Participant characteristics by tertiles of soy intake are presented in for premenopausal-luteal and postmenopausal women. Soy intake was non-significantly higher in postmenopausal compared to premenopausal-luteal women (p=0.09), with postmenopausal women making up 32.7% of the lowest tertile, but 44% of the highest soy tertile. Soy intake was associated with ethnicity in both pre- and postmenopausal women. Filipino women had the lowest frequency of soy intake (with median intakes of 0.2 and 0.5 servings per week in premenopausal-luteal and postmenopausal women, respectively) and so were over-represented in the lowest soy tertiles. In contrast, the median frequency of soy intake for each menopausal subgroup was approximately 1 serving per week among Chinese and Japanese American women. Soy intake was not significantly associated with age, study center, birthplace, or BMI in either menopausal group.
Participant Characteristics by Menopausal Status and Tertiles of Usual Adult Soy Intake
Median levels and interquartile ranges for absolute concentrations of each EM, in pmol/mg creatinine, and for EM expressed as a proportion of total EM, are presented by menopausal status in . Even within menopausal groups urinary EM concentrations can vary by factors of 10–100. Median EM concentrations are 2–10 times higher in premenopausal-luteal women compared to postmenopausal women. In contrast, interquartile ranges for %EM in pre- and postmenopausal women overlap. Most median %EM differ by less than 2 percentage points between premenopausal-luteal and postmenopausal women. One exception is estriol, which is markedly lower as a percent of total EM in postmenopausal women compared to their premenopausal-luteal counterparts (23.6% vs. 32.2%, respectively).
We tested for associations of urinary concentrations of individual EM with soy intake by modeling each EM measure using robust linear regression and adjusting for age and ethnicity (). No statistically significant associations were seen between soy intake tertiles and urinary concentrations of E1, E2, E3, or total EM in either menopausal group. In postmenopausal women, urinary levels of 2-MeOE1 were directly associated with soy intake (ptrend=0.02). Also among postmenopausal women, urinary levels of 16-ketoE2 declined significantly across soy tertiles (ptrend=0.009). These associations remained statistically significant when birthplace or BMI was added to the model. Among premenopausal-luteal women, no significant associations were observed for absolute levels of any individual EM with soy intake.
Percent difference1 in concentrations of urinary estrogens and estrogen metabolites (EM) among Asian American women in highest vs. lowest tertiles of soy intake, by menopausal status.
In we also present results of robust linear regression models for individual EM expressed as a percentage of total EM, in a combined sample of premenopausal-luteal, and postmenopausal women. %4-OHE1, %2-MeOE1, and %2-MeOE2 increased significantly, while %16-keto E2 and %16-epiE3 each had statistically significant inverse trends across tertiles of soy intake. Other %EM in these pathways had similar but non-significant associations with soy. This observation led us to examine covariation among EM.
Factor analysis with rotation resulted in extraction of four independent factors in each menopausal group (). The composition of factors was observed to be similar in premenopausal luteal- and postmenopausal women, in spite of the large differences between these groups in absolute levels of total EM. The predominant factor in both pre- and postmenopausal women, termed ‘catechols’ and accounting for 46% and 30% of the variance in EM profiles, respectively, included all three catechol estrogens (2-OHE1, 2-OHE2, and 4-OHE1) and some of their five methylated metabolites (2-MeOE1 and 3-MeOE1 in both groups, and 2-MeOE2 in premenopausal women only). The second factor in both groups (termed ‘16-hydroxylated pathway EM’) included all five EM that were initially hydroxylated at the 16-C position (16-keto E2, 16a-OHE1, 16-epiE3, E3, 17-epiE3), and accounted for 18% and 16%, respectively, of the variance in EM profiles in the two menopausal groups.
Covariation among urinary estrogens and estrogen metabolites (EM) in Asian American women by menopausal status
The next factor, termed ‘parent estrogens’, was the third factor in premenopausal women, accounting for 7% of variation in EM profiles, and the fourth in postmenopausal women, accounting for 8% of variation in EM profiles. This factor included high loadings for E1 and E2 in both menopausal groups; in postmenopausal women only, two EM from the 16-pathway also contributed in a moderate fashion (E3 and 16-EpiE3). A fourth factor termed ‘methylated catechols’ (4-MeOE1, and 4-MeOE2 in both groups, and 2-MeOE2 in postmenopausal women only) accounted for 6% and 14% of variation in pre- and postmenopausal women, respectively). The fact that methylated catechols in the 4-hydroxylation pathway did not cluster with those in the 2-hydroxylation pathway led us to define groups that examined these metabolites separately.
In we present results of robust linear regression models, fitted in each menopausal stratum and then in a combined sample, for EM grouped by metabolic pathway and expressed as a percentage of total EM. Patterns were similar in premenopausal-luteal and postmenopausal strata; accordingly there was no statistically significant effect modification of the associations by menopausal status. In the combined sample, %2-hydroxylation pathway EM and %4-pathway EM each significantly increased across increasing tertiles of soy intake; these pathways were 16% higher (ptrend= 0.02) and 19% higher (ptrend= 0.03), respectively, in the highest compared to the lowest tertile of soy intake. %16-hydroxylated pathway EM were 11% lower in the highest vs. the lowest tertile of soy intake (ptrend< 0.01). %Parent estrogens showed no significant trend across soy tertiles. %Methylated catechols, like %catechols, increased significantly across the range of soy intake (ptrend< 0.01). Although %catechol estrogens in the 2-hydroxylation and 4-hydroxylation pathways each increased significantly with soy intake, trends in %methylated catechols differed by pathway. Methylated catechols of the 2-pathway increased as a proportion of total EM across soy tertiles (ptrend<0.01) while %methylated catechols in the 4-hydroxylation pathway showed no linear trend (ptrend=0.80).
Percent difference1 in urinary EM grouped by metabolic pathway in highest vs. lowest tertiles of soy intake among premenopausal (luteal) and postmenopausal Asian American women.
Results for ratios of EM were consistent with findings for EM groups. Ratios of 2-pathway EM and 4-pathway EM to 16α-pathway metabolites significantly increased across the range of soy intake (ptrend=0.01 for each). Among all women, the ratio of catechols: methylated catechols in the 4-pathway was 20% higher in the highest soy tertile compared to the lowest (ptrend=0.11); no trend was noted in the same ratio for the 2-pathway.
Finally we considered several potential confounders (data not shown) of the EM/soy relationships. Birthplace is considered a marker of acculturation and of breast cancer risk (26
). BMI may play a causal role in breast cancer etiology and has sometimes been observed to be inversely associated with soy intake (31
). Additional adjustment of all models for birthplace and for BMI did not modify the direction or magnitude of observed associations.
shows the percent difference between highest and lowest soy tertiles in relative levels of urinary EM for each stratum defined, in turn, by menopausal status, ethnicity, birthplace, and BMI. Results suggest that associations of %EM groups with soy intake were not significantly modified by these factors. One statistically significant interaction was noted: in women with BMI below the median the %2-hydroxylation pathway showed a significant increasing trend across soy tertiles and was 16.6% higher in the highest vs. lowest soy tertile; among heavier women the %2-hydroxylation pathway also increased with soy intake but the magnitude of the effect was smaller, with a 10% increase seen in the highest vs. lowest tertile (p for interaction = 0.04). This statistically significant finding may reflect the large number of statistical tests performed.
Percent difference between highest and lowest tertiles of soy intake in relative levels of urinary estrogens and estrogen metabolites (% of total) by menopausal status, ethnicity, and BMI
Inclusion of premenopausal women in other phases of the menstrual cycle also did not modify the direction or magnitude of observed associations; however it did result in wider confidence limits for many estimates.