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From the Clinical Research Centers |
Department of Food Science and Nutrition (A.M.D., K.E.W.U., X.X., M.S.K.), University of Minnesota, St. Paul, MN, 55108 ; the Department of Obstetrics and Gynecology (J.L.), University of Minnesota, Minneapolis, Minnesota, 55455 ; Department of Obstetrics and Gynecology (W.R.P.), University of Rochester, Rochester, New York, 14642
Address correspondence and requests for reprints to: Mindy S. Kurzer, Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul, Minnesota 55108; E-mail: mkurzer{at}tc.umn.edu
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Abstract |
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 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Isoflavones, a class of phytoestrogens found primarily in soybeans, are of great interest as they have been shown to exert hormonal effects in animal studies. Estrogenic effects observed in ovariectomized rats include increased plasma prolactin (4), altered vaginal cytology (4), enhanced mammary gland proliferation (5), and increased uterine weight (5). On the other hand, a study performed in ovariectomized macaques reported no significant estrogenic effects of isoflavones on vaginal cytology (6).
Epidemiological studies have provided some support for an association between isoflavones and plasma hormones in postmenopausal women. Urinary isoflavones have been shown to correlate positively with plasma sex hormone binding globulin (SHBG) and negatively with plasma free estradiol (E2) (7, 8). In one study, consumption of phytoestrogen-containing grains and legumes correlated negatively with plasma total testosterone, but no significant correlations were found with plasma estrone (E1), SHBG, or dehydroepiandrosterone-sulfate (DHEA-S) (9).
There are no reports of hormonal effects of isoflavone consumption in postmenopausal women, although there are a few reports of the effects of soy consumption. Studies performed in postmenopausal women consuming soy have shown no significant changes in vaginal cytology (10, 11) or plasma follicle stimulating hormone (FSH) (10, 11), luteinizing hormone (LH) (10), E2 (10, 12), SHBG (10, 12), or prolactin (12). Postmenopausal women consuming soy and flaxseed (a rich source of the related phytoestrogenic lignans) showed unchanged plasma E2 but increased SHBG (13), and postmenopausal women consuming soy, flaxseed, and red clover (another rich source of isoflavones) showed decreased plasma FSH, unchanged plasma LH, and an increase in the maturation value of vaginal epithelial cells (14). Changes in menopausal symptoms have been inconsistent, with studies showing unchanged (11) or decreased number of hot flushes (15) after soy consumption, and decreased number of hot flushes after consumption of soy and flaxseed (13).
Other hormones possibly affected by soy or soy isoflavones include thyroid hormones, insulin, and cortisol. Reported effects on thyroid hormones include decreased plasma free triiodothyronine (T3) in premenopausal women fed soy isoflavones (16), increased plasma thyroxine (T4) and goiter development in rats fed soybeans (17, 18), and inhibition of thyroid peroxidase (an enzyme involved in thyroid synthesis) by isolated isoflavones in vitro (19). Improved insulin sensitivity has been observed in monkeys consuming soy protein (20), and cortisol production was decreased when genistein was added to human fetal adrenal cortical cells (21).
The primary purpose of the current study was to specifically assess the effects of soy isoflavones on plasma hormone concentrations in healthy postmenopausal women, with a particular focus on those hormones related to estrogen action. Additionally, vaginal cytology specimens and endometrial biopsies were obtained to assess estrogenic end organ responses. A randomized crossover design was used in which 18 free-living subjects consumed three soy protein isolates for 93-days each, separated by 26-day washouts. The control soy protein isolate was essentially isoflavone-free, whereas the two other soy protein isolates contained different isoflavone concentrations. Thus, we were able to isolate effects due to isoflavones from those due to non-isoflavone soy components. Baseline samples were also collected prior to the start of the study to allow for examination of effects of non-isoflavone soy components.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Potential subjects were selected after a phone questionnaire,
interview, and health screen. Exclusionary criteria included strict
vegetarian, high fiber, high soy, or low fat diets; regular consumption
of vitamin and mineral supplementation greater than the Recommended
Dietary Allowances; athleticism; cigarette smoking; antibiotic or
hormone use within six months; menstrual bleeding within 12 months;
hysterectomy or oopherectomy; FSH concentration of less than 25 IU/L;
history of chronic disorders including endocrine or gynecological
diseases; benign breast disease; regular use of medication known to
interfere with study endpoints (including aspirin); less than 90% or
more than 120% ideal body weight; weight change of more than 10 lb
within the previous year, and inability to abstain from alcoholic
beverages during the study. Twenty-three women were admitted into the
study. Four subjects dropped out during diet period one, due to
inability to comply with the study protocol, and one subject was
excluded from all analyses when her plasma hormones revealed she was
not postmenopausal. Eighteen subjects completed the study (Table 1
).
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The study protocol was approved by the University of Minnesota Institutional Review Board: Human Subjects Committee. The study consisted of three 93-day diet periods, separated by 26-day washouts. During each diet period, subjects were free-living and consumed their habitual diets supplemented with one of three soy protein powders, in a randomized crossover design. Subjects were blinded to which soy protein powder they received.
Subjects were permitted to consume their usual diets with detailed dietary instructions to minimize phytoestrogen consumption by avoiding soy, flaxseed, seeds and sprouts, and limiting beans and legumes to one serving per week in their ad libitum diets. In addition, subjects were required to avoid alcoholic beverages. Their diets were supplemented with three soy protein powders, each containing a similar macronutrient composition but a different concentration of isoflavones (Supro Brand Isolated Soy Protein, Protein Technologies International, St. Louis, MO). The three soy powders provided 0.11 ± 0.01 mg (control); 1.00 ± 0.01 mg (low-iso); and 2.00 ± 0.02 mg (high-iso) total isoflavones per kilogram of body weight per day (7.1 ± 1.1 mg, 65 ± 11 mg, and 132 ± 22 mg isoflavones per day, respectively), expressed as unconjugated phytoestrogen units. The proportions of genistein, daidzein, and glycitein averaged 58%, 33%, and 9%, respectively, with 91% of the genistein, 90% of the daidzein, and 82% of the glycitein present as their glucoside conjugates. The soy powder was provided in daily packets and kept refrigerated until the day it was to be consumed. The daily nutrient contribution of the soy powder averaged 348 kilocalories, 63 g protein, 21 g carbohydrate, and 1.9 g fat.
Study procedures
Fasting body weight was obtained in a hospital gown biweekly, and multiple skinfold thicknesses were obtained once on day 1 of the study and once between days 92–94 of each diet period. Skinfold thickness measurements were taken at triceps, biceps, suprailiac, and subscapular sites on the subject’s nondominant side. All measurements were performed by the same individual and were taken to the nearest 0.1 mm with a skinfold caliper (Cambridge Scientific Instruments, Ltd., Cambridge, MD). Body density was calculated from the sum of the four skinfold thicknesses, and a predictive equation was used to determine percent body fat (22).
Fasting blood was obtained on day 1 of the study (baseline) and on days 36–38, 64–66, and 92–94 of each diet period. Subjects were instructed to avoid soy for one week before the baseline collection. Blood was drawn in heparinized tubes, plasma was separated, and sodium azide and ascorbic acid were added to a final concentration of 0.1% each. Aliquots were frozen at -70 C until analysis.
Baseline food records, vaginal smears, and endometrial biopsies were obtained at the health screen, which was within two months before the start of the study for all subjects except two, for whom it was within four months. Additional food records were collected on days 35–37, 63–65, and 91–93 of each diet period; vaginal smears once between days 92–94 of each diet period; and endometrial biopsies once between days 92–94 of the high-iso diet only. Endometrial biopsies were optional, and 14 subjects underwent both biopsies.
Analytical methods
Plasma samples obtained on three consecutive days were pooled and, along with the baseline sample, plasma pools were analyzed for E2, E1, estrone sulfate (E1-S), LH, FSH, testosterone, androstenedione, DHEA-S, SHBG, prolactin, insulin, cortisol, total T4, free T4, total T3, free T3, thyroid binding globulin (TBG), and thyroid stimulating hormone (TSH). To reduce the effects of interassay variability, all samples from each subject were analyzed in duplicate in the same batch, along with a plasma pool control. E2, E1, E1-S, testosterone, androstenedione, DHEA-S, cortisol, insulin, and SHBG were determined by double antibody RIA using 125I-labeled hormone (Diagnostics Systems Laboratories, Inc., Webster, TX). LH and FSH were determined by immunoradiometric assay using 125I-labeled antibody (Diagnostics Systems Laboratories, Inc.). Prolactin was determined by double antibody RIA using 125I-labeled hormone (Diagnostic Products Corporation, Los Angeles). TSH was determined by immunoradiometric assay using 125I-labeled antibody (Diagnostic Products). Total T4, free T4, total T3 and free T3 were determined by single antibody-coated tube RIA using 125I-labeled hormone (Diagnostic Products). TBG was determined by double antibody RIA using 125I-labeled T4 (INCSTAR Corp., Stillwater, MN). Intra-assay variability was 1.5%, 1.0%, 1.7%, 2.3%, 1.4%, 1.1%, 1.9%, 2.0%, 1.5%, 1.5%, 1.5%, 2.5%, 2.0%, 2.1%, 1.8%, 1.6%, 1.7%, and 2.1%, and inter-assay variability was 11.6%, 21.8%, 11.4%, 18.0%, 7.2%, 12.7%, 12.4%, 5.4%, 2.9%, 22.3%, 17.9%, 7.2%, 3.1%, 11.0%, 1.8%, 10.4%, 2.1%, and 5.6% for E2, E1, E1-S, LH, FSH, testosterone, androstenedione, DHEA-S, SHBG, prolactin, insulin, cortisol, total T4, free T4, total T3, free T3, TBG, and TSH, respectively.
Food records were analyzed using Nutritionist IV, Version 4.0 (The Hearst Corporation, San Bruno, CA). For each 3-day food record, averages were calculated for energy, protein, carbohydrate, dietary fiber, fat, cholesterol, saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, and all known essential micronutrients.
Samples of vaginal epithelial cells were taken from the right and left proximal third of the vagina and fixed on a slide for histological examination. All vaginal cytology samples were evaluated by the same gynecologic pathologist, and a vaginal epithelial maturation value was calculated as the percentage of superficial cells plus half the percentage of intermediate cells.
All endometrial biopsy specimens were examined by the same gynecologic pathologist and evaluated as either proliferative or inactive.
Data and statistical analysis
Statistical analyses were performed using the Statistical Analysis System, version 6.12 (SAS Institute, Inc., Cary, NC) (23). In order to allow adaptation to each diet, days 36–38 plasma pools were excluded from the statistical analyses of the plasma hormones. Repeated measures ANOVA were performed on plasma hormones, controlling for subject, diet, and time of collection (days 64–66 vs. days 92–94). There were no significant interactions between diet and time of collection. Repeated measures ANOVA were performed on anthropometric, food record, vaginal cytology, and endometrial biopsy endpoints, controlling for subject and diet. Comparisons between each diet and baseline were evaluated using paired t-tests. Results are expressed as mean ± SD or mean ± SE. In the event of missing data, least squares means (lsmean) are presented to account for the imbalance. DHEA-S values were logarithmically transformed to satisfy the ANOVA assumption of normality and are thus presented as geometric means with 95% confidence intervals in parentheses. Significance was considered at P < 0.05.
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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. Body weight, body mass index (BMI),
and percent body fat did not change significantly during the study.
The dietary data are presented in Table 2. There were no significant differences
among the three diets in content of energy, dietary fiber, or any
macro- or micronutrients. When compared with prestudy food records,
however, records collected during the study showed significantly higher
consumption of protein (P = 0.0001) and significantly
lower consumption of carbohydrate (P = 0.005), fat
(P = 0.03), and dietary fiber (P =
0.0001). During the study, subjects also consumed significantly higher
amounts of riboflavin, vitamin B12, vitamin D,
calcium, folate, iron, magnesium, phosphorus, and pantothenic acid and
significantly lower amounts of saturated fat, polyunsaturated fat,
cholesterol, niacin, biotin, vitamin K, vitamin B6,
copper, selenium, and chromium (all P values<0.01, data not
shown).
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. E1-S was
modestly but significantly decreased by the high-iso diet, when
compared with both the control (P = 0.002) and low-iso
(P = 0.001) diets. The high-iso diet also tended to
decrease E2 and E1,
although this was not statistically significant. SHBG was significantly
increased by the high-iso diet, when compared with the control diet
(P = 0.04). For testosterone, DHEA-S and
TBG, statistically significant effects of isoflavone consumption were
noted, but the changes were modest, and no dose responses were
apparent. No significant effects of isoflavone consumption were
observed for LH, FSH, androstenedione, prolactin, cortisol, insulin, or
any thyroid hormones.
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, compared
with baseline, FSH and SHBG were significantly decreased by the low-iso
diet (P = 0.01 and 0.007, respectively), and
E2, E1,
E1-S, DHEA-S, SHBG and TBG were
significantly decreased by the high-iso diet (P = 0.04,
0.008, 0.01, 0.04, 0.01, and 0.003, respectively). Comparisons between the baseline and the isoflavone-free control diet presumably reflect nonisoflavone components of soy, although this too is complicated by differences in nutrient intake. FSH, SHBG, and TBG were significantly decreased by the control diet, when compared with baseline (P = 0.04, 0.003 and 0.03, respectively). No significant differences between baseline and control diet were observed for any other hormones.
Vaginal cytology was not significantly affected by consumption of isoflavones or other soy components. Maturation values were 38.4 ± 4.9; 41.3 ± 4.0; 36.4 ± 4.8; and 35.0 ± 4.6 (lsmean ± SE) for the baseline, control, low-iso and high-iso diets, respectively.
Endometrial biopsy results were not significantly different between the baseline and the high-iso diet. Endometrial pathology reports indicated that eight subjects were inactive at both time points, four subjects were proliferative at both time points, and two subjects were proliferative at baseline and inactive on the high-iso diet.
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Of the 18 plasma hormones and hormone-binding globulins measured, we found relatively small, but significant effects of isoflavone consumption on E1-S, testosterone, DHEA-S, SHBG, and TBG. The lowering of E1-S by the high-iso diet, in combination with trends toward lower E2 and E1, may be due to decreased peripheral aromatization of androgens, the primary source of estrogens in postmenopausal women. This finding is consistent with in vitro studies demonstrating inhibition of aromatase enzyme activity in preadipocytes exposed to phytoestrogens (24, 25). The SHBG results are consistent with epidemiologic studies showing a positive association between urinary isoflavones and plasma SHBG (7, 8) and cell culture work showing stimulation of SHBG synthesis by isoflavones in HepG2 liver carcinoma cells (26, 27). Along with a possible trend towards higher prolactin concentrations with isoflavone consumption, the SHBG results suggest an estrogenic effect of isoflavones. Nonetheless, both the estrogen and SHBG effects are too modest to suggest physiological importance. There are no obvious explanations or physiological relevance that can be attached to the increases in testosterone, DHEA-S and TBG seen with the low-iso diet.
Although the focus of the current study was on isoflavone effects, comparisons between the baseline values and the low-iso or high-iso diets reflect the combined effects of soy isoflavones and other components of soy. Despite differences in nutrient intake, and a lack of control during baseline data collection, such comparisons are comparable to those reported in previous soy intervention studies. Along these same lines, comparisons between the baseline values and the isoflavone-free control diet may reflect the effects on non-isoflavone components of soy. Compared with baseline, the concentrations of all three estrogens studied were modestly but significantly decreased by the high-iso diet, and a dose response was apparent in that for the low-iso diet, estrogen values were intermediate. Presumably, these results were due primarily to the effects of isoflavones on peripheral aromatization, and our results suggested no interference with this effect by nonisoflavone components of soy. These data are consistent with earlier studies that have reported statistically insignificant declines in E2 in postmenopausal women consuming soy (10) or a combination of soy and flaxseed (13).
Other hormone differences between baseline and the isoflavone-containing diets, even when statistically significant, were also modest in degree and unlikely to be physiologically important. Most notably, despite the apparent higher SHBG with increasing isoflavone intake, SHBG values during all three diet periods were actually lower than those at baseline. This may suggest effects of nonisoflavone soy components on SHBG. It is also possible that this effect is related to differences in nutrient consumption between baseline and the three diet periods, although the changes in fat, fiber and the protein/carbohydrate ratio were far less than those reported to influence plasma hormone and SHBG concentrations (28, 29, 30, 31). Consistent with our results, Baird et al. (10) reported a statistically insignificant decline in SHBG in postmenopausal women consuming soy. In contrast, another study reported a significant but fairly modest increase in SHBG in postmenopausal women consuming soy and flaxseed (13). It is also noteworthy that FSH was decreased by the control and the low-iso diets. Although these changes may be considered estrogenic, the small magnitude suggests minimal biological significance.
Vaginal cytology results revealed no significant effects of soy or isoflavone consumption. These data are consistent with results of other soy feeding studies (10, 11), but inconsistent with those of Wilcox et al. (14), who noted marginally but significantly increased maturation values following diets supplemented with soy flour, red clover sprouts and flaxseed for a total of six weeks. These results, along with the negative endometrial biopsy results, suggest that it is unlikely that soy or isoflavones exert clinically important estrogenic effects on the vaginal epithelium or endometrium. This is in contrast to the effects of small doses of estrogen used in hormone replacement therapy (32, 33) and the effects of tamoxifen on the vaginal epithelium and endometrium (34, 35).
Overall, the current study provides evidence for small effects of isoflavones and possibly non-isoflavone components of soy on plasma hormone concentrations in postmenopausal women. Our previous study of premenopausal women, using a comparable design, yielded largely similar results, in that the only hormonal changes noted were quite modest in degree (16). It is important to note that the small degree of these effects does not account for the probable benefits of soy consumption on estrogen-responsive conditions, such as cardiovascular disease. Our findings in no way rule out important isoflavone effects on E2-responsive gene activation or estrogen-like effects unrelated to estrogen receptor binding. Rather, our results likely illustrate the complexity of action of estrogen and estrogen-like compounds. Estrogen-responsive end organs clearly differ in their sensitivity and responses to estrogens, as a result of differing expression of estrogen receptor subtypes, differing actions of other proteins involved in gene expression (such as estrogen receptor corepressors and coactivators), and other poorly understood factors (36).
More studies are needed before dietary recommendations involving soy and/or isoflavones can be made to postmenopausal women looking for an alternative to hormone replacement therapy. Certainly, from the standpoint of cardiovascular disease and perhaps osteoporosis, soy and isoflavones appear to be promising, and our findings suggest that soy consumption is unlikely to cause endometrial hyperplasia or cancer. Though speculative, we also suspect that soy consumption would not cause pelvic pain symptoms in postmenopausal women with known endometriosis, as may occur with conventional hormone replacement therapy. On the other hand, our findings and those of others suggest that isoflavones will not effectively treat symptoms due to vaginal atrophic changes. Moreover, the use of isoflavone-rich soy to alleviate vasomotor symptoms has met with limited success (11, 13, 15).
In summary, this study suggests only modest effects of isoflavones on plasma hormones in postmenopausal women, and no significant effects on vaginal cytology or endometrial biopsy results. Thus, effects of isoflavones on plasma hormones per se are not likely to be significant mechanisms by which soy exerts estrogen-like effects in postmenopausal women. It is also unlikely that isoflavones or soy exert clinically important estrogenic effects on vaginal epithelium or endometrium. More studies are needed before dietary recommendations involving soy and/or isoflavones can be made to postmenopausal women looking for an alternative to hormone replacement therapy.
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Acknowledgments |
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Footnotes |
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Received June 4, 1999.
Accepted June 26, 1999.
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References |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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5-5
-reduction of
testosterone and the cytochrome P-450-dependent oxidation of estradiol
by dietary macronutrients in man. Proc Natl Acad Sci USA. 80:7646–7649.This article has been cited by other articles:
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 V. Jayagopal, P. Albertazzi, E. S. Kilpatrick, E. M. Howarth, P. E. Jennings, D. A. Hepburn, and S. L. Atkin Beneficial Effects of Soy Phytoestrogen Intake in Postmenopausal Women With Type 2 Diabetes Diabetes Care, October 1, 2002; 25(10): 1709 - 1714. [Abstract] [Full Text] [PDF]
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Q. Dai, A. A. Franke, F. Jin, X.-O. Shu, J. R. Hebert, L. J. Custer, J. Cheng, Y.-T. Gao, and W. Zheng Urinary Excretion of Phytoestrogens and Risk of Breast Cancer among Chinese Women in Shanghai Cancer Epidemiol. Biomarkers Prev., September 1, 2002; 11(9): 815 - 821. [Abstract] [Full Text] [PDF]
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A. H. Wu, F. Z. Stanczyk, A. Seow, H.-P. Lee, and M. C. Yu Soy Intake and Other Lifestyle Determinants of Serum Estrogen Levels among Postmenopausal Chinese Women in Singapore Cancer Epidemiol. Biomarkers Prev., September 1, 2002; 11(9): 844 - 851. [Abstract] [Full Text] [PDF]
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D. J. Jenkins, C. W. Kendall, C.-J. C Jackson, P. W Connelly, T. Parker, D. Faulkner, E. Vidgen, S. C Cunnane, L. A Leiter, and R. G Josse Effects of high- and low-isoflavone soyfoods on blood lipids, oxidized LDL, homocysteine, and blood pressure in hyperlipidemic men and women Am. J. Clinical Nutrition, August 1, 2002; 76(2): 365 - 372. [Abstract] [Full Text] [PDF]
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