In this large cross-sectional analysis of 2,165 middle-aged and elderly Chinese women, we found that urinary levels of daidzein, genistein, and glycitein correlated well with soy food intake and several other lifestyle factors. The levels of urinary isoflavones and enterodiol among women in our study were much higher than among U.S. women of comparable age, while the level of enterolactone was similar between Chinese and US women.
Soy and its products are the richest food sources of isoflavones with a ratio of daidzein, genistein, and glycitein of roughly 1:1:0.1 [18
]. After being absorbed into the blood, these compounds are mainly excreted in urine. Our results are in agreement with previous studies that have found that high concentrations of isoflavones in urine were positively associated with the consumption of soy products [19
]. Our data add further support to previous findings that isoflavones measured in a spot urine sample can serve as biomarkers for soy consumption in epidemiologic studies [22
Of the nine phytoestrogens we measured, the urinary daidzein level was the highest and concentrations of isoflavones were much higher than lignans, which is consistent with the typical Asian diet, in which soy products, rather than whole grains and cereals, contribute the bulk of phytoestrogens. The phytoestrogen levels of Chinese women in our study were higher than the average levels found among all women included in a recent study conducted in several Asian countries, including Vietnam, Cambodia, India, and Japan [23
]. However, levels of daidzein, genistein, and equol were 1.5- to 2.2- fold lower than levels reported for Vietnamese (Hanoi) and Japanese women. The isoflavonoid levels of Chinese women in our study were far higher than those of US women, consistent with the fact that soy food consumption is much higher among Chinese women than US women (the average of isoflavone intakes among Chinese women and U.S. women were 47 mg/d and 1- 6 mg/d, respectively) [4
]. Our previous studies have indicated that high soy food consumption may reduce the risk of breast cancer, and soy food intake was significantly associated with decreased risk of death and recurrence among women with breast cancer [4
]. The inverse association between soy intake and breast cancer risk was found in Asian but not Western populations in a meta-analysis by Wu et al., which suggests that protection against breast cancer may require that women consume levels of soy typically seen in Asian diets [26
Relationships between isoflavones and intakes of fruits and vegetables in previous studies have been inconsistent. In an analysis of 96 postmenopausal American women, plasma genistein levels were positively related to total servings of fruits and vegetables [28
]. Stumpf et al. did not find any change in serum daidzein or genistein concentrations among 85 middle-aged Finnish participants in a dietary intervention study after markedly increasing consumption of vegetables, fruit, and berries [29
]. In our study, we found that urinary excretions of total isoflavonoids, daidzein, genistein, glycitein, and O-DMA were weakly and inversely related to fruit intake, and genistein was also inversely associated with vegetable intake among middle-aged and elderly Chinese women. The inverse correlation between isoflavone level and fruit intake may be explained by the fact that soy food consumption is a part of traditional Chinese diet and is very affordable while fruit consumption is part of an affluent lifestyle. The reason for the inverse correlation between genistein and total vegetable intake remains to be investigated.
Urinary isoflavanoid excretion was higher among older women in our study, which is in agreement with previous reports that have suggested that older women may consume more soy-based foods for the purpose of improving their health [28
]. In our study population, urinary isoflavonoid levels were also higher among women who engaged in regular exercise, supporting the healthy lifestyle hypothesis. On the other hand, our results could also be due to older women being more likely to adhere to a traditional Chinese dietary pattern.
In accordance with the findings of Chun et al. that flavonoid-rich diets were consumed more frequently among vitamin supplement users [30
], we found that urinary excretions of total isoflavonoids and daidzein were positively associated with vitamin supplement use. Chun et al. also found that isoflavone intake was inversely associated with BMI [31
]. Although there was no significant relationship between urinary excretion of genistein or daidzein and BMI in our study, we found that urinary excretion of dihydrodaidzein, equol, dihydrogenistein, and the metabolites of daidzein and genistein were inversely related to body mass index in our study. The metabolization of daidzein occurs in the gut through the action of a particular class of bacteria Clostridium sp, Eubacterium ramulus, Bacteroides ovatus, Bifidobacterium breve, etc [8
]. The gut bacterial flora in obese individuals is known to differ from that in non-obese individuals. The lack of a direct association between daidzein and BMI could suggest that the inverse association between daidzein metabolites and BMI is a result of obesity. On the other hand, we could not exclude the possibility that daidzein and its metabolites may have a different effect on obesity. More research in this field is needed.
Equol, which possesses a more potent estrogenic activity than its precursor, daidzein, has been proposed as one of the most important components of isoflavones for disease prevention [8
]. Not all humans (only 30-60%) possess the gut flora that can produce equol [18
]. Besides the composition and activity of the intestinal microbiota, the diet has been suggested to contribute to the ability to harbor equol-producing bacteria [8
]. Asians, who are usually habitual soy consumers, seem to have a greater ability to change daidzein to equol than Western populations [34
]. In our study, 1,895 women (87.5%) had a urinary daizein level higher than 1 nmol/mg creatinine. Among these women, we estimated the prevalence of equol producers using the recommended ratio of urinary equol:daidzein higher than 0.018 as the cut-point for being an equol producer [35
]. We found that 50.3% women in our population were equol producers, which is much higher than 33% reported in a Western population but is comparable to 55.7% reported among a Japanese population in which soy product intake is also high [32
]. In our study population, there was a slightly higher percentage of equol producers among pre-menopausal women (53.2%) than post-menopausal women (48.7%).
In our study both enterodiol and enterolactone concentrations were weakly and inversely associated with BMI. Lignans can pass through the preadipocyte cell membrane [37
]; therefore, lignan concentrations in obese women may be diluted by rapid transport into cells, resulting in lower urinary excretions [37
]. However, this inverse correlation also suggests a true association. Johnsen et al. has reported that high fat intake decreased serum enterolactone levels [39
]. In contrast, we found a positive correlation between fat intake and urinary enterolactone excretion.
Tea contains high levels of lignans [40
]. In our study, urinary enterolactone concentration was positively related to tea consumption. Enterolactone concentration was higher among younger women, consistent with an earlier report that young, vegetarian women had much higher lignan excretion than their older counterparts [41
]. Urinary enterodiol excretion was higher among post-menopausal than among pre-menopausal women in our study.
Because urinary excretion of creatinine depends on muscle mass, which is influenced by body weight, gender, and age, we also estimated the urinary excretions of phytoestrogens applying the suggested body weight- and age-specific creatinine excretion rates (mg-creatinine/kg BW /day ratio [in nmol/mg creatinine] multiplied by body weight) [42
]. We found that the levels of excretion of urinary phytoestrogens derived from this method were highly correlated with levels derived without applying weight-age specific creatinine excretion levels (0.99 ≤ r ≤ 1.00). Furthermore, we found that associations between urinary phytoestrogen concentrations and lifestyle factors and dietary intakes remained largely unchanged, with the exception of the association for BMI. The correlation with BMI carries no biological meaning, however, because body weight was the major determinant for both BMI measurement and the adjustment method described above for estimating phytoestogen concentrations.
The cross-sectional study design is the major limitation of this study and makes the interpretation of the underlying mechanisms responsible for the correlations observed in our study difficult to determine. Another limitation is that a single spot urine sample may not capture the overall levels of isoflavones well given the short half-life of phytoestrogens, which is in the range of 3-10 hours [43
]. Nevertheless, we have previously shown that, in our study population, urinary isoflavone level from a single spot urine sample was relatively stable across a one-year period of time [44
]. Furthermore, we found good correlation between dietary isoflavone intake and urinary excretion level.
In summary, in this analysis of a large group of Chinese women aged 40-70 years, we found that urinary levels of isoflavones are substantially higher among Chinese women than their counterparts in the U.S., while levels of lignans for these two populations were more comparable. Urinary excretion of isoflavones significantly correlated with soy food intake but was inversely associated with fruit intake. Our finding of different correlation patterns for parent isoflavones and metabolites of isoflavones with BMI calls for further study in order to understand the nature of this association and to investigate the full spectrum of the health effects related to dietary phytoestrogen intake.