Background: Epidemiology studies have reported associations between soy intake and the risk of endocrine-related gynaecological cancers. However, to date there have been no quantitative meta-analyses reported regarding this topic.
Objectives: We investigated the quantitative associations between soy food intake and the risk of endometrial cancer and ovarian cancer by a meta-analysis of case-control studies and cohort studies.
Search strategy: We searched MEDLINE (PubMed), EMBASE and the Cochrane Library during October 2008 using common keywords related to soy intake and endometrial or ovarian cancer. Two evaluators independently reviewed and selected articles, based on predetermined selection criteria.
Selection criteria: Included studies met all of the following criteria: (1) a case-control study or cohort study (to date, no randomized controlled trials have been reported); (2) investigated the associations between 'soy or soy product intake' and 'endometrial cancer' or 'ovarian cancer'; (3) reported outcome measures with adjusted odds ratios (OR) or relative risks (RR) and 95% confidence intervals (CI).
Data collection and analysis: We investigated the associations between the overall soy intake (highest versus lowest intake) and the risk of endocrine-related gynaecological cancers (endometrial or ovarian cancer) as the main analysis. We also performed subgroup analyses by type of cancer (endometrial or ovarian), type of study design (case-control or cohort) and type of soy intake (soy foods or soy constituents).
Main results: Out of 477 articles that met our initial criteria, a total of seven epidemiology studies consisting of five case-control studies and two cohort studies were included in the final analyses. Compared with the lowest soy intake, the OR for the highest soy intake was 0.61 (95% CI, 0.53-0.72) of all endocrine-related cancers among seven studies; 0.70 for endometrial cancer (95% CI, 0.57-0.86) and 0.52 for ovarian cancer (95% CI, 0.42-0.66) in the fixed-effects meta-analyses. The subgroup analyses by study design showed similar findings among the case-control studies (OR, 0.62; 95% CI, 0.53-0.73) and the cohort studies (OR, 0.57; 95% CI, 0.36-0.90).
Author's conclusions: The results of the current study showed protective effects of soy intake on the risk for endocrine-related gynaecological cancers. Additional larger prospective studies are now needed.