Background: Increased insulin-like growth factor (IGF)-I and IGF-II concentrations are related to increased colorectal cancer risk. Isoflavones have been associated with reduced colorectal cancer risk, and may affect the IGF system because of their weak estrogenic activity. The aim of the study was to investigate the effect of isolated isoflavones on serum concentrations of IGF system components.

Materials and Methods: We conducted a randomized, placebo-controlled, double-blinded, crossover trial in four hospitals in the Netherlands to investigate the effect of an 8-week supplementation with red clover–derived isoflavones (84 mg/d) on serum IGF-I concentrations. In addition, serum concentrations of IGF-II and IGF binding proteins (IGFBP)-1, IGFBP-2, and IGFBP-3 were assessed. Normal colorectal tissue biopsies were obtained after the first intervention period and mRNA expression of IGF-I, IGF-II, IGFBP-3, and IGF-IR was evaluated. Our study population consisted of 34 postmenopausal women with a family history of colorectal cancer or a personal history of colorectal adenomas.

Results: Isoflavone supplementation did not significantly affect serum concentrations of total IGF-I (mean relative within-person difference; IGF-I, −2.0%; 95% confidence interval, −8.0% to 3.9%). IGF-II and IGFBPs were also not significantly altered after isoflavone supplementation. Colorectal tissue mRNA expression of IGF system components did not significantly differ between individuals on isoflavone supplementation and those who received placebo.

Conclusions: The results of our trial, supported by a qualitative review of soy trials published to date, suggest that isoflavones do not significantly affect circulating levels of IGF system components. Increased levels of IGF-I, as observed in most of these trials, are likely due to simultaneous protein supplementation. (Cancer Epidemiol Biomarkers Prev 2008;17(10):2585–93)

Evidence is accumulating for a protective effect of estrogenic substances against colorectal cancer (1). In line with these observations, the incidence of colorectal cancer is consistently lower in women than in men. Sex hormone replacement therapy in postmenopausal women has been shown to decrease colorectal cancer risk as well as the development of colorectal adenomas (2-4). Both sex hormone replacement therapy (5, 6) and selective estrogen receptor modulators, e.g., tamoxifen (7, 8), have been shown to reduce serum insulin-like growth factor-I (IGF-I) concentrations.

IGF-I and IGF-II are involved in cell proliferation and apoptosis, and are important in both normal and tumor growth (9). Prospective epidemiologic studies have shown that relatively high circulating concentrations of IGF-I and IGF-II are associated with increased colorectal cancer risk (10). IGF binding protein-3 (IGFBP-3), which binds 90% of circulating IGF-I in humans, is not related to risk of colorectal cancer (10, 11). However, relatively low concentrations of IGFBP-1 and IGFBP-2, which may enhance IGF bioavailability, might be associated with increased colorectal cancer risk (12, 13). Experimental evidence from mouse models has supported the rationale for cancer prevention through lowering the circulating levels of (bioavailable) IGF-I (14).

The lower incidence of several cancers including colorectal cancer in Asian countries has been attributed to the substantially higher consumption of soy foods in these countries (15, 16). Isoflavones, the main bioactive substances in soy, are a class of phytoestrogens that structurally resemble estrogens and also possess weak estrogenic activity (17). Isoflavones have been hypothesized to influence colorectal cancer risk, although data reported on this subject have thus far been inconclusive (18). In vitro and in vivo animal studies have shown that (soy) isoflavones may decrease (circulating) IGF-I concentrations (19). For humans, however, the results from various intervention studies are conflicting, possibly due to opposite effects of soy protein and soy isoflavones on serum total IGF-I concentrations (14).

We hypothesized that isolated isoflavones, through their estrogenic properties, may induce a reduction of (bioavailable) IGF-I in the circulation in postmenopausal women. Such endocrine changes may be accompanied by changes in the expression of IGF system components in normal colorectal tissue. In this randomized, controlled, crossover trial, we investigated the effect of isolated, red clover–derived isoflavone supplementation (84 mg/d) for 2 months on serum concentrations of IGF-I, IGF-II, and IGFBP-1, IGFBP-2, and IGFBP-3, as well as tissue mRNA expression of several components of the IGF system in postmenopausal women at increased risk of colorectal cancer that could potentially benefit most from this intervention.

Study Population

We selected women aged 50 to 75 years with a personal history of colorectal adenomas or at least one first-degree family member with a history of colorectal cancer. All women were postmenopausal, i.e., no menstrual cycles in the past 12 months. In case of hysterectomy, postmenopausal status was confirmed on the basis of serum follicle-stimulating hormone levels. Asymptomatic women scheduled to undergo a colonoscopy for surveillance purposes were selected from the medical registries and pathology databases, and were sent an invitation letter for participation in our study. Exclusion criteria were a history of cancer, familial adenomatous polyposis syndrome, Li-Fraumeni syndrome, chronic inflammatory bowel disease, diabetes mellitus, acromegaly, significant liver or renal disease, (partial) bowel resection, nonremissive celiac disease, diverticulitis, other severe comorbidity, laxative abuse, and the use of food supplements containing isoflavones.

Participants were recruited between November 2003 and December 2005. In total, 182 women were invited to participate in this trial (Fig. 1A). Of 138 eligible women, 39 were included in the present trial and 5 were assigned to another trial (32% response). We obtained written informed consent from all participants.

The study was conducted in four hospitals in the Netherlands: the Antoni van Leeuwenhoek Hospital in Amsterdam, the Gelderse Vallei Hospital in Ede, the Slotervaart Hospital in Amsterdam, and the Sint Antonius Hospital in Nieuwegein. The study protocol was approved by the medical ethical committees of all participating centers.

Design

We conducted a randomized, placebo-controlled, double-blinded crossover study (Fig. 1B). The total duration of the study was approximately 6 months, consisting of two 8-week intervention periods, separated by an 8-week wash-out period. Surveillance colonoscopies were always planned at the end of the first intervention period. Subjects were allocated to receive isoflavone tablets in the first intervention period and placebo tablets in the second intervention period (isoflavones-placebo, the I-P group) or vice versa (placebo-isoflavones, the P-I group), according to a randomization scheme with permuted blocks. The isoflavone tablets (Promensil, Novogen) contained an isoflavone extract derived from red clover with 42 mg of total isoflavones (25 mg biochanin, 8 mg formononetin, 4 mg genistein, and 5 mg daidzein). Subjects were asked to take two tablets per day, one with breakfast and one with dinner (total dose, 84 mg isoflavones/d). Subjects were asked to maintain their habitual lifestyle and diet, including non–isoflavone supplement use.

The sample size calculation for this randomized controlled trial was based on a pilot study in six individuals in which we observed a within-person coefficient of variation of 11% in total IGF-I concentrations in multiple serum samples drawn 3 to 12 months apart. This resulted in a required sample size of 26 participants to detect a 10% difference in serum total IGF-I concentrations between the treatment groups with 90% power.

Data and Sample Collection

Study procedures and data collection were identical to the male counterpart of this study described elsewhere (20). In summary, subjects visited the hospital at the beginning and end of both intervention periods, when body weight, and waist and hip circumference were measured. Dietary intake on the day preceding the visit was assessed using a 24-h recall, according to a standard protocol for interviewing and coding. Habitual physical activity over the 2 months preceding each visit was assessed using a validated self-administered short questionnaire. Fasting blood samples were obtained at all four time points. Fasting serum and EDTA-plasma samples were frozen and stored at −30°C until further analysis.

A surveillance colonoscopy was scheduled at the second visit, i.e., at the end of the first intervention period, after whole-gut lavage with 4 L of macrogol [Klean-Prep (Norgine BV) or Coloforte (Ipsen Farmaceutica BV)]. Biopsies from macroscopically normal mucosa were collected from the ascending colon and the rectum, and were immediately snap-frozen in liquid nitrogen and stored at −70°C until further analysis.

During both intervention periods, subjects kept a daily notebook in which they recorded information about their health, medicine use, smoking, study tablets taken, and consumption of foods rich in isoflavones. Compliance was measured by evaluating the participants' own records and by counting the number of returned tablets.

Serum Analyses

Serum total IGF-I, total IGF-II, IGFBP-1, IGFBP-2, and IGFBP-3 concentrations were measured at the end of both intervention periods. Serum total IGF-I was measured using an immunometric technique on the Immulite 1000 analyzer (Diagnostics Products Corporation). The sensitivity established in our laboratory was 12.0 μg/L, intra-assay coefficients of variation were <4.0% at 45, 150, and 370 μg/L mean serum IGF-I, and inter-assay coefficients of variation were 7.0%, 6.5%, and 7.0% at 45, 150, and 370 μg/L mean serum IGF-I, respectively. Serum IGF-II concentrations were determined in Sep-Pak C18 extracts of serum by RIA, and serum IGFBP-1, IGFBP-2, and IGFBP-3 were determined by specific RIAs. All assays were done in the same laboratory. Further details have been described previously (20).

Because the biological effectiveness of isoflavones may depend on the individual ability to biotransform isoflavone metabolites to the more potent estrogenic metabolite equol (21), equol concentrations were measured in serum samples collected after isoflavone intervention (J. Lampe; Fred Hutchinson Cancer Research Center, Seattle, WA). TR-FIA kits (Labmaster) were used as described previously (22) and fluorescence was measured with a Victor 2 model 1420 spectrofluorometer (Wallac). The sensitivity was 3.3 nmol/L and the intra-assay coefficient of variation was 6.8% at a mean equol concentration of 295 nmol/L. The IGF system may also be influenced by changes in estradiol and sex hormone-binding globulin (SHBG). Therefore, we also determined estradiol and SHBG concentrations by electrochemiluminescence immunoassays using the E170 (Elecsys module) immunoanalyzer (Roche Diagnostics).

Quantitative PCR

Total RNA was extracted from the tissue samples using RNAzolB (Campro Scientific). Total RNA (2.5 μg) was reverse-transcribed to generate first-strand cDNA (total volume 50 μL) using random hexamers and Superscript II. After 10 min at room temperature for extension of the hexameric primers, the reverse transcriptase reaction was done at 42°C for 60 min, followed by heating at 98°C for 5 min. cDNA was diluted 1:1 with RNase-free water before real-time reverse transcription-PCR.

Quantitative PCR was used to assess the levels of mRNA expression of IGF-I, IGF-II, IGFBP-3, and IGF-IR in tissue biopsies obtained from the ascending colon (n = 34) and rectum (n = 34). Primers and probes for these reactions were designed using Primer Express software (Applied Biosystems, PE; Table 1). Primers were chosen in two adjacent exons, and the fluorescent-labeled probes were selected to partially encompass both exons, to avoid DNA contamination and amplification of the homologous insulin and insulin receptor genes. PCRs were carried out using the 7500 Fast Real-time PCR System (Applied Biosystems) according to the instructions of the manufacturer (50 cycles). The content of IGF-I, IGF-II, IGFBP-3, and IGF-IR transcripts was normalized to the content of the “housekeeping gene” β-actin. A second housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), was assessed to evaluate the validity of normalization to β-actin content. Because the expression levels of both genes were highly correlated (Spearman rank correlation coefficient, r = 0.91; P < 0.001), all results were normalized to β-actin (GAPDH data not shown). For β-actin and GAPDH, standard PCR primer and probe mixtures (Applied Biosystems) were used under the same conditions as described above. Standard curves were generated using serially diluted solutions of cDNA obtained from pooled amplified RNA from 82 breast tumors. All PCR assays were conducted in duplicate for each sample.

Statistical Analyses

The main variable of interest in our statistical analysis was the relative crossover difference in the total serum IGF-I concentration after isoflavone supplementation (see formulas below), expressed as a percentage relative to the concentration after placebo treatment. Relative crossover differences in serum concentrations of IGF-II, IGFBP-1, IGFBP-2, and IGFBP-3 were secondary end points. As the sample size of the two randomized groups differed (i.e., I-P group, n = 15; P-I group, n = 19), for both randomized groups, the mean crossover difference for each IGF system component was calculated and then pooled over the two groups, to adjust for period effects. We tested whether the pooled crossover difference significantly deviated from null with a t test using the standard error of the pooled crossover differences (23). As IGFBP-1 and estradiol were not normally distributed, we tested whether the median crossover differences significantly deviated from null using a sign test. Absolute differences between serum concentrations after isoflavones and after placebo were tested using paired t tests (total IGF-I, IGFBP-2, IGFBP-3, SHBG) or Wilcoxon signed ranks tests (IGFBP-1, estradiol).

\[\begin{array}{c}\mathrm{Relative\ cross{\mbox{--}}over\ difference[{\Delta}(I{-}P)\ or\ {\Delta}(P{-}I)]}\ =\ [\mathrm{concentration\ after\ intervention}\ (\mathrm{C}_{I}){-}\mathrm{concentration\ after\ placebo}\ (\mathrm{C}_{P})]/\mathrm{C}_{P}\\\mathrm{Pooled\ cross{\mbox{--}}over\ difference}\ =\ 1/2\ (\mathrm{mean}_{{\Delta}(\mathrm{I{-}P})}\ +\ \mathrm{mean}_{{\Delta}(\mathrm{P{-}I})})\\\mathrm{SE\ of\ the\ pooled\ cross{\mbox{--}}over\ difference}\ =\ 1/2\sqrt{[}(s^{2}/n_{\mathrm{I{-}P}})\ +\ (s^{2}/n_{\mathrm{P{-}I}})]\\\mathrm{where}\ s^{2}\ =\ [(n_{\mathrm{I{-}P}}{-}1)\mathrm{SD}_{{\Delta}\mathrm{I{-}P}}^{2}\ +\ (n_{\mathrm{P{-}I}}{-}1)\mathrm{SD}_{{\Delta}\mathrm{P{-}I}}^{2}]/(n_{\mathrm{I{-}P}}\ +\ n_{\mathrm{P{-}I}}{-}2)\end{array}\]

To evaluate whether the relative crossover differences correlated with serum equol concentrations after isoflavone supplementation, a Spearman correlation coefficient was calculated.

Differences in mRNA expression of the IGF-I, IGF-II, IGF-IR, and IGFBP-3 genes in normal colorectal tissue biopsies taken after the first intervention period were compared between individuals who received isoflavone intervention (I-P group) and individuals on placebo (P-I group). Natural log-transformed data were used to normalize the data, and either two-sample t tests (IGF-I, IGF-II, IGFBP-3) or nonparametric Mann-Whitney tests (IGF-IR) were done.

Descriptive characteristics were computed for both randomized groups separately. We calculated whether relevant changes occurred in dietary and lifestyle factors known to influence the IGF system, i.e., dietary intake of macronutrients, body weight, waist and hip circumference, total physical activity score, dietary intake of products relatively rich in isoflavones, during the study period for both randomized groups separately.

P values were determined by two-sided tests, and differences were considered to be statistically significant at P < 0.05. Statistical analyses were done using SPSS 12.0 (SPSS, Inc.).

Thirty-four women finished the complete study protocol [n = 15 on isoflavones-placebo (I-P group); n = 19 on placebo-isoflavones (P-I group); Fig. 1 A]. Both groups were similar with respect to age (Table 2). However, the P-I group was slightly more overweight and consisted of more women who had never smoked. The number of participants with a family history of colorectal cancer and/or a personal history of colorectal adenomas was equally distributed among the two groups. Hormonal factors (i.e., age at menopause, parity, past use of hormone replacement therapy) also did not markedly differ between the two groups.

Isoflavone supplementation did not significantly affect serum total IGF-I and IGF-II concentrations (mean relative difference between isoflavones and placebo: IGF-I, −2.0%; 95% confidence interval, −8.0% to 3.9%; IGF-II, −1.7%; 95% confidence interval, −6.3% to 2.8%; Table 3). Additionally, the median mean relative differences in IGFBP-1, IGFBP-2, and IGFBP-3 between isoflavones and placebo did not significantly deviate from zero. However, substantial interindividual variation in IGFBP-1 changes were observed, ranging from reductions of 50% to increases of up to 500%. Interindividual variations in IGFBP-2 changes were smaller but still substantial. No correlation was found between the change in serum IGF-I concentrations and serum equol concentrations (r = −0.03, P = 0.88). Serum estradiol and SHBG were also not significantly altered by the isoflavone intervention.

In a parallel analysis, we studied tissue mRNA expression of the IGF-I, IGF-II, IGFBP-3, and IGF-IR genes measured in normal colorectal tissue biopsies taken after the first intervention period. No statistically significant differences were observed in the mRNA levels of genes between individuals on the isoflavone intervention and individuals on placebo, neither in the ascending colon nor in the rectum (Table 4).

Based on both returned tablet counts and recordings in the daily notebooks, 91% of participants (n = 31) were compliant (≥80% of tablets taken) during both the isoflavone and the placebo intervention period. Excluding participants who were not compliant (n = 3) did not markedly affect the results (data not shown). Body weight, waist and hip circumference, total physical activity score, dietary macronutrient intake, and the number of days on which products rich in isoflavones were consumed did not materially differ between the isoflavone and the placebo intervention period (data not shown).

In our randomized, placebo-controlled, double-blinded, crossover trial, red clover–derived isolated isoflavone supplementation of 84 mg/d for 2 months neither influenced circulating concentrations of IGF-I and IGF-II nor those of IGFBP-1, IGFBP-2, and IGFBP-3 in postmenopausal women at increased risk of colorectal cancer. In addition, colorectal tissue mRNA expression of IGF-I, IGF-II, IGF-IR, and IGFBP-3 did not differ between individuals on isoflavones and individuals on placebo.

This is the first randomized controlled trial investigating the effects of isolated isoflavones on circulating IGF system components in postmenopausal women at increased risk of colorectal cancer. Populations at increased cancer risk, such as our study population, are likely to benefit most from interventions aimed at lowering circulating IGF-I levels and subsequently suppressing IGF-IR signaling. We used a crossover design, which has the important advantage that our results were not affected by the generally high between-individual variation in circulating IGF-I concentrations relative to the much lower within-individual variation.

The number of participants for our study was in concordance with the a priori design (i.e., n ≥ 26). The dropout rate, although 20% had been anticipated, was 13% and unrelated to supplement intake. This resulted in an adequately powered trial to evaluate isoflavone effects (i.e., a 10% decrease) on serum IGF-I levels. Compliance, based on tablet counts and daily notebooks, was high (91%). Although serum genistein concentrations were not measured in the current trial, a strong increase in serum genistein concentrations was observed in the majority of men in the male counterpart of this trial after similar isoflavone intervention with comparable compliance (20). In that trial, serum genistein concentrations were within the range of those of subjects traditionally consuming a high-isoflavone soy-based diet (24). Furthermore, we assessed factors which are thought to potentially influence circulating concentrations of IGFs and IGFBPs (e.g., total energy and protein intake, consumption of products rich in isoflavones and lycopene, body weight, and physical activity). Because these factors did not materially change during the course of the study, any potential changes in various parameters of the IGF system would have been attributable to the isoflavone intervention. In principle, the 2-month duration of isoflavone supplementation as used in our study would have been sufficient to affect circulating IGF system components because previous human intervention studies on oral estrogens and selective estrogen receptor modulators have shown a decrease of ∼15% to 30% in serum IGF-I levels within 2 months (5, 7). In prospective epidemiologic studies, serum IGF-I concentrations for individuals in the bottom quartiles were >50% lower than for individuals in the top quartiles who were at increased risk of developing colorectal cancer (10). Although it is unknown what individual percentage of change in serum IGF-I levels is required to ultimately decrease colorectal cancer risk, we hypothesize that smaller changes, as observed in the intervention studies discussed above, may also be relevant. In the male counterpart of this trial, we observed a negative correlation between the change in serum IGF-I concentrations and serum equol concentrations (20). These results suggest that isoflavones may lower serum IGF-I concentrations only in men who are capable of metabolizing daidzein into equol. However, our results in women do not indicate a difference between equol producers and nonproducers. This may be due to differences in hormonal background, and is confirmed by studies in mice and rats that metabolize daidzein mainly to equol (21). In male mice and rats, physiologic dietary intakes of soy decreased serum IGF-I concentrations, whereas in female mice and rats this effect was not observed (25-28).

To date, 13 intervention studies in humans have evaluated the effect of isoflavone supplementation on circulating levels of IGF system components (summarized in Table 5; refs. 20, 29-40). Besides the study described in this article, only two other randomized, crossover trials evaluated the effects of isolated, red clover–derived isoflavones (20, 35). In both studies, no overall effect of isoflavones on circulating IGF system components was observed, which is in agreement with the results in our study. In eight randomized, controlled studies, an intervention with soy foods or isolated soy protein containing isoflavones was compared with either control foods, isolated milk protein, or soy protein without isoflavones. In all of these studies, the protein intake in both the intervention group and the control group had markedly increased. Consequently, six out of eight studies observed increased blood levels of IGF-I in both groups (29-32, 36, 40), which is likely due to IGF-I increasing the effects of essential amino acids in animal and soy protein (41, 42). In only one study the increase in IGF-I levels was significantly greater for the soy protein compared with the milk protein intervention group (30). Based on this qualitative review, it can be concluded that isoflavones do not substantially increase or decrease blood levels of IGF-I.

To our knowledge, studies on the effects of isoflavone supplementation on mRNA expression of IGF system components in normal human colorectal tissue have not been previously reported. Our data reveal that mRNA expression levels of IGF-I, IGF-II, IGF-IR, and IGFBP-3 in the ascending colon and the rectum do not differ between individuals on isoflavones and individuals on placebo. In contrast, in vitro studies using androgen-responsive prostate cancer cells showed that genistein, daidzein, and equol all consistently inhibit IGF-IR mRNA expression, even at relatively low levels of exposure (i.e., being comparable to human dietary intake; refs. 43, 44). In cultured HT-29 human colon cancer cells, genistein, but not daidzein, was found to reduce IGF-IR protein levels and inhibit IGF-IR signaling at relatively high pharmacologic concentrations (45). Although we could not confirm this finding for normal colorectal tissue in vivo, it must be emphasized that in our study, the effects of isoflavones were evaluated between individuals and not within individuals because only one colonoscopy was done in each participant. Substantial variability in tissue mRNA expression levels in colorectal biopsies of the same individuals sampled at different colorectal locations may occur, with a markedly higher expression level of several IGF system components in the rectum as compared with the proximal colon (46). Therefore, future studies on this subject should focus on multiple colonoscopies with multiple biopsies at fixed locations in each participant, preferably in a crossover design.

Adams et al. evaluated soy isoflavone effects on colorectal epithelial cell proliferation in a parallel trial (n = 91, men and women) comparing soy-protein powder (83 mg isoflavones/d) with ethanol-extracted soy-protein powder (containing 3 mg isoflavones/d). A colonoscopy was done before and after the 12-month intervention period. No reduction in proliferation was observed. Unexpectedly, in the sigmoid colon, an increase in cell proliferation measures was found, which was opposite to what was hypothesized (47). As mentioned previously, isoflavones as constituents of a soy protein food may have different or even opposite effects as the effects hypothesized for isolated isoflavones. Unfortunately, we were not able to study the proliferation markers in our tissue specimens because the biopsies were not oriented optimally to allow cutting slides that contained full-length crypts. The latter is required for the determination of epithelial cell proliferation (labeling index).

In conclusion, isolated isoflavones did not influence serum concentrations and tissue mRNA expression of IGF system components in our randomized, placebo-controlled, double-blind intervention study in postmenopausal women at increased risk of colorectal cancer. These results suggest that the increased serum IGF-I concentrations as observed in several previous studies investigating soy food or soy protein supplementation are most likely due to soy protein itself, and not to isoflavones. Potential effects of isoflavones on IGF-IR signaling in the colon and rectum, but also in other potential target tissues such as breast and prostate need further study.

No potential conflicts of interest were disclosed.

Grant support: Dutch Cancer Society, grant no NKI 2001-2579. The isoflavone and placebo supplements were kindly supplied by Novogen (North Ryde, Sydney, Australia).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

We thank all participants for their efforts and dedication, and all study personnel for their assistance in participant recruitment, sample handling and data collection; E. Siebelink for training and assistance in coding of 24-h recalls; M. Buning, O. Dalesio, J. Lampe, D. Linders, and O. Van Tellingen for technical support; and the gastroenterologists, medical secretaries, and endoscopy nurses for assistance and biopsy sampling during colonoscopy.

1
al Azzawi F, Wahab M. Estrogen and colon cancer: current issues.
Climacteric
2002
;
5
:
3
–14.
2
Nelson HD, Humphrey LL, Nygren P, Teutsch SM, Allan JD. Postmenopausal hormone replacement therapy: scientific review.
JAMA
2002
;
288
:
872
–81.
3
Rossouw JE, Anderson GL, Prentice RL, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Women's Health Initiative randomized controlled trial.
JAMA
2002
;
288
:
321
–33.
4
Terry MB, Neugut AI, Bostick RM, et al. Risk factors for advanced colorectal adenomas: a pooled analysis.
Cancer Epidemiol Biomarkers Prev
2002
;
11
:
622
–9.
5
Weissberger AJ, Ho KK, Lazarus L. Contrasting effects of oral and transdermal routes of estrogen replacement therapy on 24-hour growth hormone (GH) secretion, insulin-like growth factor I, and GH-binding protein in postmenopausal women.
J Clin Endocrinol Metab
1991
;
72
:
374
–81.
6
Decensi A, Bonanni B, Baglietto L, et al. A two-by-two factorial trial comparing oral with transdermal estrogen therapy and fenretinide with placebo on breast cancer biomarkers.
Clin Cancer Res
2004
;
10
:
4389
–97.
7
Decensi A, Bonanni B, Guerrieri-Gonzaga A, et al. Biologic activity of tamoxifen at low doses in healthy women.
J Natl Cancer Inst
1998
;
90
:
1461
–7.
8
Shewmon DA, Stock JL, Rosen CJ, et al. Tamoxifen and estrogen lower circulating lipoprotein(a) concentrations in healthy postmenopausal women.
Arterioscler Thromb
1994
;
14
:
1586
–93.
9
Khandwala HM, McCutcheon IE, Flyvbjerg A, Friend KE. The effects of insulin-like growth factors on tumorigenesis and neoplastic growth.
Endocr Rev
2000
;
21
:
215
–44.
10
Renehan AG, Zwahlen M, Minder C, O'Dwyer ST, Shalet SM, Egger M. Insulin-like growth factor (IGF)-I, IGF binding protein-3, and cancer risk: systematic review and meta-regression analysis.
Lancet
2004
;
363
:
1346
–53.
11
Morris JK, George LM, Wu T, Wald NJ. Insulin-like growth factors and cancer: no role in screening. Evidence from the BUPA study and meta-analysis of prospective epidemiological studies.
Br J Cancer
2006
;
95
:
112
–7.
12
Kaaks R, Toniolo P, Akhmedkhanov A, et al. Serum C-peptide, insulin-like growth factor (IGF)-I, IGF-binding proteins, and colorectal cancer risk in women.
J Natl Cancer Inst
2000
;
92
:
1592
–600.
13
Wei EK, Ma J, Pollak MN, et al. A prospective study of C-peptide, insulin-like growth factor-I, insulin-like growth factor binding protein-1, and the risk of colorectal cancer in women.
Cancer Epidemiol Biomarkers Prev
2005
;
14
:
850
–5.
14
Voskuil DW, Vrieling A, van't Veer LJ, Kampman E, Rookus MA. The insulin-like growth factor system in cancer prevention: potential of dietary intervention strategies.
Cancer Epidemiol Biomarkers Prev
2005
;
14
:
195
–203.
15
Adlercreutz H, Mazur W. Phyto-oestrogens and Western diseases.
Ann Med
1997
;
29
:
95
–120.
16
Sung JJ, Lau JY, Goh KL, Leung WK. Increasing incidence of colorectal cancer in Asia: implications for screening.
Lancet Oncol
2005
;
6
:
871
–6.
17
Setchell KD, Cassidy A. Dietary isoflavones: biological effects and relevance to human health.
J Nutr
1999
;
129
:
758
–67S.
18
Lechner D, Kallay E, Cross HS. Phytoestrogens and colorectal cancer prevention.
Vitam Horm
2005
;
70
:
169
–98.
19
Voskuil DW, Bosma A, Vrieling A, Rookus MA, Veer van 't LJ. Insulin-like growth factor (IGF)-system mRNA quantities in normal and tumor breast tissue of women with sporadic and familial breast cancer risk.
Breast Cancer Res Treat
2004
;
84
:
225
–33.
20
Vrieling A, Rookus MA, Kampman E, et al. Isolated isoflavones do not affect the circulating insulin-like growth factor system in men at increased colorectal cancer risk.
J Nutr
2007
;
137
:
379
–83.
21
Setchell KD, Brown NM, Lydeking-Olsen E. The clinical importance of the metabolite equol—a clue to the effectiveness of soy and its isoflavones.
J Nutr
2002
;
132
:
3577
–84.
22
Brouwers E, L'homme R, Al Maharik N, et al. Time-resolved fluoroimmunoassay for equol in plasma and urine.
J Steroid Biochem Mol Biol
2003
;
84
:
577
–88.
23
Hills M, Armitage P. The two-period cross-over clinical trial.
Br J Clin Pharmacol
1979
;
8
:
7
–20.
24
Howes J, Waring M, Huang L, Howes LG. Long-term pharmacokinetics of an extract of isoflavones from red clover (Trifolium pratense).
J Altern Complement Med
2002
;
8
:
135
–42.
25
Zhou JR, Gugger ET, Tanaka T, Guo Y, Blackburn GL, Clinton SK. Soybean phytochemicals inhibit the growth of transplantable human prostate carcinoma and tumor angiogenesis in mice.
J Nutr
1999
;
129
:
1628
–35.
26
Aukema HM, Housini I. Dietary soy protein effects on disease and IGF-I in male and female Han:SPRD-cy rats.
Kidney Int
2001
;
59
:
52
–61.
27
Arjmandi BH, Khalil DA, Hollis BW. Soy protein: its effects on intestinal calcium transport, serum vitamin D, and insulin-like growth factor-I in ovariectomized rats.
Calcif Tissue Int
2002
;
70
:
483
–7.
28
Zhou JR, Yu L, Mai Z, Blackburn GL. Combined inhibition of estrogen-dependent human breast carcinoma by soy and tea bioactive components in mice.
Int J Cancer
2004
;
108
:
8
–14.
29
Wangen KE, Duncan AM, Merz-Demlow BE, et al. Effects of soy isoflavones on markers of bone turnover in premenopausal and postmenopausal women.
J Clin Endocr Metab
2000
;
85
:
3043
–8.
30
Khalil DA, Lucas EA, Juma S, Smith BJ, Payton ME, Arjmandi BH. Soy protein supplementation increases serum insulin-like growth factor-I in young and old men but does not affect markers of bone metabolism.
J Nutr
2002
;
132
:
2605
–8.
31
Adams KF, Newton KM, Chen C, et al. Soy isoflavones do not modulate circulating insulin-like growth factor concentrations in an older population in an intervention trial.
J Nutr
2003
;
133
:
1316
–9.
32
Arjmandi BH, Khalil DA, Smith BJ, et al. Soy protein has a greater effect on bone in postmenopausal women not on hormone replacement therapy, as evidenced by reducing bone resorption and urinary calcium excretion.
J Clin Endocrinol Metab
2003
;
88
:
1048
–54.
33
Hussain M, Banerjee M, Sarkar FH, et al. Soy isoflavones in the treatment of prostate cancer.
Nutr Cancer
2003
;
47
:
111
–7.
34
Spentzos D, Mantzoros C, Regan MM, et al. Minimal effect of a low-fat/high soy diet for asymptomatic, hormonally naive prostate cancer patients.
Clin Cancer Res
2003
;
9
:
3282
–7.
35
Campbell MJ, Woodside JV, Honour JW, Morton MS, Leathem AJ. Effect of red clover-derived isoflavone supplementation on insulin-like growth factor, lipid and antioxidant status in healthy female volunteers: a pilot study.
Eur J Clin Nutr
2004
;
58
:
173
–9.
36
Arjmandi BH, Lucas EA, Khalil DA, et al. One year soy protein supplementation has positive effects on bone formation markers but not bone density in postmenopausal women.
Nutr J
2005
;
4
:
8
.
37
Gann PH, Kazer R, Chatterton R, et al. Sequential, randomized trial of a low-fat, high-fiber diet and soy supplementation: effects on circulating IGF-I and its binding proteins in premenopausal women.
Int J Cancer
2005
;
116
:
297
–303.
38
Maskarinec G, Takata Y, Murphy SP, Franke AA, Kaaks R. Insulin-like growth factor-1 and binding protein-3 in a 2-year soya intervention among premenopausal women.
Br J Nutr
2005
;
94
:
362
–7.
39
Woodside JV, Campbell MJ, Denholm EE, et al. Short-term phytoestrogen supplementation alters insulin-like growth factor profile but not lipid or antioxidant status.
J Nutr Biochem
2006
;
17
:
211
–5.
40
Dewell A, Weidner G, Sumner MD, et al. Relationship of dietary protein and soy isoflavones to serum IGF-1 and IGF binding proteins in the Prostate Cancer Lifestyle Trial.
Nutr Cancer
2007
;
58
:
35
–42.
41
Thissen J-P, Ketelslegers J-M, Underwood LE. Nutritional regulation of insulin-like growth factors.
Endocr Rev
1994
;
15
:
80
–101.
42
Heaney RP, McCarron DA, Dawson-Hughes B, et al. Dietary changes favorably affect bone remodeling in older adults.
J Am Diet Assoc
1999
;
99
:
1228
–33.
43
Takahashi Y, Lavigne JA, Hursting SD, et al. Using DNA microarray analyses to elucidate the effects of genistein in androgen-responsive prostate cancer cells: identification of novel targets.
Mol Carcinog
2004
;
41
:
108
–19.
44
Takahashi Y, Lavigne JA, Hursting SD, et al. Molecular signatures of soy-derived phytochemicals in androgen-responsive prostate cancer cells: a comparison study using DNA microarray.
Mol Carcinog
2006
;
45
:
943
–56.
45
Kim EJ, Shin HK, Park JH. Genistein inhibits insulin-like growth factor-I receptor signaling in HT-29 human colon cancer cells: a possible mechanism of the growth inhibitory effect of Genistein.
J Med Food
2005
;
8
:
431
–8.
46
Vrieling A, Voskuil DW, Bosma A, et al. Expression of insulin-like growth factor system components in colorectal tissue and its relation with serum IGF levels.
Growth Horm IGF Res
2008, in press.
47
Adams KF, Lampe PD, Newton KM, et al. Soy protein containing isoflavones does not decrease colorectal epithelial cell proliferation in a randomized controlled trial.
Am J Clin Nutr
2005
;
82
:
620
–6.