Objective: Several studies have demonstrated that soy isoflavone (SIF) supplementation can improve aspects of cognitive function. However, these findings remain controversial. We aimed to quantify the effects of SIF supplementation on improving cognitive function in postmenopausal women.
Methods: Databases and relevant Websites were searched for relevant studies up to March 2014. Two reviewers independently verified all potentially suitable randomized controlled trials (RCTs) using inclusion and exclusion criteria, and the quality of identified RCTs was assessed using the Jadad scale and the Risk of Bias Tool from the "Cochrane handbook for systematic reviews of interventions." Any disagreement on study quality or data extraction was resolved by consensus; a third reviewer was consulted if needed. Standardized mean differences (SMDs) in cognitive function test scores were calculated between SIF-treated and placebo-treated groups.
Results: We conducted a meta-analysis of 10 placebo-controlled RCTs of SIF supplementation (1,024 participants; treatment duration of 6 wk to 30 mo). The overall SMD in summary cognitive function test scores (0.08) was statistically significant (95% CI, 0.02-0.15; P = 0.014). The summary SMD for visual memory (0.10) was statistically significant (95% CI, 0.02-0.18; P = 0.016). In subgroup analyses, the statistically significant SMDs were as follows: 0.12 (95% CI, 0-0.25; P = 0.044) for non-US countries; 0.16 (95% CI, 0.05-0.28; P = 0.004) for mean age younger than 60 years; and 0.15 (95% CI, 0.03-0.27; P = 0.011) for treatment duration less than 12 months.
Conclusions: SIF supplementation seems to have a positive effect on improving summary cognitive function and visual memory in postmenopausal women. There may be a critical window of opportunity in initiating SIF use at an earlier age in postmenopausal women, and geography and treatment duration seem to be factors influencing the effects of SIF supplementation. All individuals in the included studies should be followed up to observe the incidence rates of Alzheimer's disease and dementia, and future studies should report any adverse effects of SIF supplementation.