Abstract

BACKGROUND

We hypothesize that premenopausal endogenous estradiol may be associated with age at menarche and adult overweight and obesity, potentially contributing to breast cancer risk.

METHODS

We assessed age at menarche by questionnaire among 204 healthy Norwegian women, aged 25–35 years. Measures of body composition included body mass index (BMI, kg/m2), waist circumference (WC, cm), waist-to-hip ratio (WHR) and fat percentage dual energy X-ray absorptiometry, (DEXA). Daily salivary 17-β-estradiol (E2) concentrations were collected throughout one entire menstrual cycle and assessed by radioimmunoassay (RIA). Linear regression analyses and linear mixed models for repeated measures were used and potential confounding factors and effect modifiers were tested.

RESULTS

Among women with an early age at menarche (≤12 years), the overall mean salivary E2 concentration increased by 3.7 pmol/l (95% confidence interval, 1.8–5.7 pmol/l) with each 9.8 cm (1 SD) increase in WC, which represents a 20.7% change in the mean for the total group. Among the same early maturers, a 1 SD (0.06) change in WHR was directly associated with a 24.0% change in mean E2 concentration for the total group.

CONCLUSIONS

Our findings support the hypothesis that early age at menarche, together with adult overweight and obesity, result in high levels of 17-β-estradiol throughout the menstrual cycle.

Introduction

The increasing breast cancer incidence worldwide follows trends towards earlier age at menarche (Onland-Moret et al., 2005), higher rates of obesity (IARC Handbook of Cancer Prevention, 2002; Ballard-Barbash et al., 2006) and unfavourable metabolic profiles (visceral obesity, glucose intolerance, hypertension and dyslipidemia). Early age at menarche (Kelsey et al., 1993; Butler et al., 2000; Key et al., 2001; Clavel-Chapelon and Gerber, 2002) and obesity (IARCHandbook of Cancer Prevention, 2002; Ballard-Barbash et al., 2006) have been identified as risk factors for breast cancer. For each 1-year delay in menarche, the risk of breast cancer decreases by around 5%, and this menarcheal effect may be stronger in premenopausal than in post-menopausal women (Key et al., 2001), indicating a time-dependent effect. Excessive body weight is associated with an increased risk of post-menopausal breast cancer, but has most often been observed to decrease the risk of premenopausal breast cancer (IARC Handbook of Cancer Prevention, 2002).

A younger age at menarche is associated with higher cumulative exposure to ovarian hormones which may play a critical role in breast carcinogenesis (Pike et al., 1993; Bernstein, 2002). Estrogens are considered to stimulate ductal growth and cell proliferation of breast epithelial cells. High levels of serum estrogens are associated with an increased risk of breast cancer among post-menopausal women (Key et al., 2002), but the role of estrogens in the etiology of premenopausal breast cancer are more complex (Key et al., 2002; Sturgeon et al., 2004; Eliassen et al., 2006) and is more difficult to study because of the complexity of measuring the cyclic variation of estrogens (Bernstein, 2002; Kaaks et al., 2005).

Adiposity appears to be both a cause and a consequence of early age at menarche (Brown et al., 1996). Studies indicate that factors influencing energy balance and metabolism, such as physical activity and body composition, are strong predictors of age at menarche (Kirchengast et al., 1998; Chavarro et al., 2005). The improvement in living conditions, nutrition and increase in energy intake, combined with a reduction in energy expenditure, may explain why age at menarche has dropped in Norway from about 16 years for women born around 1830, to 13 years for those born around 1960 (Rosenberg, 1991) and 1985 (Lien et al., 2006). Importantly, several studies indicate that early maturation seems to increase overall adult fatness (Kirchengast et al., 1998; Key et al., 2001).

Little is known about the association between adult levels of estradiol throughout an entire menstrual cycle and age at menarche and adult body composition. We have previously observed that an unfavourable metabolic profile, including being overweight (Furberg et al., 2005) and physically inactive (Jasienska et al., 2006), is associated with increased levels of free E2 throughout a menstrual cycle. In addition, we have previously observed that an unfavourable metabolic profile reflected by high-energy intake and weight gain, together with physical inactivity, increases the risk of post-menopausal breast cancer (Thune et al., 1997, 1998; Jasienska and Thune, 2001; Furberg et al., 2004).

Thus, the aims of the present study were to elucidate the association between age at menarche and premenopausal 17-β-estradiol levels throughout a menstrual cycle, and whether an unfavourable energy balance as expressed through a higher fat mass in adulthood might influence such an association.

Materials and Methods

Participants and study design

A total of 204 women, aged 25–35 years, were invited to participate in the Norwegian EBBA study during 2000–2002. Women who met the following criteria were included; self-reported regular menstruation (normal cycle length: 22–38 days within the previous 3 months), no use of steroid contraceptives and no pregnancy or lactation over the previous 6 months, no history of gynecological disorder and no chronic disorders (e.g. diabetes, hypo-/hyperthyroidism). Participants were subsequently enrolled into the study and came to the Department of Clinical Research, University Hospital of North Norway (UNN), at a scheduled time (Furberg et al., 2005).

All the participating women signed an informed consent form. The study was approved by the Regional Committee for Medical Research Ethics and the Norwegian Data Inspectorate.

Questionnaires

We used questionnaires (self- and interviewer-administered by trained personal) to collect information on age at menarche, marital status, education, ethnicity, reproductive history, lifetime total physical activity, previous use of hormonal contraceptives, family history of cancer, smoking [‘current smoker' (yes/no), ‘how many cigarettes per day'] and alcohol [‘do you drink alcohol' (yes/no), ‘units of alcohol']. In this questionnaire, all of the participants in particular answered questions about age at menarche at home, and then they went through the questions administrated by trained personal. Recall and a memory-probing aid that included a lifetime calendar were used.

The validity of the response to the questions on smoking and alcohol has previously been studied (same age group, both men and women) by measurements of serum thiocynate concentrations (Knutsen and Knutsen, 1991) and levels of serum γ-glutamyltransferans levels (Nilssen and Forde, 1991), respectively. We collected dietary data on seven different days during the menstrual cycle (Days 3–6 and 21–23) by a pre-coded food diary (Lillegaard et al., 2005).

Clinical parameters

Study participants met while fasting on three subsequent visits during the collection period: first visit (Days 1–5 of the menstrual cycle), second visit (Days 7–12) and third visit (Days 22–25) at the Department of Clinical Research, UNN, Tromsø. For the first visit, the participants met on the first day possible after onset of menstrual bleeding. Anthropometric measurements were taken with participants wearing light clothing and no footwear: height was measured to the nearest half-centimeter and weight to the nearest 0.1 kg on an electronic scale. Body mass index (BMI) was used to estimate relative weight. Waist circumference (WC) was measured in a horizontal line 2.5 cm above the umbilicus; hip circumference was measured at the largest circumference; and both were measured to the nearest 0.5 cm. Waist-to-hip ratio (WHR) was estimated. The participants underwent whole-body scan (Days 7–12) using dual energy X-ray absorptiometry (DEXA—DPX-L 2288, Lunar Radiation Corporation, Madison, WI, USA), and the percentages of total and truncal fat tissue were estimated using standard Lunar software. In this study, we have presented analysis using total tissue fat. Using truncal fat gave similar results.

Serum samples

Fasting serum blood samples were drawn from an antecubital vein in the morning on each of the three visits during the menstrual cycle. The blood was centrifuged and the serum separated. Serum concentrations of 17-β-E2 were measured in fresh sera at the Department of Clinical Chemistry, UNN (Furberg et al., 2005).

Hormone analysis

Women collected samples of their own saliva to plastic tubes pretreated with sodium azide at home once a day, preferentially in the morning, for one entire menstrual cycle (Furberg et al., 2005). They started on the first day of bleeding, according to previously established collection protocols developed at the Reproductive Ecology Laboratory at Harvard University, USA (Lipson and Ellison, 1996).

17-β-E2 concentrations were measured in daily saliva samples using an 125I-based radioimmunoassay (RIA) kit (#39100, Diagnostic Systems Laboratory, Webster, TX, USA), along with published modifications to the manufacturer's protocol (Furberg et al., 2005). All samples were run in duplicate. All samples from a particular woman were run in the same batch, with women randomly assigned to batches. CVs were calculated from high to low value pools (appropriate to the range of each steroid) that were run with each batch (Furberg et al., 2005).

The sensitivity of the E2 assay (the lowest concentration of E2 distinguishable from 0 at the 95% level) was 4 pmol/l. Average intra-assay variability (estimated from the 50% binding point of the standard curve) was 9%, and the inter-assay variability ranged from 23% for lower values (15 pmol/l) to 13% for higher values (50 pmol/l). Salivary assays have higher variability than serum assays because they are measuring levels that are one to two orders of magnitude lower in concentration. This may impact the results so that the lower values (in the tail of the cycle) will have greater variability.

Before statistical analysis, all cycles were aligned to the day of ovulation following published methods (Lipson and Ellison, 1996), based on the identification of the E2 drop at the mid-cycle (Day 0), which provides a reasonable estimate of the day of ovulation. The values for 20 consecutive days for E2 (Day −10 to +9) from each cycle, aligned on Day 0, were used in data analyses. Satisfactory identification of the mid-cycle E2 drop could not be made for 14 women, and their cycles were not aligned. In detail, to study the total variation in E2 concentration throughout a menstrual cycle and as anovulatory cycles are associated with low E2 exposure, all cycles, both anovalatory and ovulatory cycles, were included in this study.

Overall mean salivary E2 concentration was calculated for all 204 women, whereas additional indices (i.e. follicular value) were calculated for the 190 women with aligned cycles. A mean follicular value was calculated by averaging E2 values for Days −7 to −1. A mean luteal value was calculated by averaging the E2 values for cycle Days 0 to +11.

Statistical analysis

The study population was divided into three subgroups of age at menarche: ≤12, >12–14 and ≥14 years, based on the variation in age at menarche in a general Western population. The various groups of age at menarche were then compared with regard to selected characteristics of the study population. We used one-way analysis of variance for continuous variables and χ2-tests for categorical variables.

Age- and multivariate-adjusted linear regression analyses were used to study the associations between mean salivary E2 concentration (overall, follicular and luteal phases of the menstrual cycle), age at menarche and different measures of body composition. Possible interactions were studied.

In order to study whether variation in adult excess weight and fat distribution modified the association between age at menarche and salivary E2 concentrations, the body composition variables were dichotomized at the 75th percentile (75‰ BMI: ≥26.5 kg/m2, 75‰ DEXA total fat: 40%, 75‰ WC: ≥84 cm and 75‰ WHR: ≥ 0.80), and age at menarche was categorized as ≤12 and >12 years. Additionally, the groups of women in these upper quartiles of BMI, DEXA total fat, WC and WHR were defined as being overweight, and/or obese [National Institutes of Health (NIH) and World Health Organization (WHO) definition; overweight: BMI ≥ 25.0 kg/m2, obese: BMI ≥ 30.0 kg/m2].

We used a linear mixed model for repeated measures to study salivary E2 concentrations throughout the entire menstrual cycle in relation to age at menarche and body composition. Different co-variance structures were explored, and the results using heterogeneous Toeplitz are presented. Dunnett's method was used for multiple comparisons.

Age- and multivariate-adjusted results are presented. On the basis of biological plausibility and common knowledge possible covariates such as age, birthweight, energy intake, smoking, alcohol, previous use of hormonal contraceptives, time to establishment of regular cycles, age at first birth, number of children and physical activity were tested in the model. The following variables contributed and were included in the final model; age, birthweight, smoking, and physical activity. As none of the multivariate analyses gave different results compared to age-adjusted analyses, we present Fig. 1 only with age adjusted results.

Figure 1:

Age adjusted salivary 17-β-estradiol concentrations by cycle day in women categorized by anthropometric measurements [Body mass index (BMI) (A), total tissue fat (DEXA) (B), waist circumference (WC) (C), and waist-to-hip ratio (WHR) (D)] and age at menarche over and below 12 years. Linear mixed models for repeated measurement. ○, Group I: menarche ≤12 years and anthropometric measurement under 75th percentile; •, Group II: menarche ≤12 years and anthropometric measurement over 75th percentile; □, Group III: menarche >12 years and anthropometric measurement under 75th percentile; ▪, Group IV: menarche >12 and anthropometric measurement over 75th percentile. (A) Groups of BMI and age at menarche. ○, Group I: menarche ≤12 years and BMI < 26.5 kg/m2, n = 34; •, Group II: menarche ≤12 years and BMI ≥ 26.5 kg/m2, n = 15; □, Group III: menarche >12 years and BMI < 26.5 kg/m2, n = 119; ▪, Group IV: menarche >12 years and BMI ≥ 26.5 kg/m2, n = 36. (B) Groups of DEXA and age at menarche. ○, Group I: menarche ≤12 years and DEXA < 40%, n = 34; •, Group II: menarche ≤12 years and DEXA≥40%, n = 15; □, Group III: menarche >12 years and DEXA < 40%, n = 119; ▪, Group IV: menarche >12 years and DEXA ≥ 40%, n = 36. (C) Groups of WC and age at menarche. ○, Group I: menarche ≤12 years and WC < 84 cm, n = 32; •, Group II: menarche ≤12 years and WC ≥ 84 cm, n = 17; □, Group III: menarche >12 years and WC < 84 cm, n = 116; ▪, Group IV: menarche >12 years and WC ≥ 84 cm, n = 39. (D) Groups of WHR and age at menarche. ○, Group I: menarche ≤12 years and WHR < 0.08, n = 32; •p, Group II: menarche ≤12 years and WHR ≥ 0.08, n = 17; □, Group III: menarche >12 years and WHR < 0.80, n = 118; ▪, Group IV: menarche >12 years and WHR ≥ 0.80, n = 37.

Figure 1:

Age adjusted salivary 17-β-estradiol concentrations by cycle day in women categorized by anthropometric measurements [Body mass index (BMI) (A), total tissue fat (DEXA) (B), waist circumference (WC) (C), and waist-to-hip ratio (WHR) (D)] and age at menarche over and below 12 years. Linear mixed models for repeated measurement. ○, Group I: menarche ≤12 years and anthropometric measurement under 75th percentile; •, Group II: menarche ≤12 years and anthropometric measurement over 75th percentile; □, Group III: menarche >12 years and anthropometric measurement under 75th percentile; ▪, Group IV: menarche >12 and anthropometric measurement over 75th percentile. (A) Groups of BMI and age at menarche. ○, Group I: menarche ≤12 years and BMI < 26.5 kg/m2, n = 34; •, Group II: menarche ≤12 years and BMI ≥ 26.5 kg/m2, n = 15; □, Group III: menarche >12 years and BMI < 26.5 kg/m2, n = 119; ▪, Group IV: menarche >12 years and BMI ≥ 26.5 kg/m2, n = 36. (B) Groups of DEXA and age at menarche. ○, Group I: menarche ≤12 years and DEXA < 40%, n = 34; •, Group II: menarche ≤12 years and DEXA≥40%, n = 15; □, Group III: menarche >12 years and DEXA < 40%, n = 119; ▪, Group IV: menarche >12 years and DEXA ≥ 40%, n = 36. (C) Groups of WC and age at menarche. ○, Group I: menarche ≤12 years and WC < 84 cm, n = 32; •, Group II: menarche ≤12 years and WC ≥ 84 cm, n = 17; □, Group III: menarche >12 years and WC < 84 cm, n = 116; ▪, Group IV: menarche >12 years and WC ≥ 84 cm, n = 39. (D) Groups of WHR and age at menarche. ○, Group I: menarche ≤12 years and WHR < 0.08, n = 32; •p, Group II: menarche ≤12 years and WHR ≥ 0.08, n = 17; □, Group III: menarche >12 years and WHR < 0.80, n = 118; ▪, Group IV: menarche >12 years and WHR ≥ 0.80, n = 37.

Collinearity was of no concern as Pearson's correlation coefficient was ≤0.2 for all pairs of independent variables in the models. E2 measurements at the beginning and the end of the cycles are encumbered with higher coefficient of variation; therefore we included E2 measurements from cycle Day −10 to +9. SAS statistical package version 9.1. was used.

Results

The participating 204 healthy women were, on average, aged 30.7 years (24.9–35.9 years) and reported a mean age at menarche of 13.1 years (9.2–19.5 years). The mean BMI was 24.4 kg/m2 (16.9–39.8 kg/m2), WC 79.5 cm (61.0–116.0 cm) and percentage fat 34.2% (16.7–52.1%) (Table II). Mean salivary E2 concentration was 17.9 pmol/l (not given in table). Women with an age at menarche below 12 years were shorter (P = 0.03) and tended to have a higher BMI, a larger WC and a higher fat percentage than women with later age at menarche (>12–14 and ≥14 years) (Table I).

Table I.

Characteristics of the study population: means, (SDs)* or proportions. The Norwegian EBBA study (n=204)

 Menarche ≤12 years (n = 49) Menarche >12–14 years (n = 100) Menarche ≥14 years (n = 55) P-value 
Age, years 30.9 (2.8) 30.4 (2.9) 31.2 (3.5) 0.30 
Years of schooling 16.0 (3.1) 16.1 (3.0) 16.0 (3.1) 0.94 
Ethnic minority (Sami %) 12.2 6.0 7.3 0.41 
Body composition 
 Height (cm) 164.7 (5.3) 167.4 (7.0) 167.9 (6.3) 0.03 
 Body Mass Index (BMI) (kg/m225.3 (4.7) 24.3 (3.2) 23.8 (3.7) 0.11 
 Waist circumference (WC) (cm) 81.1 (12.1) 79.9 (8.9) 77.5 (8.9) 0.15 
 Waist-to-hip ratio (WHR) 0.78 (0.07) 0.77 (0.06) 0.76 (0.06) 0.20 
 Total tissue fat (DEXA) (%) 35.8 (7.8) 34.3 (7.5) 32.3 (7.4) 0.07 
Total cholesterol (mmol/L) 4.3 (0.7) 4.5 (0.8) 4.5 (0.7) 0.48 
HDL cholesterol (mmol/L) 1.5 (0.3) 1.6 (0.3) 1.6 (0.3) 0.35 
LDL/HDL ratio 1.8 (0.7) 1.8 (0.7) 1.7 (0.6) 0.85 
Saliva hormone concentrations (pmol/l)     
 Overall average 17β-estradiol (E217.8 (9.3) 17.7 (8.7) 18.4 (8.5) 0.90 
Serum hormone concentration§ (nmol/l)     
 Serum estradiol 0.16 (0.07) 0.14 (0.06) 0.15 (0.06) 0.49 
Menstrual and reproductive characteristics     
 Women with children (%) 49.0 47.0 49.1 0.96 
 Age at 1st birth (years) 24.0 (2.8) 24.7 (3.8) 24.6 (4.7) 0.79 
 Cycle length (days) 28.1 (2.9) 28.1 (3.1) 28.6 (3.5) 0.63 
Previous use of hormonal contraceptives (%) 79.6 84.0 87.0 0.59 
Energy intake (kJ/day)  7895 (2033)   8197 (1897)  8079 (1801) 0.66 
Leisure time (MET h/week) 49.8 (34.6) 52.0 (33.7) 55.5 (41.5) 0.72 
Alcohol units per week among users (n = 190) 3.0 (4.0) 2.8 (3.1) 3.8 (3.5) 0.20 
Current smokers (%) 22.5 23.0 20.0 0.91 
 Menarche ≤12 years (n = 49) Menarche >12–14 years (n = 100) Menarche ≥14 years (n = 55) P-value 
Age, years 30.9 (2.8) 30.4 (2.9) 31.2 (3.5) 0.30 
Years of schooling 16.0 (3.1) 16.1 (3.0) 16.0 (3.1) 0.94 
Ethnic minority (Sami %) 12.2 6.0 7.3 0.41 
Body composition 
 Height (cm) 164.7 (5.3) 167.4 (7.0) 167.9 (6.3) 0.03 
 Body Mass Index (BMI) (kg/m225.3 (4.7) 24.3 (3.2) 23.8 (3.7) 0.11 
 Waist circumference (WC) (cm) 81.1 (12.1) 79.9 (8.9) 77.5 (8.9) 0.15 
 Waist-to-hip ratio (WHR) 0.78 (0.07) 0.77 (0.06) 0.76 (0.06) 0.20 
 Total tissue fat (DEXA) (%) 35.8 (7.8) 34.3 (7.5) 32.3 (7.4) 0.07 
Total cholesterol (mmol/L) 4.3 (0.7) 4.5 (0.8) 4.5 (0.7) 0.48 
HDL cholesterol (mmol/L) 1.5 (0.3) 1.6 (0.3) 1.6 (0.3) 0.35 
LDL/HDL ratio 1.8 (0.7) 1.8 (0.7) 1.7 (0.6) 0.85 
Saliva hormone concentrations (pmol/l)     
 Overall average 17β-estradiol (E217.8 (9.3) 17.7 (8.7) 18.4 (8.5) 0.90 
Serum hormone concentration§ (nmol/l)     
 Serum estradiol 0.16 (0.07) 0.14 (0.06) 0.15 (0.06) 0.49 
Menstrual and reproductive characteristics     
 Women with children (%) 49.0 47.0 49.1 0.96 
 Age at 1st birth (years) 24.0 (2.8) 24.7 (3.8) 24.6 (4.7) 0.79 
 Cycle length (days) 28.1 (2.9) 28.1 (3.1) 28.6 (3.5) 0.63 
Previous use of hormonal contraceptives (%) 79.6 84.0 87.0 0.59 
Energy intake (kJ/day)  7895 (2033)   8197 (1897)  8079 (1801) 0.66 
Leisure time (MET h/week) 49.8 (34.6) 52.0 (33.7) 55.5 (41.5) 0.72 
Alcohol units per week among users (n = 190) 3.0 (4.0) 2.8 (3.1) 3.8 (3.5) 0.20 
Current smokers (%) 22.5 23.0 20.0 0.91 

*SDs=standard deviations; HDL=high-density lipoprotein; LDL=low-density lipoprotein.

Numbers may vary due to missing information.

One way analysis of variance or χ2 test.

§Blood sampling first visit (Days 1–5).

For those who have children: n = 98, in each group: 24–47–27.

Table II.

Estimated changes* in salivary 17-β-estradiol (E2) concentration (pmol/l) by 1 standard deviation (SD) increase in explanatory variables in total menstrual cycle (n = 204), follicular and luteal phase (n = 190)

Variables Mean (SD) Mean E2 change (n = 204) Mean E2 change-follicular index (n = 190)§ Mean E2 change-luteal index (n = 190)§ 
  Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P-value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P-value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P-value 
Age (years) 30.7 (3.1) −0.8 (−2.0, 0.4) 0.20 −1.0 (−2.2, 0.2) 0.12 −0.8 (−2.1, 0.5) 0.22 −0.9 (−2.2, 0.4) 0.18 −1.3 (−2.7, 0.1) 0.08 −1.3 (−2.7, 0.1) 0.07 
Menarche (years) 13.1 (1.4) −0.3 (−1.5, 1.0) 0.67 0.1 (−1.2, 1.4) 0.90 −0.4 (−1.7, 0.9) 0.53 −0.2 (−1.5, 1.1) 0.75 −0.3 (−1.7, 1.1) 0.64 0.2 (−1.3, 1.6) 0.83 
BMI** (kg/m224.4 (3.8) 2.2 (1.0, 3.4) 0.001 2.2 (1.0, 3.4) <.001 2.4 (1.2, 3.7) 0.0001 2.5 (1.3, 3.8) <.0001 1.7 (0.4, 3.1) 0.01 1.7 (0.3, 3.0) 0.01 
WC** (cm) 79.5 (9.8) 1.6 (0.4, 2.8) 0.01 1.6 (0.4, 2.8) 0.01 1.7 (0.4, 2.9) 0.01 1.7 (0.4, 2.9) <.01 1.2 (−0.2, 2.5) 0.09 1.1 (−0.2, 2.5) 0.10 
WHR** 0.77 (0.06) 1.4 (0.2, 2.6) 0.02 1.1 (−0.1, 2.3) 0.07 1.4 (0.2, 2.7) 0.03 1.3 (−0.0, 2.5) 0.06 1.2 (−0.1, 2.6) 0.08 0.9 (−0.5, 2.3) 0.20 
DEXA** (%) 34.2 (7.6) 1.4 (0.2, 2.6) 0.02 1.3 (0.1, 2.5) 0.04 1.4 (0.1, 2.7) 0.03 1.4 (0.1, 2.7) 0.03 1.0 (−0.3, 2.4) 0.14 0.8 (−0.6, 2.2) 0.28 
Variables Mean (SD) Mean E2 change (n = 204) Mean E2 change-follicular index (n = 190)§ Mean E2 change-luteal index (n = 190)§ 
  Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P-value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P-value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P-value 
Age (years) 30.7 (3.1) −0.8 (−2.0, 0.4) 0.20 −1.0 (−2.2, 0.2) 0.12 −0.8 (−2.1, 0.5) 0.22 −0.9 (−2.2, 0.4) 0.18 −1.3 (−2.7, 0.1) 0.08 −1.3 (−2.7, 0.1) 0.07 
Menarche (years) 13.1 (1.4) −0.3 (−1.5, 1.0) 0.67 0.1 (−1.2, 1.4) 0.90 −0.4 (−1.7, 0.9) 0.53 −0.2 (−1.5, 1.1) 0.75 −0.3 (−1.7, 1.1) 0.64 0.2 (−1.3, 1.6) 0.83 
BMI** (kg/m224.4 (3.8) 2.2 (1.0, 3.4) 0.001 2.2 (1.0, 3.4) <.001 2.4 (1.2, 3.7) 0.0001 2.5 (1.3, 3.8) <.0001 1.7 (0.4, 3.1) 0.01 1.7 (0.3, 3.0) 0.01 
WC** (cm) 79.5 (9.8) 1.6 (0.4, 2.8) 0.01 1.6 (0.4, 2.8) 0.01 1.7 (0.4, 2.9) 0.01 1.7 (0.4, 2.9) <.01 1.2 (−0.2, 2.5) 0.09 1.1 (−0.2, 2.5) 0.10 
WHR** 0.77 (0.06) 1.4 (0.2, 2.6) 0.02 1.1 (−0.1, 2.3) 0.07 1.4 (0.2, 2.7) 0.03 1.3 (−0.0, 2.5) 0.06 1.2 (−0.1, 2.6) 0.08 0.9 (−0.5, 2.3) 0.20 
DEXA** (%) 34.2 (7.6) 1.4 (0.2, 2.6) 0.02 1.3 (0.1, 2.5) 0.04 1.4 (0.1, 2.7) 0.03 1.4 (0.1, 2.7) 0.03 1.0 (−0.3, 2.4) 0.14 0.8 (−0.6, 2.2) 0.28 

*Linear regression analyses. Regression coefficient and 95% confidence interval (CI).

Number may vary due to missing information.

Age at entry and measurements at Days 1–5 after onset of the menstrual cycle.

§Includes women with aligned cycles only.

Adjusted for age, number of cigarettes, leisure time physical activity and birth weight.

**BMI = Body mass index; WC = Waist circumference; WHR = Waist-to-hip ratio; DEXA = Total tissue fat (%).

When studying estimated changes in overall mean salivary E2 concentrations for the total group, by changes (1 SD) in different body composition measures, we observed that an increase in any of these measures of body composition resulted in a multivariate adjusted increase in E2 concentration (Table II). For each 3.8 kg/m2 (1 SD) increase in BMI, the overall mean age-adjusted level of E2 increased by 2.2 pmol/l (95% confidence interval, CI, 1.0–3.4 pmol/l), which equals a 12.3% increase in mean overall concentration of E2 for the total group. For each 9.8 cm (1 SD) increase in WC, the overall mean concentration of E2 increased by 1.6 pmol/l (95% CI, 0.4–2.8 pmol/l), which equals an 8.9% increase in mean overall concentration of E2 for the total group. Performing the same analyses but excluding the 14 women who did not have a mid-cycle E2 drop, gave smaller estimated changes in salivary E2, but the conclusion did not change: a 1 SD increase in the different body composition variables gave a significant increase in E2 (data not shown).

We observed no difference in estimated changes in E2 by different measures of body composition between the follicular and luteal phases of the menstrual cycle (Table II).

In order to elucidate whether age at menarche influenced our results, stratified analyses by subgroups of age at menarche was performed (Table III). Women with early age at menarche (≤12 years) had an increase in overall mean E2 concentration by 3.7 pmol/l (95% CI, 1.8–5.7 pmol/l) by 9.8 cm (1SD) increase in WC (Pinteraction = 0.07); with each 0.06 (1 SD) increase in WHR, the overall mean E2 concentration increased by 4.3 pmol/l (95% CI, 2.0–6.5) (Pinteraction = 0.03) and with each 7.6% (1 SD) increase in total fat percentage, the overall mean E2 concentration increased by 2.9 pmol/l (95% CI, 0.2–5.5) (Pinteraction = 0.22). These results equal a change in mean overall concentration of E2 for the total group of 20.6, 24.0 and 16.2%, respectively. Among women with age at menarche >12–14 and ≥14 years, no significant associations were seen with adult body composition.

Table III.

Estimated changes* in salivary overall average 17-β-estradiol (E2) concentrations (pmol/l) with 95% confidence interval by 1 standard deviation (SD) increase in explanatory variable in groups of menarche (n = 204)

Variables Mean (SD) Menarche ≤12 (n = 49) Menarche >12–14 (n = 100) Menarche ≥14 (n = 55) P interaction 
  Age adjusted effect (95% CI) P- value Multivariate adjusted effect (95% CI) P- value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P- value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P- value  
Mean estradiol (pmol/l)  17.8   17.7   18.4    
BMI** (kg/m224.4 (3.8) 3.3 (1.2, 5.3) 0.002 3.4 (1.4, 5.5) 0.001 1.4 (−0.6, 3.5) 0.2 1.4 (−0.7, 3.5) 0.2 1.9 (−0.5, 4.3) 0.1 1.3 (−0.9, 3.6) 0.2 0.33 
WC** (cm) 79.5 (9.8) 3.6 (1.7, 5.6) <.001 3.7 (1.8, 5.7) <.001 0.2 (−1.8, 2.1) 0.9 0.2 (−1.8, 2.2) 0.8 1.1 (−1.5, 3.7) 0.4 0.6 (−1.8, 3.1) 0.6 0.07 
WHR** 0.77 (0.06) 4.1 (1.9, 6.3) <.001 4.3 (2.0, 6.5) <.001 0.4 (−1.3, 2.2) 0.7 0.3 (−1.5, 2.1) 0.7 0.4 (−2.2, 3.1) 0.7 −0.6 (−3.1, 2.0) 0.7 0.03 
DEXA** (%) 34.2 (7.6) 2.8 (0.2, 5.4) 0.04 2.9 (0.2, 5.5) 0.03 1.3 (−0.4, 3.0) 0.1 1.0 (−0.9, 2.8) 0.3 0.7 (−1.7, 3.1) 0.6 −0.01(−2.3, 2.3) 1.0 0.22 
Variables Mean (SD) Menarche ≤12 (n = 49) Menarche >12–14 (n = 100) Menarche ≥14 (n = 55) P interaction 
  Age adjusted effect (95% CI) P- value Multivariate adjusted effect (95% CI) P- value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P- value Age adjusted effect (95% CI) P-value Multivariate adjusted effect (95% CI) P- value  
Mean estradiol (pmol/l)  17.8   17.7   18.4    
BMI** (kg/m224.4 (3.8) 3.3 (1.2, 5.3) 0.002 3.4 (1.4, 5.5) 0.001 1.4 (−0.6, 3.5) 0.2 1.4 (−0.7, 3.5) 0.2 1.9 (−0.5, 4.3) 0.1 1.3 (−0.9, 3.6) 0.2 0.33 
WC** (cm) 79.5 (9.8) 3.6 (1.7, 5.6) <.001 3.7 (1.8, 5.7) <.001 0.2 (−1.8, 2.1) 0.9 0.2 (−1.8, 2.2) 0.8 1.1 (−1.5, 3.7) 0.4 0.6 (−1.8, 3.1) 0.6 0.07 
WHR** 0.77 (0.06) 4.1 (1.9, 6.3) <.001 4.3 (2.0, 6.5) <.001 0.4 (−1.3, 2.2) 0.7 0.3 (−1.5, 2.1) 0.7 0.4 (−2.2, 3.1) 0.7 −0.6 (−3.1, 2.0) 0.7 0.03 
DEXA** (%) 34.2 (7.6) 2.8 (0.2, 5.4) 0.04 2.9 (0.2, 5.5) 0.03 1.3 (−0.4, 3.0) 0.1 1.0 (−0.9, 2.8) 0.3 0.7 (−1.7, 3.1) 0.6 −0.01(−2.3, 2.3) 1.0 0.22 

*Linear regression analysis. Regression coefficient and 95% confidence interval (CI).

Number may vary due to missing information.

Measurements at Days 1–5 after onset of the menstrual cycle.

‖Adjusted for age, number of cigarettes, leisure time physical activity and birth weight.

**BMI = Body mass index; WC = Waist circumference; WHR = Waist-to-hip ratio; DEXA = Total tissue fat (%).

To examine whether parity influenced our result, we performed analysis stratified by parity. Among women with early age at menarche (≤12 years), the relationship between the measures of body fatness and overall E2 concentration was strongest in the subgroup of non-parous women (data not shown). With each 0.06 (1 SD) increase in WHR, overall mean E2 concentration increased by 6.1 pmol/l (95% CI, 1.5–10.9 pmol/l). However, no interaction between age at menarche, parity and the different anthropometric measurements were found, and there were small numbers in each group.

We examined the mean E2 concentrations by cycle day throughout the whole menstrual cycle in four groups stratified by age at menarche ≤12 and >12 years and body fat and distribution measures at the 75‰ (Fig. 1). Women characterized by both early age at menarche (≤12 years) and excess WC (≥84 cm, 75‰) or WHR (≥0.80, 75‰) had age adjusted mean salivary E2 profiles that were higher throughout the cycle than the rest of the study population (Fig. 1).

Using a linear mixed model for repeated measures, we examined how the mean E2 concentration by cycle day in women with both the early age at menarche and excess body weight (Group II, Fig. 1) differed from the concentrations in the other groups of women described. Fig. 1 is shown with age- adjusted results, as none of the multivariate analyses gave different results compared to age-adjusted analyses.

The difference between women with age at menarche ≤12 and BMI ≥26.5 kg/m2 (Group II, Fig. 1A) and the other groups of women described in Fig. 1A was significant for Groups I (P = 0.04) and III (P = 0.04), but not for Group IV (P = 0.43). There was no statistically significant interaction between the dichotomized menarche variable and the dichotomized variable of BMI (Pinteraction = 0.8).

There were no significant differences between women with age at menarche ≤12 and DEXA% total fat ≥40% (Group II, Fig. 1B) and the other groups of women described in Fig. 1B (Groups II and I, P = 0.17; Groups II and III, P = 0.15; Groups II and IV, P = 0.63). There was no statistically significant interaction between the dichotomized menarche variable and the dichotomized variable of DEXA% total fat (Pinteraction = 1.0).

We examined how the mean E2 levels by cycle day throughout the whole cycle in women with both age at menarche ≤12 and WC≥84 cm (Group II, Fig. 1C) differed from the levels in each of the other groups of women shown in Fig. 1C and observed significant differences for all comparisons (Groups II and I, P = 0.004; Groups II and III, P = 0.03; Groups II and IV, P = 0.01). There was a statistically significant interaction between the dichotomized menarche variable and the dichotomized variable of WC (Pinteraction = 0.02).

Finally, the same pattern was observed in women with age at menarche ≤12 and WHR ≥0.08 (Group II, Fig. 1D) and the other groups of women shown in Fig. 1D (differences between Group II in Fig. 1D and the other; Group I, P = 0.003; Group III, P = 0.04; Group IV, P = 0.007). There was a statistically significant interaction between the dichotomized menarche variable and the dichotomized variable of WHR (Pinteraction < 0.001).

Discussion

To our knowledge, this is the first study to support the hypothesis that early age at menarche in combination with adult overweight and obesity might result in an overall increase in concentrations of free biologically active E2. We observed a clear pattern, after adjustments for potential confounders, of a 16–24% increase in E2 concentration among women reporting early age at menarche in combination with adult overweight and obesity.

Several studies have suggested that lifestyle related factors, including metabolic profile, and reproductive factors as age at menarche, independently affect breast cancer risk by altering exposure to estrogens (Jasienska and Thune, 2001; Verkasalo et al., 2001; Bernstein, 2002). Women with early age at menarche have been observed to have higher levels of serum E2 throughout a cycle (Apter and Vihko, 1983, 1985) and urinary estrogen metabolites (Windham et al., 2002) than women with later age at menarche. Evidence is becoming more consistent that there is an inverse association between physical activity and estrogens (Williams, 2003; Irwin, 2006).

Additionally, several studies have observed an association between early age at menarche and adult obesity (Kirchengast et al., 1998; Laitinen et al., 2001). It has been recognized that body fat is important for the initiation and maintenance of reproductive function in women, explaining why weight gain accelerates and energy restriction delays the onset of menses (Adair and Gordon-Larsen, 2001; Dunger et al., 2005) and early maturation may increase overall adult fatness (Kirchengast et al., 1998). In our study, we also observed that women experiencing early maturation tended to also have an increased adult body fatness (P = 0.07).

The close relationship between obesity and age at menarche is supported by a recent intervention study, where age at menarche was delayed as a result of reduction in BMI and body fatness (Chavarro et al., 2005). Thus, an increase in E2 levels during late childhood and puberty may promote not only the onset of the first menstrual bleeding but also the development of higher amounts of body fat tissue. Moreover, body fat may act as a secondary hormonal gland, influencing E2 levels during the whole of adult life (Kirchengast et al., 1998). Thus, our observation that subgroups of premenopausal women, characterized by both early age at menarche and adult overweight and obesity (high BMI, DEXA, WC and WHR), experience higher E2 levels throughout their menstrual cycle is biologically plausible. Furthermore, energy expenditure may reduce serum E2 levels (Bernstein et al., 1987; Kumar et al., 2005) and biologically active free E2 throughout a menstrual cycle (Jasienska et al., 2006) supporting the hypothesis that excess weight may contribute to the increased E2 levels among premenopausal women. Of importance, is also the observation that even among our women who were relatively thin, we observed an increase in the level of E2 with increasing relative body weight (BMI, WC, WHR, DEXA total fat).

It should be noted that, by stratifying by parity, we observed the strongest inter-relationship of age at menarche, BMI, WC and E2 levels throughout a menstrual cycle for non-parous women (data not shown). However, no significant interactions were found, and due to small numbers in each group, these results need to be interpretated with caution. Furthermore, in some studies parous women were observed to have higher premenopausal E2 levels than non-parous women (Bernstein, 2002; Kaaks et al., 2005) in contrast to others (Verkasalo et al., 2001), but these studies have not been stratified by age at menarche.

To our knowledge, no studies to date have examined if the subgroup characterized by early age at menarche and excessive body weight throughout premenopausal years are at high risk for breast cancer. Furthermore, our results may seem inconsistent with studies that have found a reduced risk of premenopausal breast cancer among women who were obese as young adults (Ursin et al., 1995; IARC Handbook of Cancer Prevention, 2002; Ballard-Barbash et al., 2006). Nevertheless, several studies support our results indirectly; women with central obesity had increased risk of pre and post-menopausal breast cancer (Wu et al., 2006), newly diagnosed cases of premenopausal breast cancer had significantly higher serum total E2 levels and higher WHRs than controls (Kumar et al., 2005), and women with follicular total and free E2 levels in the top 25% had a doubling of breast cancer risk (Eliassen et al., 2006). Furthermore, premenopausal overweight women who developed breast cancer, had somewhat higher prediagnostic blood levels of E2 than those who did not develop breast cancer, which supports the importance of the levels of E2 among premenopausal women (Travis and Key, 2003). However, many prior studies have not aligned estrogen assays to the same time during the menstrual cycle making it difficult to compare results across studies (Bernstein, 2002; Kaaks et al., 2005).

Our observations of a higher E2 level among women who have both an early age at menarche and adult obesity, support studies demonstrating a modest increase in breast cancer risk associated with younger age at menarche (Kelsey et al., 1993; Bernstein, 2002). To our knowledge, these studies have not stratified by premenopausal body composition. The observed association between age at menarche and breast cancer risk is often explained due to early establishment of regular cycles and longer time between age at menarche and first full-term pregnancy, and hence the overall higher total cumulative dose of estrogen throughout life. Furthermore, age at menarche is a good marker for determining total duration of endogenous estrogen exposure (Hagemans et al., 2004). Any factor that modifies menstrual cycle patterns and reduces the levels of E2 or the frequency of ovulation, may reduce a woman's lifetime risk of developing breast cancer (Bernstein et al., 1994). The observation that obesity in premenopausal women may lower this risk of breast cancer has been hypothesized to be due to higher frequency of longer anovulatory cycles among obese women (IARC Handbook of Cancer Prevention, 2002; Ballard-Barbash et al., 2006).

Our study has several strengths, including daily saliva sampling for estimation of daily 17-β-E2 concentrations through an entire menstrual cycle. We used well-developed and validated methods and assays to characterize the women's exposure to free, biologically active E2 and performed comparisons of levels by aligned cycle days (Lipson and Ellison, 1996). This recommended approach, of examining all women at the same time during the menstrual cycle, is rarely achieved because of its logistic complexity (Bernstein, 2002; Kaaks et al., 2005). Additionally, one trained nurse traced all the participants throughout the study, and all were met in the same clinical research department at a university hospital. This standardization enhanced the quality of our data and allowed us to sample all clinical variables within the same narrow frame of the cycle for each participant, using uniform procedures. To limit any potential influence of season, women did not participate during months with no daylight (December and January).

To study if different anthropometric measurements in particular were associated with levels of E2, several measurements were included. Of notice, is the observation that even if DEXA% total fat is looked upon as a valid and objective fat measurement method, BMI, WC and WHR gave additional information. One explanation may be that our DEXA measurement does not allow an index of central obesity, as the truncal DEXA fat is measuring fat from the hips to the shoulders. Neither DEXA total fat or DEXA truncal fat, represent central adiposity in particular, in the same matter as WC and WHR.

The present population is very homogenous which may strengthen the interpretation in smaller groups, but it also underlines the need for further studies among other ethnic groups. Additionally, in order to perform stratification by age at menarche and body composition, certain subgroups were small.

Age at menarche was collected retrospectively and misclassification is possible, although several studies have found age at menarche to be recalled with high reliability (Must et al., 2002). In our study, we used both interview and a life calendar to improve the accuracy of recall, and age at menarche is in accordance with comparable observations in a large prospective study from Norway (Lien et al., 2006).

Conclusion

Reducing the lifetime levels of estrogen has been proposed as the most important step in lowering the risk of breast cancer in women. Our results suggest that women who experience early age at menarche and adult excess weight, may experience higher levels of 17-β-estradiol (E2) throughout a menstrual cycle. These findings underline the necessity to further study variation in biologically active 17-β-estradiol (E2) by subgroups of women, in order to elucidate the associations between lifestyle factors, characteristics of women and breast cancer risk.

Funding

Norwegian Cancer Society (05087, 49258); Foundation for the Norwegian Health and Rehabilitation Organizations (59010-2000/2001/2002); Aakre Foundation (5695-2000, 5754-2002).

Acknowledgements

We acknowledge each woman who participated in the Norwegian EBBA study. A special thank to Gunn Kristin Knudsen as a trained nurse, Heidi Jacobsen for technical assistance, Anna Kirsti Jenssen and Sissel Andersen for professional secretary assistance, Dr Susan Lipson at the laboratory at Harvard University, and the Clinical Research Department, UNN for the skilled and always professional setting.

References

Adair
LS
Gordon-Larsen
P
Maturational timing and overweight prevalence in US adolescent girls
Am J Public Health
 , 
2001
, vol. 
91
 (pg. 
642
-
644
)
Apter
D
Vihko
R
Early menarche, a risk factor for breast cancer, indicates early onset of ovulatory cycles
J Clin Endocrinol Metab
 , 
1983
, vol. 
57
 (pg. 
82
-
86
)
Apter
D
Vihko
R
Premenarcheal endocrine changes in relation to age at menarche
Clin Endocrinol (Oxf)
 , 
1985
, vol. 
22
 (pg. 
753
-
760
)
Ballard-Barbash
R
Friedenreich
CM
Slattery
M
Thune
I
Obesity and body composition
Cancer Epidemiology and Prevention
 , 
2006
3rd edn
New York: Oxford University Press
(pg. 
422
-
448
)
Bernstein
L
Epidemiology of endocrine-related risk factors for breast cancer
J Mammary Gland Biol Neoplasia
 , 
2002
, vol. 
7
 (pg. 
3
-
15
)
Bernstein
L
Henderson
BE
Hanisch
R
Sullivan-Halley
J
Ross
RK
Physical exercise and reduced risk of breast cancer in young women
J Natl Cancer Inst
 , 
1994
, vol. 
86
 (pg. 
1403
-
1408
)
Bernstein
L
Ross
RK
Lobo
RA
Hanisch
R
Krailo
MD
Henderson
BE
The effects of moderate physical activity on menstrual cycle patterns in adolescence: implications for breast cancer prevention
Br J Cancer
 , 
1987
, vol. 
55
 (pg. 
681
-
685
)
Brown
DE
Koenig
TV
Demorales
AM
McGuire
K
Mersai
CT
Menarche age, fatness, and fat distribution in Hawaiian adolescents
Am J Phys Anthropol
 , 
1996
, vol. 
99
 (pg. 
239
-
247
)
Butler
LM
Potischman
NA
Newman
B
Millikan
RC
Brogan
D
Gammon
MD
Swanson
CA
Brinton
LA
Menstrual risk factors and early-onset breast cancer
Cancer Causes Control
 , 
2000
, vol. 
11
 (pg. 
451
-
458
)
Chavarro
JE
Peterson
KE
Sobol
AM
Wiecha
JL
Gortmaker
SL
Effects of a school-based obesity-prevention intervention on menarche (United States)
Cancer Causes Control
 , 
2005
, vol. 
16
 (pg. 
1245
-
1252
)
Clavel-Chapelon
F
Gerber
M
Reproductive factors and breast cancer risk. Do they differ according to age at diagnosis?
Breast Cancer Res Treat
 , 
2002
, vol. 
72
 (pg. 
107
-
115
)
Dunger
DB
Ahmed
ML
Ong
KK
Effects of obesity on growth and puberty
Best Pract Res Clin Endocrinol Metab
 , 
2005
, vol. 
19
 (pg. 
375
-
390
)
Eliassen
AH
Missmer
SA
Tworoger
SS
Hankinson
SE
Endogenous steroid hormone concentrations and risk of breast cancer: does the association vary by a woman's predicted breast cancer risk?
J Clin Oncol
 , 
2006
, vol. 
24
 (pg. 
1823
-
1830
)
Furberg
AS
Veierod
MB
Wilsgaard
T
Bernstein
L
Thune
I
Serum high-density lipoprotein cholesterol, metabolic profile, and breast cancer risk
J Natl Cancer Inst
 , 
2004
, vol. 
96
 (pg. 
1152
-
1160
)
Furberg
AS
Jasienska
G
Bjurstam
N
Torjesen
PA
Emaus
A
Lipson
SF
Ellison
PT
Thune
I
Metabolic and hormonal profiles: HDL cholesterol as a plausible biomarker of breast cancer risk. The Norwegian EBBA Study
Cancer Epidemiol Biomarkers Prev
 , 
2005
, vol. 
14
 (pg. 
33
-
40
)
Hagemans
ML
van der Schouw
YT
de Kleijn
MJ
van Staveren
WA
Pop
VJ
Leusink
GL
Grobbee
DE
Indicators for the total duration of premenopausal endogenous estrogen exposure in relation to BMD
Hum Reprod
 , 
2004
, vol. 
19
 (pg. 
2163
-
2169
)
IARC Handbook of Cancer Prevention
Weight control and physical activity
2002
France
Lyon, International Agency for Research on Cancer (IARC)
Irwin
ML
Randomized controlled trials of physical activity and breast cancer prevention
Exerc Sport Sci Rev
 , 
2006
, vol. 
34
 (pg. 
182
-
193
)
Jasienska
G
Thune
I
Lifestyle, hormones, and risk of breast cancer
BMJ
 , 
2001
, vol. 
322
 (pg. 
586
-
587
)
Jasienska
G
Ziomkiewicz
A
Thune
I
Lipson
SF
Ellison
PT
Habitual physical activity and estradiol levels in women of reproductive age
Eur J Cancer Prev
 , 
2006
, vol. 
15
 (pg. 
439
-
445
)
Kaaks
R
Berrino
F
Key
T
Rinaldi
S
Dossus
L
Biessy
C
Secreto
G
Amiano
P
Bingham
S
Boeing
H
, et al.  . 
Serum sex steroids in premenopausal women and breast cancer risk within the European Prospective Investigation into Cancer and Nutrition (EPIC)
J Natl Cancer Inst
 , 
2005
, vol. 
97
 (pg. 
755
-
765
)
Kelsey
JL
Gammon
MD
John
EM
Reproductive factors and breast cancer
Epidemiol Rev
 , 
1993
, vol. 
15
 (pg. 
36
-
47
)
Key
TJ
Verkasalo
PK
Banks
E
Epidemiology of breast cancer
Lancet Oncol
 , 
2001
, vol. 
2
 (pg. 
133
-
140
)
Key
T
Appleby
P
Barnes
I
Reeves
G
Endogenous sex hormones and breast cancer in postmenopausal women: reanalysis of nine prospective studies
J Natl Cancer Inst
 , 
2002
, vol. 
94
 (pg. 
606
-
616
)
Kirchengast
S
Gruber
D
Sator
M
Huber
J
Impact of the age at menarche on adult body composition in healthy pre- and postmenopausal women
Am J Phys Anthropol
 , 
1998
, vol. 
105
 (pg. 
9
-
20
)
Knutsen
SF
Knutsen
R
The Tromso Survey: the Family Intervention study—the effect of intervention on some coronary risk factors and dietary habits, a 6-year follow-up
Prev Med
 , 
1991
, vol. 
20
 (pg. 
197
-
212
)
Kumar
NB
Riccardi
D
Cantor
A
Dalton
K
Allen
K
A case-control study evaluating the association of purposeful physical activity, body fat distribution, and steroid hormones on premenopausal breast cancer risk
Breast J
 , 
2005
, vol. 
11
 (pg. 
266
-
272
)
Laitinen
J
Power
C
Jarvelin
MR
Family social class, maternal body mass index, childhood body mass index, and age at menarche as predictors of adult obesity
Am J Clin Nutr
 , 
2001
, vol. 
74
 (pg. 
287
-
294
)
Lien
L
Dalgard
F
Heyerdahl
S
Thoresen
M
Bjertness
E
The relationship between age of menarche and mental distress in Norwegian adolescent girls and girls from different immigrant groups in Norway: results from an urban city cross-sectional survey
Soc Sci Med
 , 
2006
, vol. 
63
 (pg. 
285
-
295
)
Lillegaard
IT
Overby
NC
Andersen
LF
Can children and adolescents use photographs of food to estimate portion sizes?
Eur J Clin Nutr
 , 
2005
, vol. 
59
 (pg. 
611
-
617
)
Lipson
SF
Ellison
PT
Comparison of salivary steroid profiles in naturally occurring conception and non-conception cycles
Hum Reprod
 , 
1996
, vol. 
11
 (pg. 
2090
-
2096
)
Must
A
Phillips
SM
Naumova
EN
Blum
M
Harris
S
wson-Hughes
B
Rand
WM
Recall of early menstrual history and menarcheal body size: after 30 years, how well do women remember?
Am J Epidemiol
 , 
2002
, vol. 
155
 (pg. 
672
-
679
)
Nilssen
O
Forde
OH
The Tromso Study: the positive predictive value of gamma-glutamyltransferase and an alcohol questionnaire in the detection of early-stage risk drinkers
J Intern Med
 , 
1991
, vol. 
229
 (pg. 
497
-
500
)
Onland-Moret
NC
Peeters
PH
van Gils
CH
Clavel-Chapelon
F
Key
T
Tjonneland
A
Trichopoulou
A
Kaaks
R
Manjer
J
Panico
S
, et al.  . 
Age at menarche in relation to adult height: the EPIC study
Am J Epidemiol
 , 
2005
, vol. 
162
 (pg. 
623
-
632
)
Pike
MC
Spicer
DV
Dahmoush
L
Press
MF
Estrogens, progestogens, normal breast cell proliferation, and breast cancer risk
Epidemiol Rev
 , 
1993
, vol. 
15
 (pg. 
17
-
35
)
Rosenberg
M
Menarcheal age for Norwegian women born 1830-1960
Ann Hum Biol
 , 
1991
, vol. 
18
 (pg. 
207
-
219
)
Sturgeon
SR
Potischman
N
Malone
KE
Dorgan
JF
Daling
J
Schairer
C
Brinton
LA
Serum levels of sex hormones and breast cancer risk in premenopausal women: a case-control study (USA)
Cancer Causes Control
 , 
2004
, vol. 
15
 (pg. 
45
-
53
)
Thune
I
Brenn
T
Lund
E
Gaard
M
Physical activity and the risk of breast cancer
N Engl J Med
 , 
1997
, vol. 
336
 (pg. 
1269
-
1275
)
Thune
I
Njolstad
I
Lochen
ML
Forde
OH
Physical activity improves the metabolic risk profiles in men and women: the Tromso Study
Arch Intern Med
 , 
1998
, vol. 
158
 (pg. 
1633
-
1640
)
Travis
RC
Key
TJ
Oestrogen exposure and breast cancer risk
Breast Cancer Res
 , 
2003
, vol. 
5
 (pg. 
239
-
247
)
Ursin
G
Longnecker
MP
Haile
RW
Greenland
S
A meta-analysis of body mass index and risk of premenopausal breast cancer
Epidemiology
 , 
1995
, vol. 
6
 (pg. 
137
-
141
)
Verkasalo
PK
Thomas
HV
Appleby
PN
Davey
GK
Key
TJ
Circulating levels of sex hormones and their relation to risk factors for breast cancer: a cross-sectional study in 1092 pre- and postmenopausal women (United Kingdom)
Cancer Causes Control
 , 
2001
, vol. 
12
 (pg. 
47
-
59
)
Williams
NI
Lessons from experimental disruptions of the menstrual cycle in humans and monkeys
Med Sci Sports Exerc
 , 
2003
, vol. 
35
 (pg. 
1564
-
1572
)
Windham
GC
Elkin
E
Fenster
L
Waller
K
Anderson
M
Mitchell
PR
Lasley
B
Swan
SH
Ovarian hormones in premenopausal women: variation by demographic, reproductive and menstrual cycle characteristics
Epidemiology
 , 
2002
, vol. 
13
 (pg. 
675
-
684
)
Wu
MH
Chou
YC
Yu
JC
Yu
CP
Wu
CC
Chu
CM
Yang
T
Lai
CH
Hsieh
CY
You
SL
, et al.  . 
Hormonal and body-size factors in relation to breast cancer risk: a prospective study of 11,889 women in a low-incidence area
Ann Epidemiol
 , 
2006
, vol. 
16
 (pg. 
223
-
229
)