One possible explanation could be the regular inclusion of soy foods in Asian cuisine. Soy isoflavones act as weak estrogen receptor agonists or antagonists, depending on the cell type and estrogen concentration present.
Also, the soy isoflavones genistein and daidzein have been shown to accumulate in prostatic tissue, where they may be cytotoxic to cancer cells.
Four previous meta-analyses of epidemiological studies have reported protective associations between soy consumption and prostate cancer.
The current meta-analysis broadens the scope of investigation to include more studies, categorisation of soy foods, isoflavone measurements and evaluation of the role of soy in advanced prostate cancer.
The analysis included 30 articles, representing 266,699 participants and 21,612 cases of prostate cancer.
The included studies were case-control, nested case-control or cohort studies conducted in Asia, North America, and Europe.
Of the 30 studies, 24 included information about dietary soy intake and nine included information about circulating isoflavone levels.
The results showed that total soy food, genistein, daidzein and unfermented soy food intakes were significantly associated with a reduced risk of prostate cancer.
Tofu was the most investigated soy food in the literature and showed a protective association with prostate cancer.
Fermented soy food intake (miso and tempeh), total isoflavone intake, and circulating isoflavones were not associated with prostate cancer risk.
Based on only two studies, neither soy food intake nor circulating isoflavones were associated with advanced prostate cancer risk.
The results of this study are consistent with previous meta-analyses, demonstrating a statistically significant association between soy food consumption and decreased prostate cancer risk.