Cases and controls were well matched on age distribution (). No major differences between cases and controls existed with respect to marital status, family income, total energy intake, or use of hormone replacement therapy. However, compared with controls, cases attained higher education; had an earlier age at menarche, later age at menopause, and longer duration of menstruation; had higher body mass index and waist:hip ratio; and were less likely to take regular exercise, drink alcohol, or use oral contraceptives. Cases were also more likely to have a family history of cancer, have been pregnant fewer times, and have consumed more meat and fish. No significant differences existed between cases and controls for fruit and vegetable intake. We adjusted all subsequent analyses for age, energy intake, and the above mentioned significant variables as potential confounders.
Demographic characteristics and selected risk factors for endometrial cancer. Values are numbers (percentages) unless stated otherwise
shows the case-control comparisons for consumption of total soya foods, soya protein, and soya isoflavones. Women with endometrial cancer had a lower intake of total soya foods and almost every individual soya food investigated, as well as total soya protein and soya isoflavones, than did controls, although most differences did not reach statistical significance. The median intake of soya protein and isoflavones was 9.9 g/day and 40.5 mg/day for cases and 10.2 g/day and 42.5 mg/day for controls. The average intake of soya food among control women was comparable to that in our previous study.17
Intake of soya food and soya protein among participants in the Shanghai endometrial cancer study. Values are medians (25th to 75th centiles) unless stated otherwise
As shown in , after adjustment for dietary and non-dietary risk factors, the odds ratios for endometrial cancer associated with the highest quarter of intake of soya protein, soya fibre, soya isoflavones, and fresh soya bean intake were 0.67 (95% confidence interval 0.48 to 0.92), 0.69 (0.51 to 0.94), 0.77 (0.56 to 1.05), and 0.63 (0.46 to 0.84), compared with the lowest quarter of intake. We also observed a dose-response relation for total intake of soya protein (P = 0.01), soya fibre (P = 0.02), and fresh soya beans (P = 0.01). Additional adjustment for total fruit and vegetable intake did not alter the association (data not shown). Intakes of soya protein, soya fibre, and soya isoflavones were highly correlated, making it difficult to separate their independent effects.
Association of endometrial cancer with soya food intake
summarises the results of the stratified analysis by menopausal status (premenopausal, postmenopausal), body mass index (< 25, ≥ 25), and waist:hip ratio (≤ 0.855, > 0.855). Although high soya food intake was related to a lower risk of endometrial cancer in all strata, the inverse association between intake of soya protein and risk of endometrial cancer was more pronounced among women with a higher body mass index (P for trend 0.01) or a higher waist:hip ratio (P for trend 0.01). We observed no multiplicative interactions. The association of soya intake with endometrial cancer did not vary by menopausal status, whether users of hormone replacement therapy (35 cases and 34 controls) were included in or excluded from the analysis.
Association of endometrial cancer with soya protein intake by body mass index, waist:hip ratio, and menopausal status.* Values are odds ratios (95% confidence intervals) unless stated otherwise