
In conclusion, the researchers wrote: “The current meta-analysis of observational studies suggests that there is no higher level of evidence to support the protective effect of dietary omega-3 fatty acids on endocrine-related gynaecological cancer. Further prospective studies should be conducted to confirm the association.”
Source: Cancer Research and Treatment
https://doi.org/10.4143/crt.2018.473
“Dietary Intake of Omega-3 fatty acids and Endocrine-related Gynecological Cancer: A Meta-Analysis of Observational Studies”
Authors: Tung Hoang, et al.
