The findings support results of four previous meta-analyses, which also found an inverse relationship between soy consumption and prostate cancer (PCa) risk.
Pooled results for total soy food consumption showed a 30% risk reduction for the highest intakes compared with lowest, revealed researchers from the University of Illinois at Urbana-Champaign.
The systematic review also examined the effects of soy intake based on dietary genistein and daidzein (two components of soy food). A 10% lower PCa risk was associated with the highest intake of genistein (compared with lowest. The corresponding risk reduction for daidzein was 16%.
“This evidence from observational studies shows a statistically significant association between soy consumption and decreased PCa risk. Further studies are required to support soy consumption as a prophylactic dietary approach to reduce PCa carcinogenesis,” wrote senior author Professor John Erdman Jr.
Total soy isoflavone intake and circulating isoflavones levels were however unrelated to PCa risk.
Splitting soy foods into unfermented and fermented items revealed a big difference between the two categories. Unfermented foods included soy milk, tofu and soybeans; while fermented soy foods included miso and natto. Although high consumption of unfermented soy products was linked to a 35% PC risk reduction, the potential benefit of consuming fermented soy was not statistically significant.
The review is unique in that “No previous meta-analysis has explored potential links between soy foods and advanced PCa,”commented the researchers.
The meta-analysis found no association between either total soy intake or circulating isoflavones with the advanced form of the disease, although the scientists emphasised that “Very few studies currently exist to examine potential associations.”
Nevertheless, the researchers highlighted that the small number of studies that did investigate the cancer risk of fermented soy products showed “Wider variation in results reported by these studies than there was for studies using unfermented and other soy foods.
This wider variation could have impacted the risk outcomes. Some concerns have been expressed in the literature regarding the effects of soy fermentation on the risk of developing certain cancers, such as gastric cancer,” they explained.
The studies in this meta-analysis were all observational. Two intervention studies using isoflavones have shown some benefits in apoptosis (programmed cell death) and cell cycle in prostate tumours/ tissues. However, further research is need to support soy consumption as a therapy for the development or treatment of PCa.
Volume 10, issue 1, article no. 40 DOI: 10.3390/nu10010040
“Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis”
Authors: Catherine C. Applegate, Joe L. Rowles, Katherine M. Ranard, Sookyoung Jeon and John W. Erdman Jr.