Evidence suggests that soy foods have chemoprotective properties that may reduce the risk of certain cancers such as breast and prostate cancer. However, data involving gastrointestinal (GI) have been limited, and the evidence remains controversial. This study aims to determine the potential relationship between dietary soy intake and GI cancer risk with an evaluation of the effects of isoflavone as an active soy constituent.

Relevant studies were identified after literature search via electronic databases through May 2014. Subgroup analysis for isoflavone intake (studies n�=�10) was performed. Covariants including gender types, anatomical subsites and preparation methods were also evaluated. Pooled adjusted odds ratios (ORs) comparing highest and lowest categories of dietary pattern scores were calculated using a random effects model.

Twenty-two case–control and 18 cohort studies were included for meta-analysis, which contained a total of 633,476 participants and 13,639 GI cancer cases. The combined OR was calculated as 0.93 (95�% CI 0.87–0.99; p value heterogeneity�=�0.01), showing only a slight decrease in risk, the association was stronger for colon cancer (OR 0.92; 95�% CI 0.96–0.99; p value heterogeneity�=�0.163) and colorectal cancer (CRC) (OR 0.92; 95�% CI 0.87–0.97; p value heterogeneity�=�0.3). Subgroup analysis for isoflavone intake showed a statistically significant risk reduction with a risk estimate of 0.73 (95�% CI 0.59–0.92; p value heterogeneity�=�0), and particularly for CRC (OR 0.76; 95�% CI 0.59–0.98; p value heterogeneity�=�0).

This study provides evidence that soy intake as a food group is only associated with a small reduction in GI cancer risk. Separate analysis for dietary isoflavone intakes suggests a stronger inverse association.