Phyto-oestrogens are plant compounds structurally similar to
oestradiol, which have been proposed to have protective effects against
breast cancer. The main class of phyto-oestrogens in the Western diet is
lignans. Literature reports on the effect of
lignans in
breast cancer risk have been conflicting. We performed three separate meta-analyses to examine the relationships between (i) plant
lignan intake, (ii) enterolignan exposure and (iii) blood
enterolactone levels and
breast cancer risk. Medline, BIOSIS and EMBASE databases were searched for publications up to 30 September 2008, and 23 studies were included in the random effects meta-analyses. Overall, there was little association between high plant
lignan intake and
breast cancer risk (11 studies, combined odds ratio (OR): 0.93, 95% confidence interval (95% CI): 0.83-1.03, P=0.15), but this association was subjected to marked heterogeneity (I(2)=44%). Restricting the analysis to post-menopausal women, high levels of plant
lignan intake were associated with reduced
breast cancer risk (7 studies, combined OR: 0.85, 95% CI: 0.78, 0.93, P<0.001) and heterogeneity was markedly reduced (I(2)=0%). High enterolignan exposure was also associated with
breast cancer (5 studies, combined OR: 0.73, 95% CI: 0.57, 0.92, P=0.009) but, again, there was marked heterogeneity (I(2)=63%). No association was found with blood
enterolactone levels (combined OR: 0.82, 95% CI: 0.59-1.14, P=0.24). In conclusion, plant
lignans may be associated with a small reduction in post-menopausal
breast cancer risk, but further studies are required to confirm these results.